摘要:

R. I .C. ReviewsR.Z. C. Reviews, published twice yearly, reviews areas of chemistry of interestto the chemist who has no specialist knowledge of the field under review,but who wishes to keep abreast of the growth of chemistry as a discipline.These reviews should prove useful to students in familiarizing themselveswith a particular field.R.I.C. Reviews interprets the significance of chemistry in a wide contextand publishes articles on the economic, social and historical aspects ofchemistry, as well as on the research and applied sectors.Suggestions for future titles are welcomed. Prospective contributors shouldwrite to the Editor, enclosing a synopsis (of about 250 words) indicating thescope of their subject. The preferred length for reviews is 8000 words.Subscriptions from R.I.C. members are handled by the Royal Institute ofChemistry, 30 Russell Square, London WClB 5DT. All other subscriptionsare handled by The Chemical Society Publications Sales Office, BlackhorseRoad, Letchworth, Herts SG6 1HN.Annual Subscription: f2.50 (R.I.C. members, f l .SO

ISSN:0035-8940    

DOI:10.1039/RR97104FX001

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

STEREOVIEWING: VISUAL AIDS FOR STE REOC H EM ISTRY AN D MACRO MO LECU LAR STRUCTURES 21 lvor Smith, BSc, PhD, FRlC . . . . Courtauld Institute of Biochemistry, Middlesex Hospital Medical School, London W I Seeing in three dimensions Slides Photography, 21 Processing and mounting stereopairs for slides, 24 Stereoslide viewers, 25 . . .. . . . . .. 20 . . . . . . . . . . . . . . . . Positive prints . . Processing and mounting stereoprints, 26 Stereoprint viewers, 27 . . . . . . . . . . . . . . 30 .. . . . . .. . . .. .. 26 Stereoprojection . . The Viewmaster system . . . . . . . . . . . . . . 32 Other techniques . . . . 6 . 33 . . . . . . .. . . 32 References . . . . . . . . . . . . . . .. . . In the past it was possible to ignore the 3D aspects of chemical structure, but this is no longer true.For the study of crystal structure, conformational analysis and macromolecular structure it is essential to be able to comprehend the spatial arrangement of the atoms within the molecule. Not only must the chemist be in touch with these aspects but so equally must the biochemist, haematologist, pharmacologist and chemical pathologist. At all levels of education-from the Nuffield 0- and A-level syllabuses up to the ‘Recent advances’ series of postgraduate annuals-books and journals show two- dimensional diagrams and pictures in an endeavour to represent three- dimensional objects but, it must be stated quite openly, with little or no success. Even the most able representation is misleading.Perhaps the most often used picture in biochemistry is that of Dickerson’s myoglobinl(Fig. I ) but, in spite of its great value, it still leaves out a great deal of information because although one can see clearly what is in front and what behind, the all impor- tant information on ‘how much behind’ is missing. If one could present the data in the form of a three-dimensional representation or picture this would obviously be enormously valuable and the aim of this article is to describe and discuss all the currently available visual aid techniques for making and using such stereoviews and stereoprojections. Emphasis is placed on what is com- mercially available. It is assumed that the reader has an elementary knowledge of photography if he wishes to make his own stereopairs, but this is unneces- 19 Smith Fig.I. Dickerson’s myoglobin. sary if he only wishes to use those available. The overall procedure involves a number of quite discrete operations and these may differ depending on whether one is preparing stereoslides or stereoprints (photographs). The two sets of procedures are described under separate headings for slides and for positive prints and attention is drawn to similarities and differences as they occur. SEEING IN THREE DIMENSIONS The newborn child does not see from the first moment it opens its eyes. It has to learn to focus each eye and to co-ordinate the two, and it is only after some months that it learns to recognize objects. With growth comes experience and memory so that after a few years the child can look at, say, a photograph of a landscape and interpret the data it contains to make a three-dimensional reconstruction from the two-dimensional picture.It is much more difficult, if not impossible, to construct a three-dimensional interpretation from a flat picture which represents something for which there was no previous memory or experience. For this reason many students find molecular structure and stereochemistry incomprehensible-certainly I did when I was a student. The normal individual sees two views of an object, one with each eye, and these two views are displaced by about 6 cm from each other, i.e. the inter- ocular distance. Strictly speaking, the two views are displaced by 6 cm when the object is at infinity and the distance of displacement reduces as the object is brought nearer because of the ability of the eyes to converge.The two views are separately transmitted to the brain which then fuses or interprets them to R.I.C. Reviews 20 give a three-dimensional picture-hence a one-eyed man cannot see in stereo, The basis of any form of stereoview, therefore, is a pair of views, the one photographed by a displacement of 7 cm from the other and the two so placed that each point in the one is set 6 cm from the identical point in the other. Subsequently each eye is allowed to see only one of the two views, both of which are then separately transmitted to the brain as with normal vision. It must be stressed here that stereovision using the various devices to be described is a very personal matter amongst even those whose vision is entirely normal.Some are able to adjust their eyes and observe a stereo effect even without a viewer when looking at a stereopair on the printed page where- as others have great difficulty even with the viewer. The same is true when viewing stereoslides through the viewer or by stereoprojection on a screen. A small proportion of the population never sees stereo under these conditions. The majority can learn to see this way and, if it takes some time for this to occur, perseverance is well worthwhile. Those who wear glasses need have no fear that this is a disadvantage and they should not remove their glasses for stereoviewing.SLIDES Photography Two separate views of the object are required and these may be taken either simultaneously with a stereocamera or sequentially with a single lens camera which is moved between separate shots. I prefer a stereocamera because it is simpler and yet perfectly adequate and also because a stereomounting service is available. Many photography enthusiasts prefer the single shot method and do their own mounting, but when the photography is simply a means to a visual end then the time spent on this latter approach may be considered not worthwhile. Fig. 2. Stereocamera. Distance between midpoints of the lenses is 7cm. Because of the camera mechanics, pictures are taken on the 35 mm film at right angles to the normal direction and the size of exposure is usually 23 mm x 24 mm. 21 Smith I 1 1 -___ Layout Film I I x 2 x 3 x 4 x 5 x 6 x B 2 1 3 2 4 3 5 4 6 5 -+ 1 Pairs [ 1-1 2- I x 2 x 3 x 4 x 5 3-3 5- 5 2 6 4 Fig.3. Distribution of individual shots of stereopairs on 35 mm film in st:reocamera. The pair, I and I, present simultaneously behind the pair of lenses in the camera. B’ remains blank, i.e. unused. Stereocameras are available from Duval.2 The stereocamera (Fig. 2) has two lenses with a midpoint to midpoint distance of 7 cm and takes two photographs simultaneously on 35 mm film. The normal 20 exposure, 35 mm film will give 15 stereopairs and the 36 exposure film 28 pairs. The wind-on mechanism brings fresh film in front of each lens each time, of course, and it is an interest- ing exercise to work out how this can be done-the layout is shown in Fig.3. For most scientific uses the object will be fairly small and a close-up lens will be a necessary supplement to the camera. However, the camera view- finder remains unsupplemented and allowance must be made for this, leaving an adequate margin inside the viewfinder when observing the object. Some trial- and-error runs with cheap film will provide experience here and subsequent Fig. 4. Slide bars t o enable single lens camera t o be used for stereo. (below.) Slide bar available commercially.2 The knurled nut in front enables the slide t o be moved easily and then quickly fixed tightly in position. The camera screws on to the upper thread of the slide and the whole unit screws on t o a tripod in the usual manner.(upper right) A simple, home-made slide bar as described in the text. A, slot t o pass camera shank; B, hole t o pass camera base screw thread; C, f in female Whitworth thread; W, width of camera base. (lower right) Simple, swing-over slide bar. This bar allows a single 7 cm movement of the camera for left and right views. Only the 7 cm dimension i s important and the other figures can be adjusted to the size of the metal (Meccano) bar used to screw together the two horizontal pieces of w00d.l~ R. I.C. Reviews 22 _- . . work should be satisfactory although a small mask may be stuck around the outside edges of the viewer to eliminate the need for visual compensation. To make pairs without the use of a stereocamera, one must either move the camera or move the object.It is usually far simpler to move the camera and it is possible to buy a fairly cheap 'slide bar'2 for this purpose, or to make up an inexpensive slide bar or other device which will do the job equally well. Figure 4 shows some methods for doing this. To make a slide bar, take a piece of aluminium, say 2g in thick by 9 in long 23i n \ 'B A Smith 'C 23 by the width of the camera and at one end tap a + in female Whitworth thread centrally. This will screw on to the tripod head. Cut a slot 24 in in length to pass the shank of a camera case base screw with a slight enlargement at one end to pass the thread of the screw. It is useful to have sufficient uncovered in one position and then from that position to mark t in increases to cover the area of the bar to enable a zero position to be marked at the end of the camera full 2+ in slide.Although I said above that the two views should be taken with a displacement of the interocular distance this is true strictly only when the object is at infinity. As the object approaches, the eyes compensate by rotating inwards (this is readily observed by asking someone to hold one finger verti- cally at arm’s length and watching his eyes as the finger is brought up to the nose when the eyes are seen to ‘cross’), but the camera does not do this. The camera is made to compensate by using a larger than normal interocular distance of 7 cm for taking the shot whilst the pair is mounted at 6 cm separa- tion.However, the moving bar can be adjusted easily. It is assumed that 75 in (190 cm) or above is at infinity and the camera should then be moved along the bar the maximum distance of 2.5 in (63.5 mm)-this is known as the ‘times 30’ rule for obvious reasons. For distances closer than 75 in, measure the actual distance in inches from length to centre of object, divide by 30 and move the camera along the bar by that distance; for a 30 in separation the distance will be 1 in moved. With this method it is best to adopt a standard procedure of, say, taking the left-hand view first and then moving the camera to the right for the second view. Using the single lens camera it is also possible to copy pairs from books taking a photograph of each view separately.These pairs may be of photo- graphs or line drawings or computer graphics output although care should be taken not to infringe copyright where this applies. Processing and mounting stereopairs for slides For stereopairs taken with a stereocamera on Kodachrome, and Kodachrome only, Kodak provide a processing and mounting service and also a stereo- copying service. In both cases the product is a stereoslide and there is no service for provision of stereoprints or black and white stereo. Film is fre- quently bought at a price which includes processing but there is a slight addi- tional charge for stereomounting. Prior to these services, details should be obtained from Kodak.3 Pairs from a single lens camera must be mounted by the photographer concerned as, although mounting services are advertised they tend to be poor, particularly when the view is unfamiliar to the service mounter.Stereomounts are available from Duva12 and the cardboard mounts are to be recommended because they are much easier to work with than the glass and foil mounts. If a great deal of stereomounting is envisaged then it is possible to buy a mounting aid but possibly better to buy a stereocamera. Take the left-hand film view and position it as centrally as possible in the left-hand mounting space and stick it with Sellotape. Then place the right-hand film in the centre of the space pro- vided and measure so that the separation of a point in the middle of the view is between 5.8 and 6.2 cm from the corresponding point in the other view.Stick R . I. C. Reviews 24 down the second film. Carefully check the displacement, viewing through a slide viewer if necessary, and when satisfied that the displacement is correct, tape the two pieces of film firmly in position and stick down the mount cover. The slide mounting process is extremely tedious and, if a number have to be done, the most common result is a blinding headache and very poor slides until the knack of doing the job is found. Viewers are now readily available. Inexpensive viewers4 are fabricated from black cardboard in the form of a press-flat box (Fig. 5). Two lenses are placed at the front and two translucent screens at the back.The slide slips into a pocket and sits just in front of the screens. The viewer is picked up with one or both hands, the lenses placed to the eyes and the slide viewed either by looking through it out of a window or towards a lamp such that both views are equally illuminated. Finger pressure on the top and bottom of the box now causes it to open and the two views become visible. Adjustment of the pressure acts as a focusing mechanism and at one particular position, characteristic for the individual, the pair spring into stereo and a full three-dimensional effect is observed. The cost of such viewers is small and it is by no means impossible to consider having available up to a dozen viewers for use with small tutorial groups. Fig. 5. Stereoslide viewer.The stereoslide, seen at the top of the figure partly inserted into the viewer, has two views displaced by about 6cm from each other. Instructions on the viewer are completely explanatory. Stereoslide viewers Smith 25 Somewhat more expensive, battery or mains operated, viewers are also available2 for this purpose, but the cost would be prohibitive even for small tutorial groups. Boxes for storing stereoslides are also available.2 POSITIVE PRINTS A stereopair positive print is a pair of views either on the printed page or as a photograph and this may be in colour or in black and white. Such prints are no longer uncommon in the scientific literature or in the more modern text- bo0k;l~5~6 indeed prospective authors are urged to use stereopairs in future books as they are so easily prepared or obtained by kind permission ofjournal editors and others. In general, photography as described for slides is equally applicable here although a stereocamera is far less necessary as there is no service available for enlarging or mounting the final prints.Hence the single lens camera can be used to take the film and enlargements can be made in the normal way; even with colour this is not expensive as only the smallest enlargement is necessary. Black and white enlargements can also be made easily from colour slides, but, if stereopairs are used for this purpose, then it may be necessary to warn the processor not to remove the film from the stereomount as, otherwise, the pair will be returned unmounted or wrongly mounted; it is usually better to have a copy of the stereoslide made first and then to allow this copy to be opened for printing.Pairs photographed from books, etc. are, of course, the correct size for stereo and so the film should be re-enlarged to the original size. In enlarging, it must be remembered that the final size of the area to be viewed (not necessarily the size of the overall photo) must not be more than 6 cm wide, although it can be smaller. Such photos can be guillotined down to size during the mounting. The actual size may be conditioned by the vertical length of the figure as there is a limit to the upper length which can be taken in without the need to move the viewer. Processing and mounting stereoprints The negatives are printed and enlarged in the usual way, having made it clear to the photographer that each two constituting a stereopair must be enlarged identically and under standard conditions whether they are colour or black and white.Mounting pairs is a simple if tedious operation. Equal illumination onto both views is essential and I find good daylight better than good artificial light. Take the two views and place them adjacent or overlapping if there is a lot of free space on each side of the figure, View them through the stereoviewer and ensure that they can be adjusted to give a stereoview, i.e. that the left-hand figure is on the left and not the right, that the distance between identical points in the centres of both figures is 5.8 to 6.2 cm, etc.Note that if the two views are interchanged, then no stereo may be seen or an apparent stereo will be seen, but parts of the object seem to suddenly disappear from view. Trim the over- lapping sides if the separation distance is more than 6.2 cm. Now take one view (I usually take the left one), and stick it to a sheet of white paper so that the left and top edges of the photograph are square with the top edge of the paper (this just makes the operation a bit simpler). Then take the right-hand 26 R.Z. C. Reviews view, place it in position and adjust for optimum separation of 6 cm such that good stereo is observed with the viewer. Now carefully mark the positions of the outer edges of the photograph, remove and apply adhesive, return and carefully line up the photo within its marked position.Swift work is essential here because one must check the stereo before the adhesive dries properly in case an adjustment is still necessary. The above figures refer particularly to views taken with a 7 cm displacement on the stereocamera. However, with the smaller displacement possible on the sliding bar, the displacement may need to come down to as little as 5 cm. Again this figure may not be the best for every viewer as it is based on the average interocular distance and one chooses the ‘best’ view which will also be that which gives least eye strain. Fig. 6. Stereoprint viewer. Instructions on the viewer are self explanatory. The viewer is set up with a stereopair photograph in position (only one view of the pair can be seen) and is viewed by bending forward over the viewer so that the eyes are over the lenses and the nose fits the centre cut out.This viewer is the layflat design. Stereoprint viewers Inexpensive viewers4 have become readily available within recent years. Basically they are a lay-flat design which presses open and stands alone with the help of a centrally-placed partition (Fig. 6). Hence each eye sees only one of the Smith 27 two views, with the aid of lenses, and image fusion occurs in the brain. The partition is placed centrally between the two views, both equally illuminated, and the viewer moved around-and even raised or lowered slightly-until the stereoview comes into focus. Many people find this a difficult operation but they should persevere.If it seems difficult at first, the viewer should leave the prob- lem until early morning when the eyes are fresh and should ensure that illumination of both views is equal and strong but free from glare. A more sturdy but more expensive viewer (Fig. 7), used a great deal for aerial surveys (photogrammetry), is also available.7 This one can be adjusted for personal interocular distance as well as for vertical focus. Some people can see stereo without the aid of a viewer and as this is a useful attribute the technique will be described. Hold the slide pair up about 6-12 in from the eyes, looking through it towards a good even illumination. Focus the eyes on the distant light or infinity when three or four views will be seen.Ignore the outer two and try and fuse the inher two (still focusing on infinity) by moving the slide nearer or further away. If the two middle views are not in the same horizontal plane, rotate the slide gently until they line up. Again move the pair nearer or further and also left or right until the pair do fuse. Then slowly bring the eyes into focus on the single middle view which will now be seen to be in stereo. This often causes headaches especially if the eyes 28 Fig. 7. Viewer for print or slides. This Casella viewer is adjustable for personal interocular distance as well as vertical height above the photograph, both members of the pair being visible here. It is appreciably more expensive than the cardboard viewers shown in Figs 5 and 6.R.I.C. Reuiews are tired and should not be pursued under adverse circumstances. Daily practice in good conditions is helpful if one wishes to achieve success in this way. Stereoprints can be handled by the same technique except that lighting must be by reflection rather than transmission. Fig. 8. Stereoprojector and two Kodak Carousel projectors lined up for stereoprojection. Each member of the audience is wearing Polaroid glasses. 29 Smith STEREOPROJECTION The techniques described so far are ideal for individual viewing although they may be used for small tutorial groups without too much difficulty. However, in order to demonstrate to a whole class it is necessary to use stereoprojection onto a special film screen. The simplest procedure is to use a stereoprojector but, where this is not available, two monoprojectors may be coupled together.Only two models of stereoprojector seem to be available in the UK. A stereoslide, such as that previously described for use with slide viewers, is inserted into the projector which projects two views onto the screen. The apparatus is focused until two fairly sharp views are seen. By means of a vertical control knob, the two views are aligned in one horizontal plane. A horizontal control is now used to bring the two views into almost complete overlap. Total overlap is theoretically impossible because the two views, taken from different angles, must be slightly different, so overlap of the central area is chosen, and final focus is adjusted.Polarizing filters are now dropped into place in front of the slide views, the operator and audience put on Polaroid glasses and a 3D view is observed on the screen (Fig. 8). The audience should be warned not to watch this adjustment through their spectacles as many suffer strong headaches, but there is no adverse effect if this instruction is followed. A strange optical illusion is often observed. Sometimes one may 'see' the back of the view on the screen whereas the front projects out towards the viewer, whereas on other occasions the front is 'seen' on the screen and the back projects into the far distance; the reader will be able to think of similar optical illusions based on shadowed boxes, staircases, etc.Another interesting phenomenon which illustrates the principles involved is as follows. First, project any slide such as that of an optically active compound. Then remove the slide, rotate it 180" and replace it so that it is projected, still upright, but back to front and with the two views exchanged. The projection now appears to be the mirror image. Two Kodak Carousel S monoprojectors can be adapt- ed as described by Walsh and Lamey.8 The projectors are mounted on sub- stages as shown in Fig. 8, each sub-stage being on ratchets and ball-and- socket joints so that they can be swivelled horizontally to bring about image superimposition. Vertical adjustment is by the usual manipulation of projec- tor feet. Polaroid filters, readily obtainable from dealers in physics apparatus, are cut, framed and mounted over each lens with Sellotape.One beam is polarized at 45" to the vertical and the other at 135" as this is suitable when using the normal, commercially available Polaroid spectacles.2 Finally, the two projectors can be linked to move slides forward simultaneously. This method of projection requires two separate slides, one for each projector, and these can be readily prepared with a single lens camera and slide bar as pre- viously described but care should be taken to mark the slides so that the two members of a pair are readily found and it is known which is left and which right. The light beam passes through one Polaroid filter, is reflected from the screen back through a secondpolaroidinto the eye of theviewer (Fig.9). Thepro- jector must, therefore, have a powerful beam to overcome light losses. Further- more, the projector light-beam must fall at 90" on to the screen and definitely not at a wide angle as often happens with single slide projection. Likewise R.I.C. Reviews 30 Fig. 9. Diagram t o show light paths from two separate projectors t o the eyes. Here the left eye i s taking the view from the left projector because of the polaroids being so paired. However the left eye view could come from the right projector if the Polaroid filters in front of the projectors were interchanged which is what normally happens when a stereoprojector is used. PF = Polaroid filter, PS = Polaroid spectacles, S e screen, P = projector.Reproduced from ref. 14. maximum reflection is directly back towards the projector, reflection falls off rapidly to the sides and viewing is unsatisfactory outside an angle of about 35" each side of the screen midpoint vertical. A maximum screen size of 5 ft x 5 ft is generally adequate but 4 ft x 4 ft is often sufficient as the smaller size is more than compensated for by the stereo effect. Ordinary screens depolarize the beam and so special screens are necessary although these are cheap to prepare and a number of methods will be mentioned. If a perfectly flat and central wall is available then a suitable area is painted silver or aluminium followed by an aerosol silver spray finish. Alternatively a 1 in blockboard is covered with hardboard, given several coats of aluminium paint and finished with a silver spray.In place of aluminium paint, ordinary aluminium kitchen foil has been recommended and this is applied, matt-side uppermost, with careful rolling out and stretching. Lumaplakg silver lenticular screen material can be bought by the yard and is also very satisfactory for a permanent but moveable screen. This is affixed to hardboard-covered blockboard by means of tacks at the top of the board, unrolled and stretched with weights at the bottom until stretched well. The sides are then tacked from the top downwards, the bottom edge is tacked on and the four sides then covered with aluminium metal edging. Whatever type of screen is used, it must be kept covered when not in use and should never be touched except at the edges as dirt, oil from the hands, etc.have a depolarizing effect on the screen. THE VIEWMASTER SYSTEM Everyone will be familiar with the Viewmaster wheels and viewers-if not then they can be seen at any large camera or department store. The wheel contains seven stereopairs, the members of each pair being at opposite ends of the wheel diameter. The system is, however, complete for the production and use of stereopairs.1° The camera takes pairs of film, each about 1 1 mm square, so that the normal 20 exposure film yields 40 pairs and the 36 film about 75 pairs. The developed film can be cut with a device which places the pairs into an empty wheel. The reel is viewed through the hand viewer or stereoscope of which a number of different models are available.Alterna- tively, the reel may be projected through the stereomatic projector which is very satisfactory for small rooms but not adequate for larger lecture theatres. At present there is no suitable material for molecular structure studies. However, the Open University has done some preliminary work and it seems reasonably likely that suitable reels will become available in the future. OTHER TECHNIQUES Holographyll is a technique whereby a three-dimensional view may be captured in a special piece of glass which may be about 3 in square. When this glass is irradiated with laser light it is possible to see a three-dimensional view of the object directly without glasses, screens or any other device.Although it is currently far too expensive for general use, there can be little doubt that holograms will play a valuable part in future educational technology. Com- puter graphics12 is another valuable procedure the end products of which will shortly be available to all. It is now possible to feed data such as x-ray co- R.I. C. Reviews 32 ordinates into a computer which will subsequently plot out the model struc- ture and throw the results onto a television screen, from which a permanent photographic record can be made. The figure can be moved readily to provide other views including the second of a stereopair for further photography. Although computers are not generally available, their stereographics are already appearing in textbooks1 as well as in research journals.The graphics can be used to illustrate many points; for example, pairs may show the com- plete model of a protein, or just the backbone, or an expanded view of the active site, each from a number of different viewing positions and the scope of this type of presentation has been barely appreciated so far. Although stereoviewing is comparatively new in the biological and chemical sciences, it has been used for many years by those concerned with aerial survey, geology and various types of map making and contour surveying. Help and advice may therefore be sought from such local sources as are con- cerned with these problems. Another source of help may well be the local members of the 3D Society.13 The standard work on this subject is Stereoscopy by N.A. Valyus but other valuable works include The manual of stereophoto- graphy by K. C . M. Symons and Stereo realist manual by Morgan and Lester; all are available from Duval.2 The above discussion is based on the assumption that a far greater com- prehension of any molecular structure may be obtained from a 3D stereoview than from the usual, 2D flat view, whether it be a diagram or photograph. In my own experience, using stereoprints, stereoslides and stereoprojection, it is undoubtedly so. However, a recent study14 on two groups of students who went through a teaching programme on crystallography found that both knowledge and understanding were gained faster and more easily in the group provided with stereopictures and viewers than in the control group who were given only the usual single views.REFERENCES 1 R. E. Dickerson and I. Geis. The structure and function of proteins. New York: Harper and Row, 1969. 2 (a) Duval Studies, 217 High Road, Chiswick, London, W4; (6) Marshall Smith Ltd, 64-74 Norwich Avenue, Bournemouth, Hants. 3 Kodak Ltd, Maylands Avenue, Heme1 Hempstead, Herts. 4 Capital Biotechnic Developments, 66A Churchfield Road, London, W3. 5 A. F. Wells, The third dimension in chemistry. Oxford: Clarendon Press, 1968. 6 I. Smith, M. J. Smith and C. F. Do& Biobits and Atomunits-principles of model construction, from (4) above. 7 Casella Pocket Stereoscopes, Regent House, Britannia Walk, London, N1. 8 P.Walsh and J. K. Lamey, Times Educational Supplement and personal communication. 9 Lumaplak Screen Co, Engine Lane, Low Fell, Gateshead 9, NE9 SJJ. 10 Viewmaster, GAF Ltd, 30 Engate St, London, SE13. 11 D. R. Herriott, Scient. Amer. 1968 (Sept.), 219, 141 ; S. Pennington, Scient. Amer. 1968 (Feb.), 219, 40. 12 C. Levinthal, ‘Computer graphics’, Scient. Amer. 1966 (June), 214, 42. 13 3D Society. Secretary: Mr W. Leybourne, 5 Southfield Rd, Middlesbrough, Teesside, TS1 3BX. 14 R. F. Kempa and D. G. Holford, Educ. Chem. 1971, 8, 100, and J . Rer. Sci. Teaching, 1970, 7 , 265. Smith 33 3 STEREOVIEWING: VISUAL AIDS FORSTE REOC H EM ISTRY AN D MACRO MO LECU LARSTRUCTURESlvor Smith, BSc, PhD, FRlCCourtauld Institute of Biochemistry, Middlesex Hospital Medical School, London W ISeeing in three dimensions .. . . .. . . . . .. 20Slides . . . .Photography, 21Processing and mounting stereopairs for slides, 24Stereoslide viewers, 25Processing and mounting stereoprints, 26Stereoprint viewers, 27. . . . . . . . . . . . . . 21Positive prints . . .. . . . . .. . . .. .. 26Stereoprojection . . . . . . . . . . . . . . . . 30The Viewmaster system . . . . . . . . . . . . . . 32Other techniques . . . . 6 . . . . . . . .. . . 32References . . . . . . . . . . . . . . .. . . 33In the past it was possible to ignore the 3D aspects of chemical structure, butthis is no longer true. For the study of crystal structure, conformationalanalysis and macromolecular structure it is essential to be able to comprehendthe spatial arrangement of the atoms within the molecule.Not only must thechemist be in touch with these aspects but so equally must the biochemist,haematologist, pharmacologist and chemical pathologist. At all levels ofeducation-from the Nuffield 0- and A-level syllabuses up to the ‘Recentadvances’ series of postgraduate annuals-books and journals show two-dimensional diagrams and pictures in an endeavour to represent three-dimensional objects but, it must be stated quite openly, with little or nosuccess. Even the most able representation is misleading. Perhaps the mostoften used picture in biochemistry is that of Dickerson’s myoglobinl(Fig. I ) but,in spite of its great value, it still leaves out a great deal of information becausealthough one can see clearly what is in front and what behind, the all impor-tant information on ‘how much behind’ is missing. If one could present thedata in the form of a three-dimensional representation or picture this wouldobviously be enormously valuable and the aim of this article is to describe anddiscuss all the currently available visual aid techniques for making and usingsuch stereoviews and stereoprojections. Emphasis is placed on what is com-mercially available.It is assumed that the reader has an elementary knowledgeof photography if he wishes to make his own stereopairs, but this is unneces-Smith 1Fig. I. Dickerson’s myoglobin.sary if he only wishes to use those available. The overall procedure involves anumber of quite discrete operations and these may differ depending onwhether one is preparing stereoslides or stereoprints (photographs).The twosets of procedures are described under separate headings for slides and forpositive prints and attention is drawn to similarities and differences as theyoccur.SEEING IN THREE DIMENSIONSThe newborn child does not see from the first moment it opens its eyes. It hasto learn to focus each eye and to co-ordinate the two, and it is only after somemonths that it learns to recognize objects. With growth comes experience andmemory so that after a few years the child can look at, say, a photograph of alandscape and interpret the data it contains to make a three-dimensionalreconstruction from the two-dimensional picture.It is much more difficult, ifnot impossible, to construct a three-dimensional interpretation from a flatpicture which represents something for which there was no previous memoryor experience. For this reason many students find molecular structure andstereochemistry incomprehensible-certainly I did when I was a student.The normal individual sees two views of an object, one with each eye, andthese two views are displaced by about 6 cm from each other, i.e. the inter-ocular distance. Strictly speaking, the two views are displaced by 6 cm whenthe object is at infinity and the distance of displacement reduces as the objectis brought nearer because of the ability of the eyes to converge. The two viewsare separately transmitted to the brain which then fuses or interprets them to20 R.I.C.Reviewgive a three-dimensional picture-hence a one-eyed man cannot see in stereo,The basis of any form of stereoview, therefore, is a pair of views, the onephotographed by a displacement of 7 cm from the other and the two so placedthat each point in the one is set 6 cm from the identical point in the other.Subsequently each eye is allowed to see only one of the two views, both of whichare then separately transmitted to the brain as with normal vision.It must be stressed here that stereovision using the various devices to bedescribed is a very personal matter amongst even those whose vision isentirely normal. Some are able to adjust their eyes and observe a stereo effecteven without a viewer when looking at a stereopair on the printed page where-as others have great difficulty even with the viewer.The same is true whenviewing stereoslides through the viewer or by stereoprojection on a screen.A small proportion of the population never sees stereo under these conditions.The majority can learn to see this way and, if it takes some time for this tooccur, perseverance is well worthwhile. Those who wear glasses need have nofear that this is a disadvantage and they should not remove their glasses forstereoviewing.SLIDESPhotographyTwo separate views of the object are required and these may be taken eithersimultaneously with a stereocamera or sequentially with a single lens camerawhich is moved between separate shots. I prefer a stereocamera because it issimpler and yet perfectly adequate and also because a stereomounting serviceis available.Many photography enthusiasts prefer the single shot method anddo their own mounting, but when the photography is simply a means to avisual end then the time spent on this latter approach may be considered notworthwhile.Fig. 2. Stereocamera. Distance between midpoints of the lenses is 7cm. Because of thecamera mechanics, pictures are taken on the 35 mm film at right angles to the normal directionand the size of exposure is usually 23 mm x 24 mm.Smith 2_- . . -___I 1 Film I B 2 1 3 2 4 3 5 4 6 5 -+I x 2 x 3 x 4 x 5 x 6 xI x 2 x 3 x 4 x 5 1 Layout1-1 3-3 5- 5 1 Pairs [ 2- 2 6 4Fig. 3. Distribution of individual shots of stereopairs on 35 mm film in st:reocamera. The pair,I and I, present simultaneously behind the pair of lenses in the camera.B’ remains blank, i.e.unused.Stereocameras are available from Duval.2 The stereocamera (Fig. 2) has twolenses with a midpoint to midpoint distance of 7 cm and takes two photographssimultaneously on 35 mm film. The normal 20 exposure, 35 mm film will give15 stereopairs and the 36 exposure film 28 pairs. The wind-on mechanismbrings fresh film in front of each lens each time, of course, and it is an interest-ing exercise to work out how this can be done-the layout is shown in Fig. 3.For most scientific uses the object will be fairly small and a close-up lenswill be a necessary supplement to the camera. However, the camera view-finder remains unsupplemented and allowance must be made for this, leavingan adequate margin inside the viewfinder when observing the object.Some trial-and-error runs with cheap film will provide experience here and subsequentFig. 4. Slide bars t o enable single lens camera t o be used for stereo. (below.) Slide bar availablecommercially.2 The knurled nut in front enables the slide t o be moved easily and then quicklyfixed tightly in position. The camera screws on to the upper thread of the slide and the wholeunit screws on t o a tripod in the usual manner. (upper right) A simple, home-made slide bar asdescribed in the text. A, slot t o pass camera shank; B, hole t o pass camera base screw thread;C, f in female Whitworth thread; W, width of camera base. (lower right) Simple, swing-overslide bar.This bar allows a single 7 cm movement of the camera for left and right views. Onlythe 7 cm dimension i s important and the other figures can be adjusted to the size of the metal(Meccano) bar used to screw together the two horizontal pieces of w00d.l~22 R. I.C. Reviewwork should be satisfactory although a small mask may be stuck around theoutside edges of the viewer to eliminate the need for visual compensation.To make pairs without the use of a stereocamera, one must either move thecamera or move the object. It is usually far simpler to move the camera and it ispossible to buy a fairly cheap 'slide bar'2 for this purpose, or to make up aninexpensive slide bar or other device which will do the job equally well.Figure 4shows some methods for doing this.To make a slide bar, take a piece of aluminium, say 2g in thick by 9 in long23i n\'B 'CASmith 2by the width of the camera and at one end tap a + in female Whitworth threadcentrally. This will screw on to the tripod head. Cut a slot 24 in in length topass the shank of a camera case base screw with a slight enlargement at oneend to pass the thread of the screw. It is useful to have sufficient uncoveredarea of the bar to enable a zero position to be marked at the end of the camerain one position and then from that position to mark t in increases to cover thefull 2+ in slide. Although I said above that the two views should be taken witha displacement of the interocular distance this is true strictly only when theobject is at infinity. As the object approaches, the eyes compensate by rotatinginwards (this is readily observed by asking someone to hold one finger verti-cally at arm’s length and watching his eyes as the finger is brought up to thenose when the eyes are seen to ‘cross’), but the camera does not do this.Thecamera is made to compensate by using a larger than normal interoculardistance of 7 cm for taking the shot whilst the pair is mounted at 6 cm separa-tion. However, the moving bar can be adjusted easily. It is assumed that 75 in(190 cm) or above is at infinity and the camera should then be moved along thebar the maximum distance of 2.5 in (63.5 mm)-this is known as the ‘times 30’rule for obvious reasons. For distances closer than 75 in, measure the actualdistance in inches from length to centre of object, divide by 30 and move thecamera along the bar by that distance; for a 30 in separation the distancewill be 1 in moved.With this method it is best to adopt a standard procedureof, say, taking the left-hand view first and then moving the camera to theright for the second view.Using the single lens camera it is also possible to copy pairs from bookstaking a photograph of each view separately. These pairs may be of photo-graphs or line drawings or computer graphics output although care should betaken not to infringe copyright where this applies.Processing and mounting stereopairs for slidesFor stereopairs taken with a stereocamera on Kodachrome, and Kodachromeonly, Kodak provide a processing and mounting service and also a stereo-copying service.In both cases the product is a stereoslide and there is noservice for provision of stereoprints or black and white stereo. Film is fre-quently bought at a price which includes processing but there is a slight addi-tional charge for stereomounting. Prior to these services, details should beobtained from Kodak.3Pairs from a single lens camera must be mounted by the photographerconcerned as, although mounting services are advertised they tend to be poor,particularly when the view is unfamiliar to the service mounter. Stereomountsare available from Duva12 and the cardboard mounts are to be recommendedbecause they are much easier to work with than the glass and foil mounts.If agreat deal of stereomounting is envisaged then it is possible to buy a mountingaid but possibly better to buy a stereocamera. Take the left-hand film view andposition it as centrally as possible in the left-hand mounting space and stick itwith Sellotape. Then place the right-hand film in the centre of the space pro-vided and measure so that the separation of a point in the middle of the view isbetween 5.8 and 6.2 cm from the corresponding point in the other view. Stick24 R . I. C. ReviewFig. 5. Stereoslide viewer. The stereoslide, seen at the top of the figure partly inserted intothe viewer, has two views displaced by about 6cm from each other. Instructions on theviewer are completely explanatory.down the second film.Carefully check the displacement, viewing through aslide viewer if necessary, and when satisfied that the displacement is correct,tape the two pieces of film firmly in position and stick down the mount cover.The slide mounting process is extremely tedious and, if a number have to bedone, the most common result is a blinding headache and very poor slidesuntil the knack of doing the job is found.Stereoslide viewersViewers are now readily available. Inexpensive viewers4 are fabricated fromblack cardboard in the form of a press-flat box (Fig. 5). Two lenses areplaced at the front and two translucent screens at the back. The slide slips intoa pocket and sits just in front of the screens. The viewer is picked up with oneor both hands, the lenses placed to the eyes and the slide viewed either bylooking through it out of a window or towards a lamp such that both views areequally illuminated.Finger pressure on the top and bottom of the box nowcauses it to open and the two views become visible. Adjustment of the pressureacts as a focusing mechanism and at one particular position, characteristic forthe individual, the pair spring into stereo and a full three-dimensional effect isobserved. The cost of such viewers is small and it is by no means impossible toconsider having available up to a dozen viewers for use with small tutorialgroups.Smith 2Somewhat more expensive, battery or mains operated, viewers are alsoavailable2 for this purpose, but the cost would be prohibitive even for smalltutorial groups.Boxes for storing stereoslides are also available.2POSITIVE PRINTSA stereopair positive print is a pair of views either on the printed page or as aphotograph and this may be in colour or in black and white. Such prints are nolonger uncommon in the scientific literature or in the more modern text-bo0k;l~5~6 indeed prospective authors are urged to use stereopairs in futurebooks as they are so easily prepared or obtained by kind permission ofjournaleditors and others.In general, photography as described for slides is equally applicable herealthough a stereocamera is far less necessary as there is no service available forenlarging or mounting the final prints. Hence the single lens camera can beused to take the film and enlargements can be made in the normal way; evenwith colour this is not expensive as only the smallest enlargement is necessary.Black and white enlargements can also be made easily from colour slides, but,if stereopairs are used for this purpose, then it may be necessary to warn theprocessor not to remove the film from the stereomount as, otherwise, the pairwill be returned unmounted or wrongly mounted; it is usually better to have acopy of the stereoslide made first and then to allow this copy to be opened forprinting.Pairs photographed from books, etc. are, of course, the correct sizefor stereo and so the film should be re-enlarged to the original size.In enlarging, it must be remembered that the final size of the area to beviewed (not necessarily the size of the overall photo) must not be more than6 cm wide, although it can be smaller.Such photos can be guillotined down tosize during the mounting. The actual size may be conditioned by the verticallength of the figure as there is a limit to the upper length which can be taken inwithout the need to move the viewer.Processing and mounting stereoprintsThe negatives are printed and enlarged in the usual way, having made it clearto the photographer that each two constituting a stereopair must be enlargedidentically and under standard conditions whether they are colour or blackand white. Mounting pairs is a simple if tedious operation. Equal illuminationonto both views is essential and I find good daylight better than good artificiallight.Take the two views and place them adjacent or overlapping if there is a lot offree space on each side of the figure, View them through the stereoviewer andensure that they can be adjusted to give a stereoview, i.e.that the left-handfigure is on the left and not the right, that the distance between identical pointsin the centres of both figures is 5.8 to 6.2 cm, etc. Note that if the two views areinterchanged, then no stereo may be seen or an apparent stereo will be seen,but parts of the object seem to suddenly disappear from view. Trim the over-lapping sides if the separation distance is more than 6.2 cm. Now take oneview (I usually take the left one), and stick it to a sheet of white paper so thatthe left and top edges of the photograph are square with the top edge of thepaper (this just makes the operation a bit simpler).Then take the right-hand26 R.Z. C. ReviewFig. 6. Stereoprint viewer. Instructions on the viewer are self explanatory. The viewer is setup with a stereopair photograph in position (only one view of the pair can be seen) and isviewed by bending forward over the viewer so that the eyes are over the lenses and the nosefits the centre cut out. This viewer is the layflat design.view, place it in position and adjust for optimum separation of 6 cm such thatgood stereo is observed with the viewer. Now carefully mark the positions ofthe outer edges of the photograph, remove and apply adhesive, return andcarefully line up the photo within its marked position. Swift work is essentialhere because one must check the stereo before the adhesive dries properly incase an adjustment is still necessary.The above figures refer particularly to views taken with a 7 cm displacementon the stereocamera.However, with the smaller displacement possible on thesliding bar, the displacement may need to come down to as little as 5 cm.Again this figure may not be the best for every viewer as it is based on theaverage interocular distance and one chooses the ‘best’ view which will also bethat which gives least eye strain.Stereoprint viewersInexpensive viewers4 have become readily available within recent years.Basically they are a lay-flat design which presses open and stands alone with thehelp of a centrally-placed partition (Fig.6). Hence each eye sees only one of theSmith 2Fig. 7. Viewer for print or slides. This Casella viewer is adjustable for personal interoculardistance as well as vertical height above the photograph, both members of the pair beingvisible here. It is appreciably more expensive than the cardboard viewers shown in Figs 5 and6.two views, with the aid of lenses, and image fusion occurs in the brain. Thepartition is placed centrally between the two views, both equally illuminated,and the viewer moved around-and even raised or lowered slightly-until thestereoview comes into focus. Many people find this a difficult operation but theyshould persevere. If it seems difficult at first, the viewer should leave the prob-lem until early morning when the eyes are fresh and should ensure thatillumination of both views is equal and strong but free from glare.A moresturdy but more expensive viewer (Fig. 7), used a great deal for aerial surveys(photogrammetry), is also available.7 This one can be adjusted for personalinterocular distance as well as for vertical focus.Some people can see stereo without the aid of a viewer and as this is a usefulattribute the technique will be described. Hold the slide pair up about 6-12 infrom the eyes, looking through it towards a good even illumination. Focus theeyes on the distant light or infinity when three or four views will be seen.Ignore the outer two and try and fuse the inher two (still focusing on infinity)by moving the slide nearer or further away.If the two middle views are not inthe same horizontal plane, rotate the slide gently until they line up. Againmove the pair nearer or further and also left or right until the pair do fuse.Then slowly bring the eyes into focus on the single middle view which willnow be seen to be in stereo. This often causes headaches especially if the eyes28 R.I.C. Reuieware tired and should not be pursued under adverse circumstances. Dailypractice in good conditions is helpful if one wishes to achieve success in thisway. Stereoprints can be handled by the same technique except that lightingmust be by reflection rather than transmission.Fig. 8. Stereoprojector and two Kodak Carousel projectors lined up for stereoprojection.Each member of the audience is wearing Polaroid glasses.Smith 2STEREOPROJECTIONThe techniques described so far are ideal for individual viewing although theymay be used for small tutorial groups without too much difficulty.However, inorder to demonstrate to a whole class it is necessary to use stereoprojectiononto a special film screen. The simplest procedure is to use a stereoprojectorbut, where this is not available, two monoprojectors may be coupled together.Only two models of stereoprojector seem to be available in the UK.A stereoslide, such as that previously described for use with slide viewers, isinserted into the projector which projects two views onto the screen. Theapparatus is focused until two fairly sharp views are seen. By means of avertical control knob, the two views are aligned in one horizontal plane.Ahorizontal control is now used to bring the two views into almost completeoverlap. Total overlap is theoretically impossible because the two views, takenfrom different angles, must be slightly different, so overlap of the central areais chosen, and final focus is adjusted. Polarizing filters are now dropped intoplace in front of the slide views, the operator and audience put on Polaroidglasses and a 3D view is observed on the screen (Fig. 8). The audience shouldbe warned not to watch this adjustment through their spectacles as many sufferstrong headaches, but there is no adverse effect if this instruction is followed.A strange optical illusion is often observed. Sometimes one may 'see' theback of the view on the screen whereas the front projects out towards theviewer, whereas on other occasions the front is 'seen' on the screen and theback projects into the far distance; the reader will be able to think of similaroptical illusions based on shadowed boxes, staircases, etc.Another interestingphenomenon which illustrates the principles involved is as follows. First,project any slide such as that of an optically active compound. Then removethe slide, rotate it 180" and replace it so that it is projected, still upright, butback to front and with the two views exchanged. The projection now appearsto be the mirror image. Two Kodak Carousel S monoprojectors can be adapt-ed as described by Walsh and Lamey.8 The projectors are mounted on sub-stages as shown in Fig.8, each sub-stage being on ratchets and ball-and-socket joints so that they can be swivelled horizontally to bring about imagesuperimposition. Vertical adjustment is by the usual manipulation of projec-tor feet. Polaroid filters, readily obtainable from dealers in physics apparatus,are cut, framed and mounted over each lens with Sellotape. One beam ispolarized at 45" to the vertical and the other at 135" as this is suitable whenusing the normal, commercially available Polaroid spectacles.2 Finally, thetwo projectors can be linked to move slides forward simultaneously. Thismethod of projection requires two separate slides, one for each projector, andthese can be readily prepared with a single lens camera and slide bar as pre-viously described but care should be taken to mark the slides so that the twomembers of a pair are readily found and it is known which is left and whichright.The light beam passes through one Polaroid filter, is reflected from thescreen back through a secondpolaroidinto the eye of theviewer (Fig.9). Thepro-jector must, therefore, have a powerful beam to overcome light losses. Further-more, the projector light-beam must fall at 90" on to the screen and definitelynot at a wide angle as often happens with single slide projection. Likewise30 R.I.C. ReviewFig. 9. Diagram t o show light paths from two separate projectors t o the eyes. Here the left eyebecause of the polaroids being so paired. However the left eye view could come from the rightprojectors were interchanged which is what normally happens when a stereoprojector is used.PFS e screen, P = projector. Reproduced from ref. 14maximum reflection is directly back towards the projector, reflection falls offrapidly to the sides and viewing is unsatisfactory outside an angle of about 35"each side of the screen midpoint vertical. A maximum screen size of 5 ft x 5 ftis generally adequate but 4 ft x 4 ft is often sufficient as the smaller size is morethan compensated for by the stereo effect. Ordinary screens depolarize thebeam and so special screens are necessary although these are cheap to prepareand a number of methods will be mentioned. If a perfectly flat and central wallis available then a suitable area is painted silver or aluminium followed by anaerosol silver spray finish.Alternatively a 1 in blockboard is covered withhardboard, given several coats of aluminium paint and finished with a silverspray. In place of aluminium paint, ordinary aluminium kitchen foil has beenrecommended and this is applied, matt-side uppermost, with careful rollingout and stretching. Lumaplakg silver lenticular screen material can be boughtby the yard and is also very satisfactory for a permanent but moveable screen.This is affixed to hardboard-covered blockboard by means of tacks at the topof the board, unrolled and stretched with weights at the bottom until stretchedwell. The sides are then tacked from the top downwards, the bottom edge istacked on and the four sides then covered with aluminium metal edging.Whatever type of screen is used, it must be kept covered when not in use andshould never be touched except at the edges as dirt, oil from the hands, etc.have a depolarizing effect on the screen.THE VIEWMASTER SYSTEMEveryone will be familiar with the Viewmaster wheels and viewers-if notthen they can be seen at any large camera or department store.The wheelcontains seven stereopairs, the members of each pair being at opposite endsof the wheel diameter. The system is, however, complete for the productionand use of stereopairs.1° The camera takes pairs of film, each about 1 1 mmsquare, so that the normal 20 exposure film yields 40 pairs and the 36 filmabout 75 pairs. The developed film can be cut with a device which places thepairs into an empty wheel.The reel is viewed through the hand viewer orstereoscope of which a number of different models are available. Alterna-tively, the reel may be projected through the stereomatic projector which isvery satisfactory for small rooms but not adequate for larger lecture theatres.At present there is no suitable material for molecular structure studies.However, the Open University has done some preliminary work and it seemsreasonably likely that suitable reels will become available in the future.OTHER TECHNIQUESHolographyll is a technique whereby a three-dimensional view may becaptured in a special piece of glass which may be about 3 in square. When thisglass is irradiated with laser light it is possible to see a three-dimensional viewof the object directly without glasses, screens or any other device.Although itis currently far too expensive for general use, there can be little doubt thatholograms will play a valuable part in future educational technology. Com-puter graphics12 is another valuable procedure the end products of which willshortly be available to all. It is now possible to feed data such as x-ray co-32 R.I. C. Reviewordinates into a computer which will subsequently plot out the model struc-ture and throw the results onto a television screen, from which a permanentphotographic record can be made. The figure can be moved readily to provideother views including the second of a stereopair for further photography.Although computers are not generally available, their stereographics arealready appearing in textbooks1 as well as in research journals. The graphicscan be used to illustrate many points; for example, pairs may show the com-plete model of a protein, or just the backbone, or an expanded view of theactive site, each from a number of different viewing positions and the scope ofthis type of presentation has been barely appreciated so far.Although stereoviewing is comparatively new in the biological and chemicalsciences, it has been used for many years by those concerned with aerialsurvey, geology and various types of map making and contour surveying.Help and advice may therefore be sought from such local sources as are con-cerned with these problems. Another source of help may well be the localmembers of the 3D Society.13 The standard work on this subject is Stereoscopyby N. A. Valyus but other valuable works include The manual of stereophoto-graphy by K. C . M. Symons and Stereo realist manual by Morgan and Lester;all are available from Duval.2The above discussion is based on the assumption that a far greater com-prehension of any molecular structure may be obtained from a 3D stereoviewthan from the usual, 2D flat view, whether it be a diagram or photograph. Inmy own experience, using stereoprints, stereoslides and stereoprojection, it isundoubtedly so. However, a recent study14 on two groups of students whowent through a teaching programme on crystallography found that bothknowledge and understanding were gained faster and more easily in the groupprovided with stereopictures and viewers than in the control group who weregiven only the usual single views.REFERENCES1234567891011121314R. E. Dickerson and I. Geis. The structure and function of proteins. New York: Harperand Row, 1969.(a) Duval Studies, 217 High Road, Chiswick, London, W4;(6) Marshall Smith Ltd, 64-74 Norwich Avenue, Bournemouth, Hants.Kodak Ltd, Maylands Avenue, Heme1 Hempstead, Herts.Capital Biotechnic Developments, 66A Churchfield Road, London, W3.A. F. Wells, The third dimension in chemistry. Oxford: Clarendon Press, 1968.I. Smith, M. J. Smith and C. F. Do& Biobits and Atomunits-principles of modelconstruction, from (4) above.Casella Pocket Stereoscopes, Regent House, Britannia Walk, London, N1.P. Walsh and J. K. Lamey, Times Educational Supplement and personal communication.Lumaplak Screen Co, Engine Lane, Low Fell, Gateshead 9, NE9 SJJ.Viewmaster, GAF Ltd, 30 Engate St, London, SE13.D. R. Herriott, Scient. Amer. 1968 (Sept.), 219, 141 ; S. Pennington, Scient. Amer. 1968(Feb.), 219, 40.C. Levinthal, ‘Computer graphics’, Scient. Amer. 1966 (June), 214, 42.3D Society. Secretary: Mr W. Leybourne, 5 Southfield Rd, Middlesbrough, Teesside,TS1 3BX.R. F. Kempa and D. G. Holford, Educ. Chem. 1971, 8, 100, and J . Rer. Sci. Teaching,1970, 7 , 265.Smith33

ISSN:0035-8940    

DOI:10.1039/RR9710400019

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

THE HENRYS OF MANCHESTER Sixth Grove Lecture W. V. Farrar, BSc, PhD, and Kathleen R. Farrar, BSc, PhD Dept of History of Science and Technology, UMIST, Manchestei I E. L. Scott, MSc Stamford High School, Stamford, Lincs Thomas Henry’s early life Manchester life in the late 1700s 40 . . .. .. . . . . . . 35 . . .. . . . . . . 37 Henry’s magnesia . . * . .. . . .. . . . . . . William Henry . . .. . . . . .. . . . . .. 42 Later generations . . . . . . .. . . . . . . . . 45 Most history is about great men, and the history of science is no exception. Lavoisier, Dalton, Berthollet, Berzelius, set the course of 19th century chemistry; we know and we write a lot about such people, because they are very important. But let us think of the other side of the coin-of the people whose courses were set by the great men, the scientists of second rank, the ‘common scientists’.It is these people, after all, who do 99 per cent of all science, who carry on the daily business of science. They teach, they work in industry, they write textbooks; they do their bits of research which once in a lifetime may touch greatness-or may not. The Henry family of Manchester were such ‘common scientists’, and we have found them a very rewarding study. They did all the things just mentioned, and ran medical practices as well; one of them, William Henry, just touched the edge of fame with ‘Henry’s law’ about the solubility of gases. They were all in contact with a great man, John Dalton, who owed to them more than he ever admitted, or perhaps even knew.They fit very well T. S . Eliot’s lines about J. Alfred Prufrock: No, I am not Prince Hamlet nor was meant to be. Am an attendant lord, one that will do To swell a progress, start a scene or two Advise the prince. .. THOMAS HENRY’S EARLY LIFE Thomas Henry was born in Wrexham, the son of a dancing-master. Now one would not have thought that a dancing-master in 18th century Wrexham would make a very fat living; but his father intended to send Thomas to Oxford, and then into the Church. There is in fact a little mystery about Thomas’s father, and we have a suspicion (it is no more) that he may have been a natural son of Lord Bulkeley, who ruled Anglesey almost like an independent kingdom. Farrar, Farrar and Scott 35 W i I liam (? I 700- I 774) Thomas (I 734- I8 16) Ellis (1743-1815) I C William (1774-1836) Peter ( 1769- I 808) Thomas (I 767- 1798) I William Charles (18044892) I I Simplified family tree of the Henry family.Only male members who lived t o maturit) are shown. However this may be, there was enough money to think of sending Thomas ta the University, and it is said that a horse had even been hired to take him there; but at the last minute the family got cold feet ‘with the uncertainty of eventual success’. The horse was sent back to the stables and the disappointed boy apprenticed to a local apothecary. Halfway through his time, Thomas’s master died, and he finished his apprenticeship over the border at Knutsford in Cheshire.In either Wrexham or Knutsford his interest in chemistry was aroused, improbable though it may seem, by reading the Latin text of Boer- haave’s Elementae chemiue, a book which, as his son said, ‘was not calculated to present the science in its most fascinating aspect’. But it is difficult to think of a better education that he could have had in chemistry in those days; Boerhaave and the apothecary’s shop-no university could have taught him more. R.I.C. Reviews 36 When he was about 20, Thomas left Knutsford to become assistant to a leading apothecary of Oxford. Two important things happened to him there. His master’s practice lay among University men, and he met some of his old schoolfellows from Wrexham Grammar School, who were friendly towards him, and enabled him to continue his education in an informal fashion.Secondly, he became acquainted with another apothecary, Samuel Glass, who made magnesium carbonate for medicinal purposes; we shall hear more of him later. After a few years at Oxford, Thomas declined an offer of a partner- ship, went back to Knutsford, started his own apothecary’s business and married a relative of his former master. He must have prospered, for in 1764 he bought a practice in fashionable St Ann’s Square, Manchester-and this is where the story really begins. Manchester in 1764 was a mere country town (pop. 17000). Its almost explosive growth had barely begun; but its growth and its wealth were such that an able (and personable) young apothecary could not fail to prosper, and Thomas practised very profitably for nearly 50 years.Unlike most men in such a position, however, he did not rest content with professional success and a growing family; he soon began to extend his energies in other directions-into industrial ventures, original work in chemistry, and the educational and intellectual life of Manchester. William Henry (left), father of Thomas Henry, shown above as a young man. MANCHESTER LIFE IN THE LATE 1700s To deal with the last matter first, at some time in the 1770s Thomas Henry left the Established Church, and became a Unitarian. This was a large, influential, wealthy (but rather unpopular) sect ifi Manchester at that time, which wor- Farrar, Farrar and Scott 37 shipped at Cross Street Chapel.This religious dissent, coupled with his interest in science, brought Thomas on to the fringe of the Lunar Society, aninformal group of experimenters and speculators centred in the Birmingham area, and on the personality of Matthew Boulton, Watt’s partner. He became friendly with many of the ‘Lunaticks’ including Priestley (on whose recommendation he became FRS in 1775), James Watt and his son (also called James), and Josiah Wedgwood the potter. Manchester at that time must have held as many men of real intellectual distinction as London itself. There was Thomas Percival, Charles White, John Ferriar, all with an honourable place in the history of medicine; the mathe- matician Henry Clarke, the Unitarian minister Thomas Barnes, wealthy gentlemen of wide interests like T.B. Bayley; and also industrialists con- sciously seeking a scientific basis for their craft-Thomas Henry himself, John Wilson the dyer, Thomas Cooper the bleacher, later James Watt junior (who came to work in Manchester) and the Scottish engineer Peter Ewart. Many of these, but not all, were Unitarians; many, again not all, radical or even republican in politics, with admiration for the recent American revolution. It seems that a number of these people, with the example of the Lunar Society in mind, began to meet in each other’s houses of an evening, to hear or read a paper on some scientific or philosophical topic. But unlike the Lunar Society, which did not outlive its founders, the Manchester group felt the need of some formal and permanent organization; this was formed in 1781 as the Manchester Literary and Philosophical Society (the ‘Lit and Phil’), with regular meetings in a room at the back of Cross Street Chapel.Percival was president, and he, Barnes, and Thomas Henry were the real moving spirits. In 38 R.I.C. Reviews spite of the heavy Unitarian and Radical bias among its members, the Society was anxious not to be identified in the public mind with religious or political faction; religion and politics were forbidden topics of discussion. It was of course into this congenial gathering of dissenting scientists who left their political views at the door, that John Dalton was received when he came to Manchester in 1793.Thomas Henry’s first paper read to the Lit and Phil was ‘An essay on the consistency of literary and philosophical interests with commercial pursuits’- a plea for a ‘liberal’ education for the sons of a mercantile and manufacturing community. Barnes also spoke to the same effect, and in 1783 the Lit and Phil tried to put its ideals into practice by founding the Manchester College of Arts and Sciences, in which Thomas Henry lectured on chemistry, dyeing, and bleaching. This institution, with its evening lectures to young artisans, fore- shadowed in many respects the Mechanics’ Institutes of the 1820s. Perhaps it came before its time, for its life was short; the last advertisements appeared in the Manchester newspapers in the autumn of 1787. One account of its decline was that it was due to ‘a superstitious dread of the tendency of science to unfit young men for the ordinary details of business’ ; but Thomas Henry himself, in a letter written at the time, had no doubt that ‘bigotry and political rage’ was the cause.The College, in fact, split the Lit and Phil into two factions, and led to wholesale resignations. Above and left: Manchester in the late 1700s. The College should not be confused (though it often is) with the Manchester Academy, founded in 1786 by the same group of people, with Barnes as Principal. This was a Dissenting Academy of the ordinary kind, largely residential, a direct successor to Warrington Academy which had foundered a Farrar, Farrar arid Scott 39 short time before.This was the Academy at which Dalton taught; Thomas Henry and his son, Thomas junior, continued their chemical lectures in it until young Thomas went to America in 1794. This time the Lit and Phil were a t great pains to dissociate themselves from the Academy, by a public dis- claimer in the newspapers. About Thomas Henry’s research work I shall not say much, because it was of no lasting importance. He, and indeed all the Henrys, were concerned with the chemistry of gases-pneumatic chemistry. Thomas was maturing as a chemist just at the time when there was a dawning realization that there were different gases; different chemical individuals, not just bad air, and good air, and air with a smell; and he was interested in working out some of the implications of this fascinating new idea.‘Fixed air’ (COz) was the most avail- able of the gases, and he and Percival did some experiments on the effect of COZ on the growth of plants which might, in more expert hands, have led them to an understanding of photosynthesis; but they missed it, and the success went to Priestley and Ingenhousz. There was also his enthusiasm for pneumatic medicine; these new gases would be a means of introducing chemi- cal substances into the body through the lungs, and this might have interesting medical consequences. Indeed, in the (lucky) hands of Humphry Davy, it led a little later to the discovery of the effects of nitrous oxide, though not, curiously, to the discovery of anaesthesia. But we suspect that Henry only made his unfortunate patients cough and splutter.Thomas Henry was not a great scientist, nor even a good one. He was an enthusiastic amateur, with an incurable optimism about what science could do, and a burning ambition to spread a knowledge of science all through the com- munity, especially to people working in industry, from mill-owner to artisan. This he shared with his colleagues in the Lit and Phil (and with revolutionary France); and he and his friends prepared the ground in Manchester for the next generation, the real professionals-his son William, and John Dalton. HENRY’S MAGNESIA We cannot leave Thomas Henry without saying a little about the source of the family’s wealth; the magnesia factory. The 18th century, whatever else it may have been, was the great century of overeating, and indigestion powders were the best-selling lines in the apothecary’s shop.You can ease the pangs of stomach acidity by taking chalk, but this causes constipation; soon after 1700 a new (at first secret) remedy was found in magnesium carbonate, which neutra- lizes acidity effectively and is also a mild laxative. Among the people who made magnesium carbonate for sale was the apothecary Samuel Glass, who had a little factory on Cowley Marsh, on the outskirts of Oxford, when Thomas Henry was there as a young man. Thomas later said himself, with devastating candour, how he had lived in the neighbourhood of ‘a gentleman . . . celebrated as the preparer of the most genuine magnesia.. . never having been able myself to make magnesia comparable to his. . . I was desirous of gaining some intelligence as to his process; and was at last so fortunate as to obtain some useful hints’. With the ‘hints’ obtained from Mr Glass, whether with that gentleman’s R. I. C. Reviews 40 knowledge or not, Henry began the production of magnesia in Manchester in 1772. However, the next year, with typical 18th-century rancour, he put out a pamphlet entitled Strictures on Glass’s magnesia, alleging that not only did the 4 oz bottles contain a mere l i o z , but that the contents ‘so puffed in every newspaper’ contained a great deal of chalk. This, as Henry no doubt intended, started a battle of pamphlets between himself and the successors to Glass (Glass probably being dead by this time).Much learned-sounding abuse was flung by both sides; but the interesting thing is this-Henry knew instinctively what he meant by a pure substance. His opponents, who were not chemists, did not. The concepts of ‘chemical purity’ and ‘chemical individuality’ are absolutely basic to the development of chemistry, and it is fascinating to see them being hammered out on the anvil of a minor commercial squabble. At first, Henry, like Glass, made magnesium carbonate. But soon he found that the oxide was an even more satisfactory product. It was more effective on a weight basis, and caused no distressing evolution of gas on contact with stomach acid. He also found (and this, kept a close secret, was the only genuine discovery in the whole story) that if the heating of the carbonate was done in a certain way, then the resulting oxide was not fluffy and hygroscopic, but heavy and granular, and would form a dispersion simply by stirring with water.This was Henry’s Magnesia, a product whose real and imaginary virtues were so loudly extolled by Thomas as to earn him the local nickname of ‘Magnesia Henry’. Thomas Henry, j r (left) and his brother Peter (above). Thomas Henry had three sons; the two eldest must have been a sad dis- appointment to him. Young Thomas made a promising start, but either he, or perhaps his father, could not decide what he was to do for a living, and he had a bewildering succession of jobs and courses of training. He was also friendly Farrar, Farrar and Scott 41 with Thomas Cooper and James Watt junior, young men of very radical politics; and Manchester, in the 1790s, with the French Revolution going sour after its early idealism, was an uncomfortable place for radicals.Cooper’s newspaper, the Manchester Herald, had its premises wrecked; Thomas Walker, a most respectable merchant and a member of the Lit and Phil, was tried for treason (a hanging matter) and only acquitted when the prosecution witnesses admitted perjury. Old Thomas kept his head well down, but young Thomas may not have been so prudent; in the end he sailed rather hurriedly for America, though his motives for doing so were not entirely political-he had been engaged in a chemical enterprise in Anglesey, on the edge of the great copper mines of Parys Mountain, and had contracted heavy debts which he had no prospect of paying. In America, he attached himself to the circle around Priestley, who went into exile about the same time, after the Birming- ham riots had wrecked his house and laboratory.Priestley seems not to have been able to do much for young Thomas, who eventually became a ship’s surgeon, and died of fever in the West Indies in 1798. Of the other son, Peter, we know almost nothing. The other Henrys are curiously silent about him in their letters, and we suspect that he may have been the black sheep of the family. He was trained as a chemist, but joined the army, and as Captain Peter Henry died of fever in India in 1808. WILLIAM HENRY This left only the youngest son, William, who turned out to be the most important, scientifically, of the family.A childhood accident, in which he was seriously injured by a falling beam, made him an ailing and bookish boy; for the rest of his life he was seldom free for long from pain and illness, which led eventually to his tragic death. He went to Edinburgh as a medical student in 1797, but after a year his father called him home (this must have been the time when he realized that young Thomas and Peter were not going to be of much use), took him into partnership, and put him in charge of the magnesia factory. In 1805, William Henry returned to Edinburgh to complete his studies, and took his MD. The years between these two periods at university were the busiest and most productive in his life.He diversified the magnesia business in two directions which are both interesting. About 1803 the Henrys started to make soda water (COz in water under pressure) for medicinal use. There is nothing startling about this; many others were doing the same. But it is interesting to remember that it was a manufacturer of soda water who dis- covered Henry’s Law, which is about the solubility of gases in liquids under pressure. Secondly, from 1799 to about 1802, William Henry was making alkali (soda) by a process which he was anxious not to disclose. We can, however, eliminate most of the likely possibilities for various reasons, and we think it probable that he was operating the first Leblanc process in Britain.Outside the factory, he played an active part in the early days of the gas industry, advising Boulton and Watt when they installed gas lighting into a cotton mill for the first time (Phillips and Lee, Salford, 1805). He published the first few editions of his successful textbook, The elements ofchemistry ; he ran a R. I. C. Reviews 42 William Henry, the youngest son. medical practice; he married and started a large family. Not least, he col- laborated with Dalton during the crucial years when Dalton was thinking out the atomic theory, and laying the foundations of all subsequent chemistry. Farrar, Farrar and Scott 43 When Dalton came to the Manchester Academy in 1793, he was not a chemist; he was a meteorologist. Up to 1800 and beyond, his reputation rested on his studies of weather.But it seems that, from his studies of the atmosphere, and the problem of how the different gases in the air stayed mixed, instead of separating out into layers, he had begun to entertain vivid and concrete speculations about the particles of these gases and the forces between them. He was not the first person to do so; like most English scientists, he was following reverently in the footsteps of Newton. But Dalton was not content merely to touch his hat to Newton. He had a great simplicity of mind which forced him to ask crude, naive questions about atoms. How big are they? How many of them are there? What different kinds exist, and how do they differ from one another? His attempts to answer these questions brought him to chemistry, especially the chemistry of gases; and it must have been from his friends Thomas and William Henry, pneumatic chemists both, that Dalton learnt his chemistry-though as a chemist he never became more than barely competent.In his early work on gases, Dalton discovered his law of partial pressures, working hand-in-glove with William Henry, who was doing his solubility work at the same time. The interdependence of their work at this period is shown by Henry’s Law, which is very simple (solubility varies as pressure) and was studied in a very simple apparatus; but the interpretation of the results was far from simple. This was because, in those days, pure gases were hardly to be had.All were mixed with more or less air, which greatly confuses the experimental results-but they can be brought into order by a knowledge of the law of partial pressures. About this time (1803) it seems to have dawned upon Dalton, perhaps through talking about chemistry to Henry, that the answer to one of his naive questions lay ready to hand in analytical chemistry. He had already grasped the idea that each different element (as recently listed by Lavoisier) consisted of one particular kind of atom, and that these kinds of atom differed in weight. Now he saw that he could determine the relative weights of atoms quite easily from the results of chemical analysis. Thus, if a compound AB contains 63 per cent by weight of element A, and 37 per cent of B, then ~~~ weight of atom A weight of atom B = 63/37 and by taking any atom (say hydrogen) as unity, one can construct a table of atomic weights.Dalton published the first such rudimentary table towards the end of 1803. Now to do this, and to press on with this major breakthrough, he wanted two things. First, he would want to know all that had been done in chemical analysis up to that time, and Henry could tell him. Secondly, published R. I . C. Reviews analyses would not be enough; Dalton would want to do his own, and he must have apprenticed himself to Henry to learn how to do it. We need not go on with the later development of the atomic theory, leading to the publication of Dalton’s New system in 1808. The point is made; at a vital stage in the evolu- tion of the atomic theory a chemist was needed, William Henry was at hand to help, and he helped generously.44 After William Henry finally came back, as Dr Henry, from Edinburgh in 1807, he took up research again; but it is clear from his private letters that his main preoccupations were the magnesia factory and his family-research had to take a back seat. He did a lot of work on the analysis of coal-gas, how its composition varied according to the variety of coal used, temperature of carbonization, and so on; tedious stuff, but essential for the industry, and it remained standard work for 40 years or more. His work on controversial matters, such as the nature of chlorine (some thought it an element, some an oxide) is marred by a sort of intellectual timidity, or perhaps excessive fair mindedness, which is also apparent in his textbook.Seeing both sides of a question may be a great virtue, but it seldom makes for productive science. Even on the question of the atomic theory, in whose birth he had played such a vital part, he did not fully commit himself until the last editions of his text- book, in the late 1820s. Perhaps William Henry’s best work, certainly the most interesting, was done towards the end of his life, during the great cholera epidemic of 1832. There is not space here to describe this work, even in summary fashion. It must suffice to say that in those days before bacteria had been discovered, Henry developed a chemical theory of contagion which, though wrong, is ingenious and plausible, and made sense of a great many confusing medical observations.On the basis of this theory he constructed a steam-heated apparatus for dis- infecting clothes and bedding. If this had been widely used during the epidemic it might have saved many lives. It is what is now called a ‘sterilizer’ but this word implies the killing of something living, and Henry did not think he was doing that; he thought he was isomerizing (by moderate heat) the complex molecules of the supposed chemical contagions into harmless isomerides. He was, in his way, groping towards the structural organic chemistry that came in the 1860s. At the time of this work on cholera, William Henry was already a very sick man. He spent the last years of his life in almost continual pain; partly due to his childhood accident, partly to painful tumours on his hands.From certain hints, it seems that he may have had periods of actual insanity. In August 1836, his family packed him off to the British Association meeting in Bristol, along with John Dalton, in the hope that’the change and the company might do him good. But when he came back he was quite distracted; in the early morning of September 2, 1836, he crept downstairs to the private chapel attached to his house and shot himself. LATER GENERATIONS Only one of William Henry’s sons lived to maturity. This was William Charles Henry, a young man of great scientific promise, but a rather baffling per- sonality. Unlike his father and grandfather, he was born into a wealthy house- hold with an assured social position, and with a scientific tradition going back two generations.He was a pupil of Dalton when at the height of his powers, then he studied medicine at Edinburgh. His earliest research work was on the physiology of the nervous system but, like Thomas and William before him, he seems to have turned away from medicine to chemistry. In 1835 he went off to Farrar, Farrar amd Scott 45 Germany for a year to turn himself into a proper chemist, thus pioneering a trail which was to become very well worn as the century went on. He went to Rose’s laboratory in Berlin and, more importantly, to Liebig in Giessen. German university life had the same effect on Charles Henry as it did on hundreds of other young Englishmen-for the rest of his life he looked back on those few months in Giessen as a sort of Arcadia, which could be remembered but never recaptured.He came back to Manchester in the summer of 1836, full of enthusiasm for chemical research; there were even plans for Liebig to come to Manchester and collaborate with him on a study of the effect of pressure on gas reactions. The importance of such a study, if it had ever been carried out, need hardly be emphasized. But he came back to find his father at the end of his tether, and the laboratory in a shambles; then a few weeks later came the tragic end described above. His father’s suicide shook Charles Henry to the core. In his letters to Liebig there is no more about plans for research, and he becomes sour about science in general; ‘the only science pursued in this town is that of making money, and I do not hear of much that is new in London’.Liebig did come to visit him the next year, and was mightily impressed by the opulence of the Henry household. He had seen nothing like it in Germany; washbasins in the bedrooms, well-trained servants all over the place, frequent large(but not very tasty) meals. But there was no talk of scientific collabora- tion, and Liebig was in fact witnessing the end of an era-the Henrys in Manchester. In September 1837, Charles Henry retired, from science and even from business, at the ripe age of 33. The family moved to the house and estate of Haffield in Herefordshire, and there for 55 years Henry lived the life of a Liebig’s laboratory about 1850.R.I.C. Reviews 46 wealthy country gentleman, with a dilettante interest in classical literature and a taste for foreign travel. The only noteworthy thing that he did, slowly and with obvious reluctance, was to write a life of Dalton. As Dalton’s pupil, friend, and medical adviser, he had the opportunity to write one of the greatest of scientific biographies. All that need be said is that he failed; the resultant book was a very disappointing one. The magnesia factory was left in the hands of a manager, and Henry seldom saw it again. The stuff evidently sold well, and we know that it brought in a considerable income to the family. It must have lived on its reputation (‘brand loyalty’ we call it nowadays) for it was hardly ever advertised. The process was never improved, cheapened, or altered in any way; it was carried on by skilled workmen who were familiar with every detail, but who had no knowledge of chemistry.When Charles Henry died, the factory passed to his son, Frank, a professional soldier (none of his 12 children showed the slightest trace of any interest in science), then to Frank‘s sons, Gilbert and Vivian. We have been fortunate enough to tjace one of the last employees of the factory, Mr David Dobson, who was able to describe in vivid detail how the place was run in the 1920s, a quaint and pleasing survival of an 18th century pharmaceutical firm. About once a year, Mr Dobson would go to the station to meet Gilbert Henry, an immensely tall and dignified man in a long astrakhan coat, and conduct him with due ceremony to the factory to inspect the books, and taste the magnesia for traces of lime, to ensure that the standards of 1772 were being kept up.Quaint and pleasing it may have been, but no longer profitable; the com- petition of firms like Boots hit hard. After a pathetic attempt to restore its fortunes by distributing fancy blotting-pads, T. & W. Henry was finally sold to BDH in 1933 for the derisory sum of E100. After a brief period as ‘Henry’s Garage’ the factory lay derelict for many years. Early last summer a bulldozer levelled the site; the very end of the story of the Henrys of Manchester. ACKNOWLEDGEMENT The authors are grateful to Mr Frederick Henry and Mr Gerard Henry for providing photographs of portraits in the possession of the family.The quotation from T. S. Eliot is reproduced by kind permission of Messrs Faber and Faber. 47 Farrar, Farrrir and Scott Sixth Grove LectureTHE HENRYS OF MANCHESTERW. V. Farrar, BSc, PhD, and Kathleen R. Farrar, BSc, PhDDept of History of Science and Technology, UMIST, Manchestei IE. L. Scott, MScStamford High School, Stamford, LincsThomas Henry’s early life . . .. .. . . . . . . 35Manchester life in the late 1700s . . .. . . . . . . 37Henry’s magnesia . . * . .. . . .. . . . . . . 40William Henry . . .. . . . . .. . . . . .. 42Later generations . . . . . . .. . . . . . . . . 45Most history is about great men, and the history of science is no exception.Lavoisier, Dalton, Berthollet, Berzelius, set the course of 19th centurychemistry; we know and we write a lot about such people, because they arevery important.But let us think of the other side of the coin-of the peoplewhose courses were set by the great men, the scientists of second rank, the‘common scientists’. It is these people, after all, who do 99 per cent of allscience, who carry on the daily business of science. They teach, they work inindustry, they write textbooks; they do their bits of research which once in alifetime may touch greatness-or may not. The Henry family of Manchesterwere such ‘common scientists’, and we have found them a very rewardingstudy. They did all the things just mentioned, and ran medical practices aswell; one of them, William Henry, just touched the edge of fame with ‘Henry’slaw’ about the solubility of gases.They were all in contact with a great man,John Dalton, who owed to them more than he ever admitted, or perhaps evenknew. They fit very well T. S . Eliot’s lines about J. Alfred Prufrock:No, I am not Prince Hamlet nor was meant to be.Am an attendant lord, one that will doTo swell a progress, start a scene or twoAdvise the prince. ..THOMAS HENRY’S EARLY LIFEThomas Henry was born in Wrexham, the son of a dancing-master. Now onewould not have thought that a dancing-master in 18th century Wrexham wouldmake a very fat living; but his father intended to send Thomas to Oxford, andthen into the Church.There is in fact a little mystery about Thomas’s father,and we have a suspicion (it is no more) that he may have been a natural son ofLord Bulkeley, who ruled Anglesey almost like an independent kingdom.Farrar, Farrar and Scott 3W i I liam (? I 700- I 774)IThomas I C (I 734- I8 16) Ellis (1743-1815)Thomas(I 767- 1798) Peter William (1774-1836)I ( 1769- I 808)William Charles (18044892)ISimplified family tree of the Henry family. Only male members who lived t o maturit)are shown.However this may be, there was enough money to think of sending Thomas tathe University, and it is said that a horse had even been hired to take himthere; but at the last minute the family got cold feet ‘with the uncertainty ofeventual success’.The horse was sent back to the stables and the disappointedboy apprenticed to a local apothecary. Halfway through his time, Thomas’smaster died, and he finished his apprenticeship over the border at Knutsford inCheshire. In either Wrexham or Knutsford his interest in chemistry wasaroused, improbable though it may seem, by reading the Latin text of Boer-haave’s Elementae chemiue, a book which, as his son said, ‘was not calculatedto present the science in its most fascinating aspect’. But it is difficult to thinkof a better education that he could have had in chemistry in those days;Boerhaave and the apothecary’s shop-no university could have taught himmore.36 R.I.C. ReviewWilliam Henry (left), father of ThomasHenry, shown above as a young man.When he was about 20, Thomas left Knutsford to become assistant to aleading apothecary of Oxford.Two important things happened to him there.His master’s practice lay among University men, and he met some of his oldschoolfellows from Wrexham Grammar School, who were friendly towardshim, and enabled him to continue his education in an informal fashion.Secondly, he became acquainted with another apothecary, Samuel Glass, whomade magnesium carbonate for medicinal purposes; we shall hear more ofhim later. After a few years at Oxford, Thomas declined an offer of a partner-ship, went back to Knutsford, started his own apothecary’s business andmarried a relative of his former master. He must have prospered, for in 1764he bought a practice in fashionable St Ann’s Square, Manchester-and this iswhere the story really begins.Manchester in 1764 was a mere country town (pop. 17000).Its almostexplosive growth had barely begun; but its growth and its wealth were suchthat an able (and personable) young apothecary could not fail to prosper, andThomas practised very profitably for nearly 50 years. Unlike most men in sucha position, however, he did not rest content with professional success and agrowing family; he soon began to extend his energies in other directions-intoindustrial ventures, original work in chemistry, and the educational andintellectual life of Manchester.MANCHESTER LIFE IN THE LATE 1700sTo deal with the last matter first, at some time in the 1770s Thomas Henry leftthe Established Church, and became a Unitarian.This was a large, influential,wealthy (but rather unpopular) sect ifi Manchester at that time, which wor-Farrar, Farrar and Scott 3shipped at Cross Street Chapel. This religious dissent, coupled with his interestin science, brought Thomas on to the fringe of the Lunar Society, aninformalgroup of experimenters and speculators centred in the Birmingham area, andon the personality of Matthew Boulton, Watt’s partner. He became friendlywith many of the ‘Lunaticks’ including Priestley (on whose recommendationhe became FRS in 1775), James Watt and his son (also called James), andJosiah Wedgwood the potter.Manchester at that time must have held as many men of real intellectualdistinction as London itself.There was Thomas Percival, Charles White, JohnFerriar, all with an honourable place in the history of medicine; the mathe-matician Henry Clarke, the Unitarian minister Thomas Barnes, wealthygentlemen of wide interests like T. B. Bayley; and also industrialists con-sciously seeking a scientific basis for their craft-Thomas Henry himself, JohnWilson the dyer, Thomas Cooper the bleacher, later James Watt junior (whocame to work in Manchester) and the Scottish engineer Peter Ewart. Many ofthese, but not all, were Unitarians; many, again not all, radical or evenrepublican in politics, with admiration for the recent American revolution.It seems that a number of these people, with the example of the LunarSociety in mind, began to meet in each other’s houses of an evening, to hear orread a paper on some scientific or philosophical topic.But unlike the LunarSociety, which did not outlive its founders, the Manchester group felt the needof some formal and permanent organization; this was formed in 1781 as theManchester Literary and Philosophical Society (the ‘Lit and Phil’), withregular meetings in a room at the back of Cross Street Chapel. Percival waspresident, and he, Barnes, and Thomas Henry were the real moving spirits. In38 R.I.C. ReviewAbove and left: Manchester in the late 1700s.spite of the heavy Unitarian and Radical bias among its members, the Societywas anxious not to be identified in the public mind with religious or politicalfaction; religion and politics were forbidden topics of discussion.It was ofcourse into this congenial gathering of dissenting scientists who left theirpolitical views at the door, that John Dalton was received when he came toManchester in 1793.Thomas Henry’s first paper read to the Lit and Phil was ‘An essay on theconsistency of literary and philosophical interests with commercial pursuits’-a plea for a ‘liberal’ education for the sons of a mercantile and manufacturingcommunity. Barnes also spoke to the same effect, and in 1783 the Lit and Philtried to put its ideals into practice by founding the Manchester College of Artsand Sciences, in which Thomas Henry lectured on chemistry, dyeing, andbleaching. This institution, with its evening lectures to young artisans, fore-shadowed in many respects the Mechanics’ Institutes of the 1820s.Perhaps itcame before its time, for its life was short; the last advertisements appeared inthe Manchester newspapers in the autumn of 1787. One account of its declinewas that it was due to ‘a superstitious dread of the tendency of science to unfityoung men for the ordinary details of business’ ; but Thomas Henry himself, ina letter written at the time, had no doubt that ‘bigotry and political rage’ wasthe cause. The College, in fact, split the Lit and Phil into two factions, and ledto wholesale resignations.The College should not be confused (though it often is) with the ManchesterAcademy, founded in 1786 by the same group of people, with Barnes asPrincipal. This was a Dissenting Academy of the ordinary kind, largelyresidential, a direct successor to Warrington Academy which had foundered aFarrar, Farrar arid Scott 3short time before.This was the Academy at which Dalton taught; ThomasHenry and his son, Thomas junior, continued their chemical lectures in it untilyoung Thomas went to America in 1794. This time the Lit and Phil were a tgreat pains to dissociate themselves from the Academy, by a public dis-claimer in the newspapers.About Thomas Henry’s research work I shall not say much, because it wasof no lasting importance. He, and indeed all the Henrys, were concerned withthe chemistry of gases-pneumatic chemistry. Thomas was maturing as achemist just at the time when there was a dawning realization that there weredifferent gases; different chemical individuals, not just bad air, and good air,and air with a smell; and he was interested in working out some of theimplications of this fascinating new idea. ‘Fixed air’ (COz) was the most avail-able of the gases, and he and Percival did some experiments on the effect ofCOZ on the growth of plants which might, in more expert hands, have ledthem to an understanding of photosynthesis; but they missed it, and thesuccess went to Priestley and Ingenhousz.There was also his enthusiasm forpneumatic medicine; these new gases would be a means of introducing chemi-cal substances into the body through the lungs, and this might have interestingmedical consequences. Indeed, in the (lucky) hands of Humphry Davy, it led alittle later to the discovery of the effects of nitrous oxide, though not, curiously,to the discovery of anaesthesia.But we suspect that Henry only made hisunfortunate patients cough and splutter.Thomas Henry was not a great scientist, nor even a good one. He was anenthusiastic amateur, with an incurable optimism about what science could do,and a burning ambition to spread a knowledge of science all through the com-munity, especially to people working in industry, from mill-owner to artisan.This he shared with his colleagues in the Lit and Phil (and with revolutionaryFrance); and he and his friends prepared the ground in Manchester for the nextgeneration, the real professionals-his son William, and John Dalton.HENRY’S MAGNESIAWe cannot leave Thomas Henry without saying a little about the source of thefamily’s wealth; the magnesia factory.The 18th century, whatever else it mayhave been, was the great century of overeating, and indigestion powders werethe best-selling lines in the apothecary’s shop. You can ease the pangs ofstomach acidity by taking chalk, but this causes constipation; soon after 1700 anew (at first secret) remedy was found in magnesium carbonate, which neutra-lizes acidity effectively and is also a mild laxative. Among the people who mademagnesium carbonate for sale was the apothecary Samuel Glass, who had alittle factory on Cowley Marsh, on the outskirts of Oxford, when ThomasHenry was there as a young man. Thomas later said himself, with devastatingcandour, how he had lived in the neighbourhood of ‘a gentleman .. . celebratedas the preparer of the most genuine magnesia. . . never having been ablemyself to make magnesia comparable to his. . . I was desirous of gainingsome intelligence as to his process; and was at last so fortunate as to obtainsome useful hints’.With the ‘hints’ obtained from Mr Glass, whether with that gentleman’s40 R. I. C. ReviewThomas Henry, j r (left) and his brotherPeter (above).knowledge or not, Henry began the production of magnesia in Manchester in1772. However, the next year, with typical 18th-century rancour, he put out apamphlet entitled Strictures on Glass’s magnesia, alleging that not only did the4 oz bottles contain a mere l i o z , but that the contents ‘so puffed in everynewspaper’ contained a great deal of chalk.This, as Henry no doubt intended,started a battle of pamphlets between himself and the successors to Glass(Glass probably being dead by this time). Much learned-sounding abuse wasflung by both sides; but the interesting thing is this-Henry knew instinctivelywhat he meant by a pure substance. His opponents, who were not chemists,did not. The concepts of ‘chemical purity’ and ‘chemical individuality’ areabsolutely basic to the development of chemistry, and it is fascinating to seethem being hammered out on the anvil of a minor commercial squabble.At first, Henry, like Glass, made magnesium carbonate. But soon he foundthat the oxide was an even more satisfactory product. It was more effective ona weight basis, and caused no distressing evolution of gas on contact withstomach acid.He also found (and this, kept a close secret, was the only genuinediscovery in the whole story) that if the heating of the carbonate was done in acertain way, then the resulting oxide was not fluffy and hygroscopic, but heavyand granular, and would form a dispersion simply by stirring with water. Thiswas Henry’s Magnesia, a product whose real and imaginary virtues were soloudly extolled by Thomas as to earn him the local nickname of ‘MagnesiaHenry’.Thomas Henry had three sons; the two eldest must have been a sad dis-appointment to him. Young Thomas made a promising start, but either he, orperhaps his father, could not decide what he was to do for a living, and he hada bewildering succession of jobs and courses of training.He was also friendlyFarrar, Farrar and Scott 4with Thomas Cooper and James Watt junior, young men of very radicalpolitics; and Manchester, in the 1790s, with the French Revolution going sourafter its early idealism, was an uncomfortable place for radicals. Cooper’snewspaper, the Manchester Herald, had its premises wrecked; Thomas Walker,a most respectable merchant and a member of the Lit and Phil, was tried fortreason (a hanging matter) and only acquitted when the prosecution witnessesadmitted perjury. Old Thomas kept his head well down, but young Thomasmay not have been so prudent; in the end he sailed rather hurriedly forAmerica, though his motives for doing so were not entirely political-he hadbeen engaged in a chemical enterprise in Anglesey, on the edge of the greatcopper mines of Parys Mountain, and had contracted heavy debts which hehad no prospect of paying.In America, he attached himself to the circlearound Priestley, who went into exile about the same time, after the Birming-ham riots had wrecked his house and laboratory. Priestley seems not to havebeen able to do much for young Thomas, who eventually became a ship’ssurgeon, and died of fever in the West Indies in 1798.Of the other son, Peter, we know almost nothing. The other Henrys arecuriously silent about him in their letters, and we suspect that he may havebeen the black sheep of the family. He was trained as a chemist, but joined thearmy, and as Captain Peter Henry died of fever in India in 1808.WILLIAM HENRYThis left only the youngest son, William, who turned out to be the mostimportant, scientifically, of the family.A childhood accident, in which he wasseriously injured by a falling beam, made him an ailing and bookish boy; forthe rest of his life he was seldom free for long from pain and illness, which ledeventually to his tragic death. He went to Edinburgh as a medical student in1797, but after a year his father called him home (this must have been the timewhen he realized that young Thomas and Peter were not going to be of muchuse), took him into partnership, and put him in charge of the magnesiafactory.In 1805, William Henry returned to Edinburgh to complete his studies, andtook his MD.The years between these two periods at university were thebusiest and most productive in his life. He diversified the magnesia business intwo directions which are both interesting. About 1803 the Henrys started tomake soda water (COz in water under pressure) for medicinal use. There isnothing startling about this; many others were doing the same. But it isinteresting to remember that it was a manufacturer of soda water who dis-covered Henry’s Law, which is about the solubility of gases in liquids underpressure. Secondly, from 1799 to about 1802, William Henry was makingalkali (soda) by a process which he was anxious not to disclose. We can,however, eliminate most of the likely possibilities for various reasons, and wethink it probable that he was operating the first Leblanc process in Britain.Outside the factory, he played an active part in the early days of the gasindustry, advising Boulton and Watt when they installed gas lighting into acotton mill for the first time (Phillips and Lee, Salford, 1805).He published thefirst few editions of his successful textbook, The elements ofchemistry ; he ran a42 R. I. C. ReviewWilliam Henry, the youngest son.medical practice; he married and started a large family. Not least, he col-laborated with Dalton during the crucial years when Dalton was thinking outthe atomic theory, and laying the foundations of all subsequent chemistry.Farrar, Farrar and Scott 4When Dalton came to the Manchester Academy in 1793, he was not achemist; he was a meteorologist.Up to 1800 and beyond, his reputation restedon his studies of weather. But it seems that, from his studies of the atmosphere,and the problem of how the different gases in the air stayed mixed, instead ofseparating out into layers, he had begun to entertain vivid and concretespeculations about the particles of these gases and the forces between them. Hewas not the first person to do so; like most English scientists, he was followingreverently in the footsteps of Newton. But Dalton was not content merely totouch his hat to Newton. He had a great simplicity of mind which forced himto ask crude, naive questions about atoms. How big are they? How many ofthem are there? What different kinds exist, and how do they differ from oneanother? His attempts to answer these questions brought him to chemistry,especially the chemistry of gases; and it must have been from his friendsThomas and William Henry, pneumatic chemists both, that Dalton learnt hischemistry-though as a chemist he never became more than barely competent.In his early work on gases, Dalton discovered his law of partial pressures,working hand-in-glove with William Henry, who was doing his solubilitywork at the same time.The interdependence of their work at this period isshown by Henry’s Law, which is very simple (solubility varies as pressure) andwas studied in a very simple apparatus; but the interpretation of the resultswas far from simple. This was because, in those days, pure gases were hardlyto be had. All were mixed with more or less air, which greatly confuses theexperimental results-but they can be brought into order by a knowledge ofthe law of partial pressures.About this time (1803) it seems to have dawned upon Dalton, perhapsthrough talking about chemistry to Henry, that the answer to one of his naivequestions lay ready to hand in analytical chemistry.He had already graspedthe idea that each different element (as recently listed by Lavoisier) consistedof one particular kind of atom, and that these kinds of atom differed inweight. Now he saw that he could determine the relative weights of atoms quiteeasily from the results of chemical analysis. Thus, if a compound AB contains63 per cent by weight of element A, and 37 per cent of B, thenweight of atom Aweight of atom B = 63/37 ~~~and by taking any atom (say hydrogen) as unity, one can construct a table ofatomic weights.Dalton published the first such rudimentary table towards theend of 1803.Now to do this, and to press on with this major breakthrough, he wanted twothings. First, he would want to know all that had been done in chemicalanalysis up to that time, and Henry could tell him. Secondly, publishedanalyses would not be enough; Dalton would want to do his own, and he musthave apprenticed himself to Henry to learn how to do it. We need not go onwith the later development of the atomic theory, leading to the publication ofDalton’s New system in 1808. The point is made; at a vital stage in the evolu-tion of the atomic theory a chemist was needed, William Henry was at hand tohelp, and he helped generously.44 R.I . C. ReviewAfter William Henry finally came back, as Dr Henry, from Edinburgh in1807, he took up research again; but it is clear from his private letters that hismain preoccupations were the magnesia factory and his family-research hadto take a back seat. He did a lot of work on the analysis of coal-gas, how itscomposition varied according to the variety of coal used, temperature ofcarbonization, and so on; tedious stuff, but essential for the industry, and itremained standard work for 40 years or more. His work on controversialmatters, such as the nature of chlorine (some thought it an element, some anoxide) is marred by a sort of intellectual timidity, or perhaps excessive fairmindedness, which is also apparent in his textbook.Seeing both sides of aquestion may be a great virtue, but it seldom makes for productive science.Even on the question of the atomic theory, in whose birth he had played such avital part, he did not fully commit himself until the last editions of his text-book, in the late 1820s.Perhaps William Henry’s best work, certainly the most interesting, wasdone towards the end of his life, during the great cholera epidemic of 1832.There is not space here to describe this work, even in summary fashion. It mustsuffice to say that in those days before bacteria had been discovered, Henrydeveloped a chemical theory of contagion which, though wrong, is ingeniousand plausible, and made sense of a great many confusing medical observations.On the basis of this theory he constructed a steam-heated apparatus for dis-infecting clothes and bedding.If this had been widely used during the epidemicit might have saved many lives. It is what is now called a ‘sterilizer’ but thisword implies the killing of something living, and Henry did not think he wasdoing that; he thought he was isomerizing (by moderate heat) the complexmolecules of the supposed chemical contagions into harmless isomerides. Hewas, in his way, groping towards the structural organic chemistry that came inthe 1860s.At the time of this work on cholera, William Henry was already a very sickman. He spent the last years of his life in almost continual pain; partly due tohis childhood accident, partly to painful tumours on his hands. From certainhints, it seems that he may have had periods of actual insanity. In August 1836,his family packed him off to the British Association meeting in Bristol, alongwith John Dalton, in the hope that’the change and the company might do himgood.But when he came back he was quite distracted; in the early morning ofSeptember 2, 1836, he crept downstairs to the private chapel attached to hishouse and shot himself.LATER GENERATIONSOnly one of William Henry’s sons lived to maturity. This was William CharlesHenry, a young man of great scientific promise, but a rather baffling per-sonality. Unlike his father and grandfather, he was born into a wealthy house-hold with an assured social position, and with a scientific tradition going backtwo generations. He was a pupil of Dalton when at the height of his powers,then he studied medicine at Edinburgh.His earliest research work was on thephysiology of the nervous system but, like Thomas and William before him, heseems to have turned away from medicine to chemistry. In 1835 he went off toFarrar, Farrar amd Scott 4Liebig’s laboratory about 1850.Germany for a year to turn himself into a proper chemist, thus pioneering a trailwhich was to become very well worn as the century went on. He went to Rose’slaboratory in Berlin and, more importantly, to Liebig in Giessen. Germanuniversity life had the same effect on Charles Henry as it did on hundreds ofother young Englishmen-for the rest of his life he looked back on those fewmonths in Giessen as a sort of Arcadia, which could be remembered but neverrecaptured.He came back to Manchester in the summer of 1836, full of enthusiasm forchemical research; there were even plans for Liebig to come to Manchesterand collaborate with him on a study of the effect of pressure on gas reactions.The importance of such a study, if it had ever been carried out, need hardly beemphasized.But he came back to find his father at the end of his tether, andthe laboratory in a shambles; then a few weeks later came the tragic enddescribed above. His father’s suicide shook Charles Henry to the core. In hisletters to Liebig there is no more about plans for research, and he becomessour about science in general; ‘the only science pursued in this town is that ofmaking money, and I do not hear of much that is new in London’.Liebig did come to visit him the next year, and was mightily impressed bythe opulence of the Henry household. He had seen nothing like it in Germany;washbasins in the bedrooms, well-trained servants all over the place, frequentlarge(but not very tasty) meals. But there was no talk of scientific collabora-tion, and Liebig was in fact witnessing the end of an era-the Henrys inManchester.In September 1837, Charles Henry retired, from science and evenfrom business, at the ripe age of 33. The family moved to the house and estateof Haffield in Herefordshire, and there for 55 years Henry lived the life of a46 R.I.C. Reviewwealthy country gentleman, with a dilettante interest in classical literature anda taste for foreign travel. The only noteworthy thing that he did, slowly andwith obvious reluctance, was to write a life of Dalton. As Dalton’s pupil,friend, and medical adviser, he had the opportunity to write one of thegreatest of scientific biographies. All that need be said is that he failed; theresultant book was a very disappointing one.The magnesia factory was left in the hands of a manager, and Henry seldomsaw it again. The stuff evidently sold well, and we know that it brought in aconsiderable income to the family. It must have lived on its reputation (‘brandloyalty’ we call it nowadays) for it was hardly ever advertised. The process wasnever improved, cheapened, or altered in any way; it was carried on by skilledworkmen who were familiar with every detail, but who had no knowledge ofchemistry. When Charles Henry died, the factory passed to his son, Frank, aprofessional soldier (none of his 12 children showed the slightest trace of anyinterest in science), then to Frank‘s sons, Gilbert and Vivian. We have beenfortunate enough to tjace one of the last employees of the factory, Mr DavidDobson, who was able to describe in vivid detail how the place was run in the1920s, a quaint and pleasing survival of an 18th century pharmaceutical firm.About once a year, Mr Dobson would go to the station to meet GilbertHenry, an immensely tall and dignified man in a long astrakhan coat, andconduct him with due ceremony to the factory to inspect the books, and tastethe magnesia for traces of lime, to ensure that the standards of 1772 werebeing kept up.Quaint and pleasing it may have been, but no longer profitable; the com-petition of firms like Boots hit hard. After a pathetic attempt to restore itsfortunes by distributing fancy blotting-pads, T. & W. Henry was finally soldto BDH in 1933 for the derisory sum of E100. After a brief period as ‘Henry’sGarage’ the factory lay derelict for many years. Early last summer a bulldozerlevelled the site; the very end of the story of the Henrys of Manchester.ACKNOWLEDGEMENTThe authors are grateful to Mr Frederick Henry and Mr Gerard Henry for providingphotographs of portraits in the possession of the family. The quotation fromT. S. Eliot is reproduced by kind permission of Messrs Faber and Faber.Farrar, Farrrir and Scott 4

ISSN:0035-8940    

DOI:10.1039/RR9710400035

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

EXPERIMENT, IMAGINATION AND MEANING Inductive and Hypothetico-deductive Method in Science D. W. Theobald Dept of Chemistry, UMIST, Monchester I Vision and technique Bacon, Hume and Mill . . Philosophical problems for hypothetico-deduction . . . . . . 59 . . . . . . .. . . . . . . 50 . . . . . . . . .. . . 51 Testing in science . . .. .. . . . . . . . . . . 57 Hypotheses Chemistry and scientific method 65 .. . . .. .. . . . . . . . . 61 . . .. . . . . . . References . . .. . . . . .. . . . . . . . . 66 We had the experience but missed the meaning, And approach to the meaning restores the experience In a different form, ... T. S. Eliot, The Dry Salvages The moment we reflect philosophically upon the knowledge we have of the world about us, two questions emerge.One, how do we acquire such know- ledge? And two, how do we guarantee that what passes for knowledge is in fact knowledge? The first is a question about discovery, the second a question about justification and proof. Of course these are strictly speaking philosophi- cal questions, but the answers to them are of interest, indeed importance, to scientists. To explain this a little further, let us take a simple example, an example which we shall return to frequently throughout this essay. Suppose we are interested in rusting, both from a theoretical point of view and perhaps with the practical aim of preventing its occurrence and the inconvenience it causes. First we shall want to find out the conditions under which rusting occurs so that we can discover its causes, and then we shall require some procedure for establishing or proving that we have found out correctly.And the questions I am asking are-one, what are the procedures we use for discovery, and two, what are our methods of proof? How do we make, and how do we check up on our discoveries ? Various answers have been given to these questions by many philosophers and scientists from Bacon1 in the sixteenth century and Mi112 in the nineteenth, to Popper3 and Hanson4 among others in the present century. Nowadays the discussion usually revolves around the question whether science works inside an inductive mould as Bacon and Mill appeared to believe, or inside an hypothetico-deductive mould as Popper and others maintain. Perhaps I should give the terms ‘inductivd and ‘hypothetico-deductive’ some brief preliminary explanation at once.According to inductive method, causal relationships are claimed to be dis- coverable from the repeated coincidence of certain observable properties. Thus, observational generalizations, such as that rusting is always noticed when water is present, are supposed to entitle us to assert for example, that water is a cause of rust. Induction starts with no causal commitment. The 49 Theobald 4 inductivist is entirely open-minded about the causes of rusting before he starts observing. Causes he believes, will emerge from observations without teasing. Hypothetico-deductive method on the other hand, starts with a causal hypo- thesis about rusting, such as that water is always required, and then proceeds via the deduction of observable consequences to subject this hypothesis to the knock-down argument of designed, experimental test.The hypothetico- deductivist then, is not entirely open-minded about the causes of rust when he starts observing. Causes, he believes, are revealed only by teasing. A good deal of philosophical and scientific opinion nowadays is directed squarely against inductive method, witness Popper’s Logic of scientzjic discovery3 and Meda- war’s more recent attack Induction and intuition in scienti3c thought.5 My own view, which I hope will emerge during the rest of this essay, is that it would be wrong to suppose that there was a single scientific method appro- priate to all stages in the development of a scientific enquiry.Scientific know- ledge is often the product of both induction and hypothetico-deduction, but used at different moments in an investigation. In this essay I want to examine some aspects of both inductive and hypothetico-deductive method to see whether or not they separately constitute adequate methods of discovery and proof. Fuller and more systematic accounts can be found in the works cited in the bibliography.293 I shall also touch upon a few aspects of the debate of wider interest, which as far as I know are not discussed by other recent writers on this subject. VISION AND TECHNIQUE The contrast between induction and hypothetico-deduction reflects the con- trast between the two ways we have of considering our experience of the world.Either we can take experience at its face value, or we can assume it to be the sign of some hidden reality which is not directly observable. Either appearances are reality or they are not. This is recognizable as a very ancient philosophical debate, which some may think is hardly worth attention these days. But such issues have a ghostly habit of reappearing in new guises to trouble us. Which is precisely what has happened here. The first disjunct corresponds roughly to the philosophy behind induction, the second to that behind hypothetico- deduction. The inductivist tries to make sense of the phenomenal surface of the world by establishing observational generalizations which he thinks will allow him inter aka to plan his life at the surface of things more effectively.However, generalizations of this sort, though they usually enable one to predict, do not usually enable one to understand. If I know that iron always rusts in the presence of water, then I can predict the occurrence of rusting. If I change the pressure on a gas, Boyle’s Law enables me to predict the change in its volume under certain conditions. But neither inductive generalization taken alone enables me to understand either the corrosion of iron or the compression of gases, that is, to specify their causes. We do not get to know any more about rusting just by observing more and more examples of it. The scientist has to have some idea or hypothesis about the formation of rust and then test his idea experimentally, if he is to further his understanding of such corrosion. On the other hand, we do not get to know anything whatever about rust unless we do 50 R.I. C. Reviews observe some examples of it to begin with. It looks then as though we may need both induction and hypothetico-deduction to succeed with any scientific problem. The hypothetico-deductivist, on the other hand, usually sees the visible world as a clue to some hidden reality which is not directly observable and which has to be discovered by the trial and error of intelligent and imaginative hypothesis. An hypothesis which stands the test of experiment provides the scientist, for the moment at any rate, with both an explanation for the visible world as it is, and an apparatus for successful prediction.The difference between induction and hypothetico-deduction then lies in this-that the former involves diligent and discriminating perception, while the latter involves this and the exercise of the imagination in formulating hypotheses. Induction needs technique, hypothetico-deduction vision and technique. We should note here that both induction and hypothetico-deduction run risks. Induction runs the risk of being trivial since it may be unable to get beyond the level of commonplace generalization. Per contra hypothetico-deduction runs the risk of being fanciful, of considering hypotheses whose metaphysical and physical implications are hard to accept. I have said that the hypothetico-deductivist often explains the phenomenal world by appealing to descriptions of inscrutable processes of which it is the sign.This immediately raises a number of interesting philosophical questions which do not concern the inductivist. Is the world of inscrutable things and processes really inscrutable ? If not, in what senses can it be observed ? Is it real in the same sense that the visible world is acknowledged real? What is the relation between descriptions of this hidden world and descriptions of the visible world? I shall return to some of these peripheral questions later on in this essay. Science then, taken as an explanation of visible reality, cannot rely upon induction alone. Induction can be a useful method of suggesting that there is a connection between various features of the visible world, but it cannot explain and, as we shall see, neither can it establish that connection.This is a matter for hypothetico-deduction. However we can safely say that a bit of induction is often the excuse for a bit of hypothetico-deduction. BACON, HUME AND MILL Discussions of scientific method are found in Aristotle (Posterior analytics) and various mediaeval philosophers such as Grosseteste and Roger Bacon. Such discussions became increasingly important as mediaeval technology grew more important, and Francis Bacon can be regarded as the Renaissance climax to this interest. So, for the purposes of this essay, I shall consider inductive reasoning as discussed by Francis Bacon in his Novum organum (1620), and then subsequently by Mill in his System of Zogic (1843).Bacon is much the more attractive writer although less thorough than Mill, differences which reflect the very different backgrounds of the two writers. What the sciences stand in need of (wrote Bacon) is a form of induction which shall analyse experience and take it to pieces, and by a due process of exclusion and rejection lead to an inevitable conclusion.6 Theobald 51 Induction (wrote Mill) may be defined as the operation of discovering and proving general propositions.7 Note the emphasis here in both Bacon and Mill on proof and discovery as one and the same operation. We shall come to dispute this in due course. Between the seventeenth century arguments of Bacon and the nineteenth century arguments of Mill on behalf of induction, come the eighteenth century criticisms of Hume in the Treatise (1738)s and the Enquiries (1777).9 I shall consider Hume’s point of view fairly briefly here because I think that it is perhaps of more direct interest to the philosopher than the scientist.Hume was concerned at a very fundamental level with how we could form simple generalizations, with how we could pass from statements about our experience of a limited number of examples of say, rusting, to the assertion of a generaliza- tion about it; from ‘This piece of iron x, y, z, and that piece of iron x, y, z, and . . .’ to ‘All pieces of iron x, y, z’. Hume was therefore concerned with the first stage of induction as a search for causes, namely how we get our observa- tional generalizations in the first place.He regarded any form of argument which was not deductive as specious, and since no inductive argument from particular to general is deductive, he would not allow it as a legitimate source of knowledge. It should be said here in passing that Hume did not make it at all clear in what sense he regarded a deduction as a source of knowledge. But whatever his thoughts on this, he would not consent to induction as a method of discovering true generalizations, nor therefore as a sound method of dis- covering the causes of phenomena. In recent times Russell expressed much the same point of view. What is called induction (he wrote in The principles of mathematics) appears to me to be either disguised deduction, or a mere method of making plausible guesses.In deduction, discovery and proof of the truth of that discovery are one and the same thing, whereas in induction they are not. It was this inter aka that Hume was concerned to point out. But let us be clear exactly what it was that Hume was saying. Hume was saying that induction provides no proof of the correctness of its conclusions, although of course these conclusions may in fact be correct. Induction was neither valid nor invalid argument, because strictly speaking it was no argu- ment at all. Hume would not allow that in an argument a conclusion can be supported by certain premises without following from them, and that argu- ments of this form may be sound or unsound although logically invalid.For soundness and validity are not at all the same thing. A good and persuasive argument can be sound without having deductive validity, although it cannot have deductive validity without being sound.10 Moreover as I hinted above, it is arguable whether a valid deductive argument gives us fresh information, although an inductive argument claims to and often does. All new information is subject to correction by further empirical evidence, and so therefore is any inductive conclusion. A valid deductive con- clusion is not corrigible in this way. But the fact that inductive conclusions are corrigible does not mean that induction can never be a source of knowledge.R.I.C. Reviews 52 All it means is that we have to be prepared to look to other than induction for a proof of the correctness of an inductive conclusion. To this we shall return shortly. I think we need say no more here about Hume’s criticisms of the starting point of inductive method in science, the formation of generalizations. For the problem we are faced with is this. When we have got some inductive generaliza- tions about say, rusting, which we believe to be true, can we get any further along the road to understanding and explaining the incidence of rusting from these alone? It is not difficult to see that the answer to this is that we cannot. Thus suppose pace Hume we do manage to formulate some generalizations about the conditions under which rust seems to occur.A conviction that these are in fact causal conditions is defensible only if there is a theoretical, explana- tory background into which the generalizations fit. What I mean here is that if an inductive generalization can be interpreted within some theoretical frame- work, then the connection it asserts is likely to be causally significant. We no longer have to consider the generalization as merely the expression of a re- markable set of observational coincidences as Hume contended was all we were ever entitled to do. For example the causal implications of generaliza- tions about rusting can be understood in terms of the ionic and molecular theories of chemistry in a way which enables us to see why x, y, z are necessary and sufficient conditions for rusting. Ionic and molecular theory explains because inter alia it asserts that there are such things as molecules and ions which obey the general laws of matter and electricity, and which can be related to the fundamental substratum of chemistry, the elements and their recogniz- able powers of valency.We shall have to bear in mind this question of theoreti- cal backing in what follows, for without it we are thrown back upon Hume’s remarkable series of coincidences. And now for a closer look at Bacon and Mill. Here we shall see just how important theoretical preconceptions actually are in framing generalizations, and why therefore Bacon’s and Mill’s suggestions as to how scientists should proceed are insufficient.Both Bacon and Mill believed that we could acquire the knowledge sufficient to control our environment only if we had no pre- conceptions whatsoever about what we might find. There are good reasons why Bacon and Mill thought like this. Bacon lived at a time when mediaeval habits of mind were slowly being replaced by what we would regard as more modern habits of mind. Thus knowledge from authority was being replaced by knowledge from observation, what Bacon himself called the ‘Presentation of instances to the understanding’.ll Bacon was inclined to distrust theorizing on the grounds that whereas nearly everyone could make observations, by no means everyone could invent theories. Theories and hypotheses, ‘the fume of subtile, sublime or delectable speculation’, was open to all the old mediaeval abuses.Moreover, in Bacon’s day there were considerable practical difficulties in following up investigations to any depth, and Bacon no doubt thought that it was better to concentrate in the circumstances upon what could be done easily and effectively, namely observing and reflecting inductively upon the simple surface aspects of things. There is however in Bacon a detectable ambival- ence on this point, but then this is perhaps no more than a reflection of the total ambivalence of his age. Thus he writes apparently in support of induction: Theobald 53 The human understanding is of its own nature prone to abstractions . . . but to resolve nature into abstractions is less to our purpose than to dissect her into parts.12 whereas elsewhere he writes what could be taken to be support for hypothetico- deduction : For like as a man’s disposition is never well-known till he be crossed, nor Proteus ever changed shapes till he was straitened and held fast; so the passages and variations of nature cannot appear so fully in the liberty of nature, as in the trials and vexations of art.13 Mill of course was educated in the philosophical tradition of English empiricism which had not been developed when Bacon was writing.This philosophy supposes that all our knowledge has its source in experience, but more controversially that the content of knowledge can never be logically richer than the content of experience. So it is not surprising that Mill had little time for truths supposedly known by intuition. And one might guess that he had a similar attitude towards the imagination. Like Locke before him, he had no belief in innate ideas or in innate differences between men.It was almost inevitable then that he should decide upon a scientific method which left little room for that in which men clearly do differ, imagination. Besides this, Mill was interested in devising an acceptable scientific method for the developing social sciences,14 and this could only be done, he thought, by con- centrating upon empirical generalization, for it was far too dangerous and provocative to theorize about matters such as these. Mill was not unopposed in his advocacy of inductive method. Among nineteenth century controversial- ists on scientific method, Jevons, Pierce and Whewell all disagreed with him.Whewell for example in his History of the inductive sciences (1 837) formulated a primitive version of hypothetico-deduction later to be elaborated by Popper in his important Logic ofscientijic discovery (1935, 1959). Pierce and Jevons took issue with Mill when he suggested that induction was a method of dis- covering causal connection and at the same time a rigorous method of proving it. But Mill found some support in Pearson who in his Grammar of science (1892) urged that the collection of facts into general bundles was the proper aim of science, that facts ‘were the one thing needful to form the minds of reasoning animals’ .I5 Pearson’s claim raises the question as to what facts are, whether facts are free from all theoretical or conceptual colouring, whether facts are untreated reports of direct experience.Is the assumption of Mill and Pearson following Locke that there is such a thing as pure, naive observation valid? I think that most philosophers nowadays would argue that all observation is theory- loaded at some point;16 and that if we have no theoretical background against which to observe, then our observations will be mechanical acts of sentience, and so meaningless. Kant made this point in his criticisms of empiri- cal philosophy at the end of the eighteenth century. He wrote in the Prolegomena . . . although all judgements of experience are empirical, i.e.have their ground in immediate sense-perception, yet all empirical judgments are not 54 R. I. C. Re views conversely for that reason judgments of experience, but in addition. . . , special concepts which have their origin wholly apriori in pure understand- ing must still be added.. . and it is these which make the judgement of experience objectively valid.17 Let us now look at Mill’s discussion of induction. Mill, like Bacon, discusses four inductive methods in his System of logic (1843). I shall consider only two of them, since the criticisms I shall level at these can be levelled similarly at the others. One method Mill calls ‘the method of agreement’, and it works as follows. We survey all the examples of the phenomenon P we can, and try to see what factor (or factors) F they seem to have in common.This factor Fcan then be taken as the cause or part of the cause of the phenomenon P, as reflected in generalizations of the form ‘For any F, P’ (sufficient condition) or ‘For any P, I;’ (necessary condition). (From exercising the method of agree- ment alone we cannot tell which. As we shall see later-p. 58-we can decide only by attempting to falsify the generalizations.) This seems clear enough, even to conforming with common practice, but upon reflection there are several evident difficulties with it. For example, how in fact shall we hit upon the common factor F, when as far as Mill’s instructions are concerned, we ought to approach the matter with open minds. If we are true to the philosophy of the method we ought to pay attention to every possible causal factor F.Suppose we are trying to find out why iron rusts. According to this Millian method, we ought to examine every aspect of those samples of rusty iron which we have before us-their size, shape, origin, position, purity, history, environment, owner and so on. Clearly this will not do. The only way we can make the method work is to decide what factors are relevant to outbreaks of rusting and what are not. We must have some idea what we are looking for, or we cannot begin to look. We must have some idea about what might be the causes of rusting before we begin; our search in other words, has to be guided by some tentative theory of rust. By itself therefore, the method constitutes an ineffective means of discovery.(The same sorts of criticism can be made of Mill’s complementary ‘method of difference’.)lg Even if we do succeed in discovering some F which seems to be universally correlated with rusting, we cannot rule out the possibility that both P and F may be caused by something else. For example suppose we found that water is always present when iron rusts. We could not say that water was a cause of rust, for as far as we can tell while using the method, the rust might be the cause of the water, or both the rust and the water might be the effects of some third and unidentified influence. This no doubt seems absurd to us, but only because we already have some theory of rusting in mind. If we approach the matter in true Millian manner, it is not at all absurd.Theobald As I have already said, we might be able to tell whether F is sufficient, necessary or necessary and sufficient condition of P by testing our generaliza- tions, but such testing according to Mill is not necessary if we have followed the inductive method properly. But, testing apart, a causal analysis becomes possible only when we have some theory or hypothesis with which we can assess the significance of the correlations we observe, with which we can separate real causal connection from fortuitous conjunction. Mill we must suppose, 55 like Hume before him, read no more into the notion of cause than that of invariable suc~ession.~~ Mill calls the second method I consider ‘the method of concomitant varia- tion’. This at first sight seems close to aspects of contemporary scientific practice, and was indeed close to the aspirations of nineteenth century philo- sophers of science like Mach who wished to limit science to establishing func- tional relationships without further enquiry into their physical significance. The method requires us to find some Fwhich varies in an orderly manner when the phenomenon P in which we are interested varies.This can be formulated in quantitative terms, though of course Bacon in the seventeenth century would have found it difficult to exploit this possibility. But what, we may ask, is meant by ‘orderly’ here? What is an orderly variation? Since there is no such thing as absolute or complete disorder, any set of data will exhibit some order.The question is: is it significant order? Moreover the observation of simultaneous variations in P and F does not of itself establish any causal connection, for both variations may be governed by the variation of a third, unidentified factor. Nor can we tell whether F i s a necessary, sufficient or necessary and sufficient condition of P. Only some theoretical preconceptions can help us to answer these questions. For example we might observe that the more a piece of iron rusts, the less oxygen there is in the immediate surroundings. Yet we cannot infer from this alone that the oxygen causes the iron to rust, though such an inference might be justified if we took our generalization in the light of some hypothesis about rusting.We must conclude then that although Mill’s methods may discover causes to us, they do not prove their discoveries. Mill’s claim that his inductive methods constituted methods of discovery and proof at one and the same time is false. For proof at least, we shall have to turn to the logically more rigorous hypothetico-deduction. The limitation of inductive science then, as I have described it, is that even if we do discover true correlations which may be useful predictively, we shall still be unable to prop up these predictions with satisfactory explanations. Here one is led to think of one of Mill’s primary concerns, the social sciences, where inductive generalization still plays an important part.There are of course difficulties in getting even as far as reliable generalizations in the social sciences for various reasons, generally well known; the fact that human beings may be predisposed to behave in a certain way by external conditions, but they are not usually constrained so to behave; the fact that how they behave will depend upon their evaluation of those conditions; the fact that control experi- ments in the social sciences are not possible as they are in the physical sciences; the fact that society is arguably not a system in the way that a physical system is a system; and so on. It may be that these difficulties rule out the formation of sensible hypotheses in the social sciences, thus confining social scientists to making inductive generalizations with limited predictive usefulness and little real explanatory power.Unless society is a system, it will not be possible to form clear ideas as to what universal forces may be at work within it. It might even be argued that the failure to find such forces suggests increasingly that society is not a system. Ironically therefore Mill seems to have developed his inductive philosophy, which in the event turns out to be inadequate, with R.I.C. Reviews 56 perhaps the most intractable subject matter he could find at the back of his mind. Induction by itself then will not do as a source of explanation and under- standing. The laws of chemical composition do not by themselves help us to understand chemical changes. Hypothetico-deduction on the other hand, according to which we form relevant hypotheses of cause and structure and then attempt to reject them seriatim by appealing to their experimental implications is in some ways nearer the mark.I say in some ways because although it gives us a clear criterion of validity by its emphasis on falsification, it does not help us to understand the generation of hypotheses. And here we may find ourselves back with all the informality of induction. I am inclined to think that in chemical practice for example, scientists work in a variety of ways, which may now be inductive and now hypothetico-deductive. But I shall return to this point later (p. 65). TESTING IN SCIENCE I want now to turn to the important question of how we test scientific claims, because this rather nicely illustrates some of the important differences between induction and hypothetico-deduction.20 It hardly needs saying that some rigorous testing of any empirical claim is a vital part of scientific method.Now there would seem to be two ways of testing a scientific statement; one is to try to confirm or verify it, the other is to try to falsify it. Both are used, though which depends quite a lot upon the science concerned and more than that, upon the context of the testing. Thus physics in contrast perhaps to botany is a full-time matter of looking for trouble. We ought to remember here that testing, whether confirmation or falsifica- tion, has no proper place in inductive method because this method is supposed by its advocates, incorrectly, to supply its own validation.Nevertheless let us suppose we are going to test both an hypothesis and a generalization. As an inductive generalization we may consider (a) ‘All damp iron rusts’; and as an hypothesis (b) ‘If iron is damp, then it rusts’. If we think about rust in terms of the hypothesis (b), then it is, I maintain, more natural to consider testing by falsification rather than by confirmation. On the other hand, if we think of rust in terms of the general statement (a), then I would say that confirmation seems the more appropriate procedure for testing. For if our claim is a properly formed inductive generalization, then we should not be expecting to find any falsifying cases, but merely confirming examples. And if it is an hypothesis, it stands accepted until shown to be false. ‘All damp iron rusts’ suggests then, that we look for further examples of water with rust.‘If iron is damp, then it rusts’ suggests rather that we look for cases where damp iron does not rust. I want to take this a little further. The general statement (a) ‘All damp iron rusts’ can be converted into a logical equivalent (al) ‘All rust-free iron is dry’. LogicaZly (al) is equivalent to (a). And so we might think that what confirms (a) ought to confirm (al). But (a) is confirmed by damp, rusty iron, and (al) by dry, rust-free iron. And it must strike us as odd that in order to confirm (a) ‘All damp iron rusts’, it seems that all we need to do is observe dry, rust-free iron (which confirms (al) the logical equivalent of (a)).It strikes us as odd because Theobald 57 such an observation does not seem to bear at all critically upon the original general statement (a). This peculiar situation shows us two important things. One, the fact that two statements such as (a) and (al) are logically equivalent does not mean that they are methodologically equivalent. And two, as long as we think of testing a statement in terms of corroboration, we shall be depen- dent upon the logical form our statement takes. This can be very uneconomi- cal. If we are interested in the properties of a small class of Xs, then it will be extravagant, other things being equal, to divert our interest to what will then be a very large class of non-Xs.Suppose we now try to falsify (a) and (al). In the case of (a) we shall need to observe damp, rust-free iron, and in the case of (al) rust-free, damp iron. In other words, to falsify either (a) or (al) requires the same observation. The falsification of a generalization then does not depend upon the logical form of the generalization. Moreover the observation which falsifies (a) and (al) also falsifies (b). It seems then that as far as the methodology of testing is con- cerned we should concentrate upon the falsification of hypotheses and general statements rather than upon their confirmation. Now we had occasion to note above that falsification is more appropriate to (b), while confirmation is more appropriate to (a) and (al).It seems then that when we are concerned with checking the validity of general statements of inductive origin they are better construed as hypotheticals open to falsification. So although we may acquire general knowledge by some inductive generalizing process, Millian or Baconian, its validity is best established by attempting to falsify related hypo- theses. As far as testing is concerned, we must desert induction for hypothetico- deduction. It might be worth digressing here for a moment to point out that some philosophers of science have argued that general statements and hypotheses are not real statements at all, but merely licences for forming such. For example, ‘All damp iron rusts’ or ‘If iron is damp, then it rusts’ licenses us to assert ‘This sample of iron is damp and so will rust’.The claim in effect is that the meaning of hypotheses and general statements is properly to be found in the meaning of the singular observation statements whose truth or falsity would confirm or falsify them. This is the philosophy of logical positivism, whose faults I cannot go into here,21 except to say that the identification of the meaning of a statement with the procedure for testing it has never been persua- sively established. In particular, for the positivist the meaning of an hypothesis which mentions an entity such as an atom which is not directly observable, is to be found in the meaning of the observation statements which confirm or falsify it. The scene is then set for the positivist to say that such a physical object is nothing more than a collection of observable properties.It is the sort of thing it is because it has the particular properties it has. But it could equally well be argued that an object has the properties it does have because it is the sort of thing it is. Now this is an important move, because it leaves the nature of the object open to be investigated. If an object is nothing but a collection of observable properties, if it has no nature, indeed if there is really no such thing as ‘it’, what reason can we have to investigate it beyond what we have already observed? Indeed what reason can we have even to start investigation? And besides, if an object has R.I.C. Reviews 58 no nature apart from its properties, what sort of properties are we to consider looking for? Any? If so we are back with the problems of inductive method discussed earlier, of not knowing how to start or when to stop.Positivism is not a philosophy for research. In a later development of positivism, instrumentalism, the argument was changed somewhat. A theory, it was contended, was simply a rule for inferring one observation statement from another. A theory asserted nothing whatso- ever about what the world was made of. But the same counter move can be made here-it is because a theory asserts something in fact about the world that it is a successful rule for making inferences. Let us now return to testing. There is a simple reason why falsification is in logic the proper way of bearing critically upon an hypothesis.I have so far merely discussed some methodological reasons why this is so. Suppose a scientific argument is generally of the form ‘Given Tand A , then B’, where Tis a theory from which given A , B may be deduced. Now if B is true, the con- junction Tand A can be true or false. For any true proposition can be implied by either a true proposition or a false proposition. On the other hand, if B is false, then Tand A can be only false. For no true proposition can imply a false proposition. So if B is a deductive consequence of T and A , and if B is observed to be false, then T and A can be inferred to be false also. And if A are observed initial conditions, the theory T must be false. We may conclude therefore that falsification is the proper way to test an hypothesis. ‘The aim of science is not to open a door to infinite wisdom, but to set a limit to infinite error’ as Brecht has Galileo say.22 There is a further point worth making in connection with testing.In ordinary life we are usually content to treat sufficient conditions as causes, whereas in science we usually reserve this description for necessary and sufficient condi- tions. Now the confirmation of (a) tells us nothing about whether dampness is a necessary, sufficient or necessary and sufficient condition of rust. But the falsification of (a) or (b) tells us at least that dampness is not a sufficient condi- tion of rust. If then we were to falsify (c) ‘All rusty iron is damp’, or ( d ) ‘If iron is rusty, it is damp’, this would tell us at least that rust was not a sufficient condition of dampness and therefore that dampness was not a necessary condition of rust.If attempts to falsify (a), (b), (c) and ( d ) fail (which we know in actual fact they would not), then dampness can be taken to be a necessary and sufficient condition of rust, at least until further evidence is forthcoming. PHILOSOPHICAL PROBLEMS FOR HYPOTHETICO-DEDUCTION I have tried to show how, although knowledge may be obtained fortuitously by an inductive method, the critical evaluation of that knowledge proceeds best by trying to falsify relevant hypotheses. I mentioned at the beginning of this essay that induction remains content with the world at its face value, whereas the invention and testing of hypotheses is not limited in this way.The content of an hypothesis is not limited by our ordinary experience of the world. It is in fact difficult to see how many of the most important principles and theories of physical science could have arisen by any generalizing operation alone. How for example could atomic theory have arisen in this way? And how can the role of conservation principles be interpreted inductively ? Theobald 59 Since the content of an hypothesis is not limited by our everyday experi- ence, we soon have to face the question : are atoms, ions, electrons and so on, which are features of many physical hypotheses, simply fictions or are they real existents? Are they merely grammatical ciphers in a scientific language, or do they actually designate bits of the real physical world? Are atoms and ions real in the sense that we take the furniture of our laboratory to be real? Per- haps I should point out that in a logical sense it does not make any difference to the properties of a thing whether it is a real existent or not.I mean by that that logically speaking, existence adds nothing to anything. If I describe a piece of chemical apparatus in detail, and then add ‘But such a piece of apparatus does not exist’, all I do is to lose the attention of my audience. I do not change the properties of the apparatus. So whether atoms or ions are real existents or not makes no difference to the predictive power of any atomic hypothesis, though it must make a difference to its persuasive power of explanation, its power to help us understand.Hypothetico-deduction then raises certain philosophical problems not raised by induction. It is often said that we never observe atoms in that sense of ‘observe’ appropriate to ordinary common-sense observation. But is this so ? I am inclined to think not. I think that observing an atom or an ion or an electron involves a perfectly proper sense of ‘observe’, a sense which I hope to explain. The atomic theory is designed to explain all the visible aspects of macroscopic change, and therefore atoms themselves cannot be invested with ordinary, visible properties. But this does not mean that they cannot be observed. Imagine looking into the clear night sky and seeing a comet.You say to yourself ‘There’s a comet’ or perhaps ‘I can see a comet’. Rather more im- probably you might say ‘I can see the light-track of a comet’, even though in a sense that is what you do see. You can quite properly say these things because you undoubtedly know a little astronomical science and probably also some- thing about the appearance of moving luminous bodies, and because you are confident that astronomers have some sort of astronomical tests up their sleeves to confirm or falsify such claims. The argument ‘I saw an A ; A is B ; so I saw a B’ is generally valid only if you know that A is B. That is, if you saw A , you are entitled to say you saw B, only if you know that A was B.From the fact that you saw a man, you are entitled to say that you saw a laughing man, only if you know the man was laughing. Of course you need not have observed his laughing, someone might have told you. You can be entitled to say you saw a laughing man without being entitled to say that you saw a man laughing. To take another example, if I saw a piece of apparatus, I am entitled to say I saw an ir spectrometer only if I know it is an ir spectrometer. But I need not have seen that it was an ir spectrometer; someone, an expert, may have told me that that was what I was seeing. To return to comets: if we see a light-track in the sky, we are entitled to say we see the light-track of a comet because we know from astronomers that such is in fact the light-track of a comet.And seeing the light-track of a comet is seeing a comet making a light-track. It is in fact seeing a comet. What else could possibly count as seeing it? And no one on this account would dream of saying that comets are not real. The same sort of considerations apply to seeing a-particles in cloud- R.Z. C. Reviews 60 chambers or molecules in electron micro~copes.2~ One reason why these hypothetical entities are bona fide candidates for the description ‘real’ is that like the comet, there is a particular conceptual background which makes certain claims to see them legitimate and significant. There is an accepted reservoir of knowledge which makes the second premise in the argument I outlined above true. To take a chemical example, to see a chemical change occurring is to see an electronic rearrangement going on, once we know all about chemical theory.The important point is that what we say we can see depends upon our know- ledge. As does therefore what we can say is real. Against this some philosophers have argued that to say that we see an electronic rearrangement going on, if taken literally, is to say something mis- leading if not false. It is argued that whereas seeing a chemical change going on in a flask is clear enough, to say that this is to see electrons rearrange is to hide the fact that an inference has been made in accordance with some physical theory, an inference from what goes on in the flask to the fate of the electrons. This has been the view of many instrumentalist and positivist philosophers of science.24 But given the setting of atomic theory, seeing a chemical change in progress just is seeing the electrons rearranging, and moreover that is all there is to seeing the electrons rearranging.The confusion arises from failing to notice the difference between seeing inside a theory and seeing outside it. Both are legitimate seeings, and neither involves making inferences of any sort. Seeing is not just opening one’s eyes-seeing is an achievement. Note how frequently ‘see’ is used with ‘can’ and how rarely if at all in the present tense, ‘I see. . .’ or ‘I am seeing. . .’. For to see is to invest what you notice with some significance. And this is what theories and hypotheses are for.All observation is dyed with theory to a greater or lesser extent, and it was a cardinal and curiously romantic error of Mill, Pearson and others to suppose that there was such a thing as simple, naive observation, and therefore that there was such a thing as a fact which was not charged with some theoretical preconceptions. HYPOTHESES I have argued that although it may suggest correlations between observable properties, induction cannot establish causal connection. And so it cannot supply profound understanding. We need to devise hypotheses and test them critically in order to explain and understand. The actual form any hypothesis takes and the particular suggestions it makes will depend a great deal upon our present knowledge. For any hypothesis which is to extend our present knowledge will begin by suggesting analogies between new physical systems and systems with whose behaviour we are already familiar.Rarely therefore will an hypothesis depart completely from previous ideas. This might be taken to imply that the refutation of an hypothesis will call the whole of the science concerned into question. But this is not necessarily so. As has been said, falsification is the sign only that something has gone wrong, not that every- thing has gone wrong. On the occasions when an hypothesis does depart radically from previous ideas the result is a conceptual revolution in science, examples of which will be very familiar. To the chemist the reorientation of chemical thought by Lavoisier in the eighteenth century, by Dalton in the Theobald 61 nineteenth, and by Bohr and later quantum theorists in the twentieth century are perhaps the most important.Sometimes, metaphysical conviction can lead to the retention of hypotheses even when the evidence is against them, even when they have been technically falsified. Witness the persistence of alchemical ideas despite their many failures,25 and the reluctance of chemists, for example Priestley, to abandon phlogiston theory even after the work of Cavendish and Lavoisier. As another example we may recall that MendelCef persisted with his classification of the elements despite the fact that at the time his periodic arrangement failed in several places. Sometimes metaphysical conviction makes us continue with an hypothesis when there is no clear evidence for or against it.Dalton’s atomic hypothesis was maintained in the face of a great deal of initial quantitative uncertainty, and an inability to see precisely how to put it to conclusive test. In fact there was a gradual decline of interest in atomic theory during the middle of the nineteenth century until the work of Avogadro and Gay-Lussac was re- presented by Cannizzaro, and shown to be compatible with Dalton’s hypo- thesis after some modifications. These few historical points show that dis- covery, justification, judgement, imagination and metaphysical conviction become inextricably mixed during the evolution of scientific ideas. They also show that hypothetico-deduction is not a scientific method which commands the scientist’s unswerving allegiance. As I pointed out in the introduction to this essay, science like any other human activity cannot always be conducted successfully by rule of logic alone. But the most interesting question to do with hypotheses has yet to be asked. How do we form hypotheses ? How does our guided imagination work ?4 There are very difficult questions even to begin to answer.We can certainly make a survey of the sorts of person who have had the necessary imaginative frame of mind to devise hypotheses, and we can certainly make a survey of the physical and intellectual conditions which predispose (but not determine) a man to be imaginative. But I doubt whether any statistical findings of this sort about imaginative thinking will leave us any nearer either predicting or engineering its occurrence.And such a survey would in any case do little to clarify what imagination is or how it works in the business of discovery. And it is to aspects of the philosophy of the imagination I wish to turn now. All our attempts to come to terms with the experienced world depend upon our fitting this experience into an orderly, though flexible matrix of general concepts whose expression is usually verbal. As I have already remarked, scientific knowledge which is linguistic and propositional, requires more than a keen eye for phenomenal incident. To know is to create order and structure in the world by means of language. Knowledge is not something absolute, just ‘out there’ for quick capture, but is, as Coleridge realized, something to be created and recreated because language is subject to constant, creative renewal by philosophers, poets and scientists.‘The relation of language to thought and reality is not a passive ref? ection, but an active and tendentious reaction’ wrote the literary critic and poet Christopher Caudwell.26 This I think is not such a bad definition of scientific practice. We can see this more surely if we reflect upon how we come by the concepts R.I.C. Reviews 62 we have. Let us take the concept ‘chemist’ as an example. A common view of the origin of our concept ‘chemist’ would be this-all we do is to make a survey of all chemical persons we know, and draw up an inventory of what they have in common.As a result we have a concept ‘chemist’ the meaning of which is decided, that is defined, by a collection of common properties. But clearly this will not do, attractively simple as it seems. And it will not do for the same reasons that Mill’s method of agreement will not do. For until we have at least some concept ‘chemist’ in our minds, we can have no idea who are chemical persons, and therefore no idea what common characteristics we are to look for. This is an important point. Suppose we take the concept ‘red’. It is often argued that this concept is acquired by noticing and naming what is common to a set of red objects. But this can be done only if we are quite clear that we are dealing with culuur as opposed to say shape, size or weight.We must be clear what sort of thing we are attending to before we start. Concepts are not acquired independently, but in relation to other concepts. Another way of putting this would be to say that language is a system of concepts, not a collection. We cannot and do not acquire concepts by diligent stock-taking. The concept ‘chemist’ like all other concepts is first and foremost a mental invention, but one whose meaning will change and develop as our experience of the class of objects designated by the concept increases. It is not easy to say precisely why we invent concepts, nor how they and our experience develop side by side. We may feel (inductively) that there is some uniformity in our experience which needs charting, or we may wish to discover whether there is.These are matters clearly for imaginative judgement, for hypothesis and testing, not simply for an inductive logic. What has been said here of the concepts ‘chemist’ and ‘red’ is, I think, true of all concepts. They are first and foremost mental inventions and they designate open classes. Whether or not some particular item of experience falls under a given concept is a matter both for our perception and our judgement, but not for our perception alone. It is a matter of weighing the evidence for or against inclusion. This looseness in the application of language to the world is vitally important, for it allows language to hover over our experience of the world, and settle upon it not only as an instrument of definition, but as an instrument of promise, expectation and revaluation.Concepts are like Axes After whose stroke the wood rings, And the echoes! Echoes travelling Off from the centre like horses.27 In scientific contexts it is this looseness which allows us to overlook disagree- ment between hypothesis and observation without being guilty of logical inconsistency.28 For, as we have seen, theories and hypotheses are not always refuted by such disagreements. Concepts then do not mirror reality; rather in an important way they create the reality we talk about. Experience does not give our concepts meaning; we give them meaning, and by so doing we endow experience with meaning. This Theobald 63 is an extremely important point which explains why I head this essay with some words from Eliot’s Dry Salvages, and which is at the heart of the conflict between induction and hypothetico-deduction.‘Concepts lead us to make in- vestigations ; are the expression of our interest, and direct our interest’ wrote Wittgenstein.29 This can be said equally of any hypothesis in science. My remarks about concepts in the preceding paragraphs, although they need more elaboration and defence than I can give them here, can be directly translated into the terms of the induction/hypothetico-deduction controversy. For hypotheses are not elicited from experience, they are invented to make sense of experience and, like concepts-themselves in some ways hypotheses of persis- tence, identity, resemblance and uniformity-they are modified in the light of it.The object of ordinary language, like its scientific development, is to tie down experience into labelled bundles, to reduce the apparent variety in experience by collecting it into generalities. The scientific imagination tries to simplify, though not, as I have tried to show, exclusively by induction. The poetic imagination as I understand it, tries to complicate. ‘What we have gotten by this revolution is a great deal of good sense. What we have lost is a world of fine fabling’ wrote Richard Hurd, Bishop of Worcester, of the scien- tific enlightenment in the eighteenth century. The poet, the Blake who sees scientists as generalizing idiots, thinks of simplification as restricting the pos- sibilities for seeing the world, as numbing our sensitivity to small but impor- tant differences in things. ‘We have art in order not to die of the truth’ wrote Nietz~che.~~ Scientific imagination is more controlled and has less room for manoeuvre than the artistic imagination, and yet in the long run it may carry more universal conviction. For the fact that the scientific imagination works within the confines of previous knowledge and opinion guards scientific hypotheses against some of the subjective extravagancies which Bacon, Mill and others feared.New ideas in science are either rejected or become accepted as textbook truths, but new ideas from the artistic imagination are always fresh to each generation. They are not assimilable to a body of knowledge.This is because in science the meaning of a term is designed to be independent of the context in which it is used and so becomes the expression of a timeless gener- ality in nature whereas in poetry, say, the meaning depends very much upon that context and requires revaluation when that context changes. I have already mentioned the importance of analogy in science. The scien- tist’s imagination works by seeing in the apparently new much that is like the already familiar. The artist works by seeing in the apparently familiar much that is actually new. The effectiveness of all analogy as Coleridge pointed out31 is a complex and subtle matter of ‘the balance or reconciliation of opposite or discordant qualities; of sameness with difference; of the general with the concrete, the individual with the representative; the sense of novelty and freshness, with old and familiar objects; a more than usual state of order .. .’. Once we have accepted an analogy, once it convinces us, we are necessarily converted to a reconstructed reality. ‘The expression of a change of aspect is the expression of a new perception and zt the same time of the perception’s being ~ n c h a n g e d . ’ ~ ~ We cannot look at gases in the same way once we havesup- posed that they consist of bits of matter in Newtonian motion. We cannot look at human beings and chairs in the same way once we accept that they both R.I.C. Reviews 64 have legs, nor at clouds in the same way once we have seen ‘a cloud that’s dragonish.. .’.33 In science this sort of imaginative move is part and parcel of forming and testing hypotheses. We start by seeing A as B and we then act on this sugges- tion to find out whether in fact A is B, whether the analogy stands up to examination. Imagining is always preliminary to knowing. Imagining a gas as a collection of Newtonian particles is preliminary to knowing that a gas is in fact more or less just that; and also to seeing why it is onZy more or less just that. We can see then that hypothetico-deduction is an extension of aspects of the philosophy of concept formation into the methodology of science. I have tried to show that induction is by itself methodologically inadequate, and in some respects philosophically naive.This does not mean to say however that a Mill- ian approach to a problem is always inappropriate, as some critics seem to suppose. I return to a point I made earlier which was that it is not possible to legislate about how scientists should go about the business of discovery. It is possible to legislate only about how they should check their discoveries, how they should check their claims to know And here the verdict is clearly in favour of hypothetico-deduction. Hypothetico-deduction will evidently find favour with logicians and mathematically-minded physical scientists on account of its logical rigour, but it is not so clear that it will always find favour with the more experimental scientist for whom the chance observation, the remarkable coincidence is of great moment.One is sometimes tempted to think that those scientists who argue often overbearingly for hypothetico-deduction have themselves lost contact with the uncertainties and chances of day to day work in the laboratory. CHEMISTRY AND SCIENTIFIC METHOD I want now to say a few words about the position of the chemist in all this. Many important discoveries have been made in chemistry (as indeed they have in the other physical sciences) from the odd observation, the peculiar coinci- dence, so that although chemists may need the logic of hypothetico-deduction to put their ideas to the test, they often rely upon the informal consequences of induction to generate their ideas in the first place.The chemist is traditionally a scientist who often simply wants to know what occurs when A is mixed with B, besides wanting to produce a theoretical explanation for it. Although admittedly we may wish to explain the reaction once we have mixed A with B, it is inductive curiosity which leads us to the mixing in the first place. In chemistry many more things have been discovered than have been explained, and there is no sign that discovery is dependent for its impetus upon theory and explana- tion as it undoubtedly is in physics. Chemistry is still in many respects an informal science. And as I have already remarked, important discoveries come from such informality. I think that chemical curiosity is much less dis- ciplined than physical curiosity, and it is probably this element of ‘idle curiosity’ which makes chemistry so successful in its relations with technology.For it is the chance rather than the systematic discovery which is often interesting to the technologist. Theobald 65 5 It is a notorious fact that in theoretical physics one experimental result is often all that is necessary, indeed in practice is often all that is available to evaluate an hypothesis and if need be reject it. On the other hand in chemistry there is more often than not a vast array of experimental results which can be brought to bear upon an hypothesis. Usually no single result bears upon a chemical hypothesis in the way a single result can bear upon an hypothesis in physics. This is reflected in the fact that theories and models in chemistry are generally much looser formulations than those in physics.The chemist is concerned with modifying details of a well-authenticated molecular model, not with rejecting it. On the other hand the physicist at his level of physical enquiry is concerned much more with making a conclusive choice between rival models. Accordingly there is not such a tight relationship between hypothesis and observation in chemistry as there seems to be in physics. These things are all a reflection of the fact that chemistry is still in some measure an inductive science. Chemistry then stands between physics on the one hand and the field biological sciences on the other in being an area of inquiry peculiarly appro- priate for the exercise of both inductive and hypothetico-deductive method.However chemists will continue to enjoy this stimulating position only if they resist the temptation to suppose that they must model their science either on physics or biology. This, as I have argued elsewhere,34 is seriously to suppress the imagination besides, as I believe, being logically and methodologically indefensible. There is more I believe in heaven and earth than is comprehended in the philosophy of physics. But fortunately there is no reason to suppose that chemistry must succumb to relations with either of its suitors. Those who would have it do so will I think find relations become very strained, and I suggest they would do well to remember Tristram’s remarks about his father: he was systematical, and, like all systematic reasoners he would move heaven and earth and twist and torture everything in nature to support his hypothesis.In a word he was serious and be content to remain chemists. ACKNOWLEDGMENT The excerpt from T. S . Eliot’s The Dry Salvages is reproduced by permission of Faber and Faber. REFERENCES 1 Novum organum (1 620) ; The advancement of learning (1 605). 2 System of logic (1843). 3 Logic of scientific discovery (1935, 1959). 4 Patterns of discovery (1958); The concept of the positron (1963). 5 Induction and intuition in scientific thought (1969). 6 Thegreat instauration (1607), Plan of (trans. by J. Spedding, R. L. Ellis and D. D. Heath). 7 System of logic, book 3, chapter 1.8 Treatise of human nature (1738). 9 Enquiry concerning human understanding (1 777). 10 See my Introduction to the philosophy of science (1968) for further discussion and references. 11 Novum organum, Aphorisms, book 2, 15 (trans. by J. Spedding, R. L. Ellis and D. D. Heath) . 12 Ibidem, book 1, 51. R . I. C. Reviews 66 13 Advancement of learning, book 2. 14 System of logic, book 6. 15 C. Dickens, Hard times, book 1, chapter 1. 16 See, for example, Hanson, op. cit. 17 Prolcgomena to any future metaphysics (1783), para. 18 (trans. by P. G. Lucas). 18 These methods are discussed more fully in most books on scientific method, e.g. E. Nagel, An introduction to logic and scientific method (1934). 19 System of logic, book 3, chapter 5.20 See my Introduction to the philosophy of science, chapter 4. 21 See J. 0. Urmson, Philosophical analysis (1956), and G. J. Warnock, English philosophy since 1900 (1958, 1969) for useful discussions. 22 Scene 9. 23 See my ‘Observation and reality’, Mind, 1967, 76, 198. 24 See E. Nagel, The structure of science (1961). 25 See my ‘Alchemy; a philosophical reappraisal’, Technology and Society, 1965, 2, 135. 26 Illusion and reality (1 937). 27 Sylvia Plath, ‘Words’ in Ariel(l948). 28 T. S. Kuhn, The structure of scientific revolutions (1962). 29 Philosophical investigations (1 958), part 1, 570. 30 Goethe is an excellent example of a man torn in the two directions. See E. Heller, The disinherited mind (1952). 31 Biographia literaria 1817, 2, 12.32 Philosophical investigations, part 2, xi. 33 Antony and Cleopatra, act 4, sc. 12. 34 See my Introduction to the philosophy of science, chapter 6. 67 Theobald EXPERIMENT, IMAGINATION AND MEANINGInductive and Hypothetico-deductive Method in ScienceD. W. TheobaldDept of Chemistry, UMIST, Monchester IVision and technique . . . . . . .. . . . . . . 50Bacon, Hume and Mill . . . . . . . . . . .. . . 51Testing in science . . .. .. . . . . . . . . . . 57Philosophical problems for hypothetico-deduction . . . . . . 59Hypotheses .. . . .. .. . . . . . . . . 61Chemistry and scientific method . . .. . . . . . . 65References . . .. . . . . .. . . . . . . . . 66We had the experience but missed the meaning,And approach to the meaning restores the experienceIn a different form, ...T.S. Eliot, The Dry SalvagesThe moment we reflect philosophically upon the knowledge we have of theworld about us, two questions emerge. One, how do we acquire such know-ledge? And two, how do we guarantee that what passes for knowledge is infact knowledge? The first is a question about discovery, the second a questionabout justification and proof. Of course these are strictly speaking philosophi-cal questions, but the answers to them are of interest, indeed importance, toscientists. To explain this a little further, let us take a simple example, anexample which we shall return to frequently throughout this essay. Suppose weare interested in rusting, both from a theoretical point of view and perhaps withthe practical aim of preventing its occurrence and the inconvenience it causes.First we shall want to find out the conditions under which rusting occurs sothat we can discover its causes, and then we shall require some procedure forestablishing or proving that we have found out correctly.And the questionsI am asking are-one, what are the procedures we use for discovery, and two,what are our methods of proof? How do we make, and how do we check up onour discoveries ? Various answers have been given to these questions by manyphilosophers and scientists from Bacon1 in the sixteenth century and Mi112 inthe nineteenth, to Popper3 and Hanson4 among others in the present century.Nowadays the discussion usually revolves around the question whether scienceworks inside an inductive mould as Bacon and Mill appeared to believe, orinside an hypothetico-deductive mould as Popper and others maintain.Perhaps I should give the terms ‘inductivd and ‘hypothetico-deductive’ somebrief preliminary explanation at once.According to inductive method, causal relationships are claimed to be dis-coverable from the repeated coincidence of certain observable properties.Thus, observational generalizations, such as that rusting is always noticedwhen water is present, are supposed to entitle us to assert for example, thatwater is a cause of rust.Induction starts with no causal commitment. TheTheobald 49inductivist is entirely open-minded about the causes of rusting before he startsobserving.Causes he believes, will emerge from observations without teasing.Hypothetico-deductive method on the other hand, starts with a causal hypo-thesis about rusting, such as that water is always required, and then proceedsvia the deduction of observable consequences to subject this hypothesis to theknock-down argument of designed, experimental test. The hypothetico-deductivist then, is not entirely open-minded about the causes of rust when hestarts observing. Causes, he believes, are revealed only by teasing. A good dealof philosophical and scientific opinion nowadays is directed squarely againstinductive method, witness Popper’s Logic of scientzjic discovery3 and Meda-war’s more recent attack Induction and intuition in scienti3c thought.5My own view, which I hope will emerge during the rest of this essay, is thatit would be wrong to suppose that there was a single scientific method appro-priate to all stages in the development of a scientific enquiry.Scientific know-ledge is often the product of both induction and hypothetico-deduction, butused at different moments in an investigation. In this essay I want to examinesome aspects of both inductive and hypothetico-deductive method to seewhether or not they separately constitute adequate methods of discovery andproof. Fuller and more systematic accounts can be found in the works cited inthe bibliography.293 I shall also touch upon a few aspects of the debate ofwider interest, which as far as I know are not discussed by other recent writerson this subject.VISION AND TECHNIQUEThe contrast between induction and hypothetico-deduction reflects the con-trast between the two ways we have of considering our experience of the world.Either we can take experience at its face value, or we can assume it to be thesign of some hidden reality which is not directly observable.Either appearancesare reality or they are not. This is recognizable as a very ancient philosophicaldebate, which some may think is hardly worth attention these days. But suchissues have a ghostly habit of reappearing in new guises to trouble us. Which isprecisely what has happened here. The first disjunct corresponds roughly tothe philosophy behind induction, the second to that behind hypothetico-deduction. The inductivist tries to make sense of the phenomenal surface of theworld by establishing observational generalizations which he thinks will allowhim inter aka to plan his life at the surface of things more effectively. However,generalizations of this sort, though they usually enable one to predict, do notusually enable one to understand.If I know that iron always rusts in thepresence of water, then I can predict the occurrence of rusting. If I change thepressure on a gas, Boyle’s Law enables me to predict the change in its volumeunder certain conditions. But neither inductive generalization taken aloneenables me to understand either the corrosion of iron or the compression ofgases, that is, to specify their causes. We do not get to know any more aboutrusting just by observing more and more examples of it. The scientist has tohave some idea or hypothesis about the formation of rust and then test his ideaexperimentally, if he is to further his understanding of such corrosion.On theother hand, we do not get to know anything whatever about rust unless we do50 R. I. C. Reviewobserve some examples of it to begin with. It looks then as though we mayneed both induction and hypothetico-deduction to succeed with any scientificproblem.The hypothetico-deductivist, on the other hand, usually sees the visibleworld as a clue to some hidden reality which is not directly observable andwhich has to be discovered by the trial and error of intelligent and imaginativehypothesis. An hypothesis which stands the test of experiment provides thescientist, for the moment at any rate, with both an explanation for the visibleworld as it is, and an apparatus for successful prediction.The differencebetween induction and hypothetico-deduction then lies in this-that the formerinvolves diligent and discriminating perception, while the latter involves thisand the exercise of the imagination in formulating hypotheses. Inductionneeds technique, hypothetico-deduction vision and technique. We should notehere that both induction and hypothetico-deduction run risks. Induction runsthe risk of being trivial since it may be unable to get beyond the level ofcommonplace generalization. Per contra hypothetico-deduction runs the riskof being fanciful, of considering hypotheses whose metaphysical and physicalimplications are hard to accept.I have said that the hypothetico-deductivist often explains the phenomenalworld by appealing to descriptions of inscrutable processes of which it is thesign.This immediately raises a number of interesting philosophical questionswhich do not concern the inductivist. Is the world of inscrutable things andprocesses really inscrutable ? If not, in what senses can it be observed ? Is it realin the same sense that the visible world is acknowledged real? What is therelation between descriptions of this hidden world and descriptions of thevisible world? I shall return to some of these peripheral questions later on inthis essay.Science then, taken as an explanation of visible reality, cannot rely uponinduction alone.Induction can be a useful method of suggesting that there is aconnection between various features of the visible world, but it cannot explainand, as we shall see, neither can it establish that connection. This is a matterfor hypothetico-deduction. However we can safely say that a bit of inductionis often the excuse for a bit of hypothetico-deduction.BACON, HUME AND MILLDiscussions of scientific method are found in Aristotle (Posterior analytics)and various mediaeval philosophers such as Grosseteste and Roger Bacon.Such discussions became increasingly important as mediaeval technologygrew more important, and Francis Bacon can be regarded as the Renaissanceclimax to this interest. So, for the purposes of this essay, I shall considerinductive reasoning as discussed by Francis Bacon in his Novum organum(1620), and then subsequently by Mill in his System of Zogic (1843).Bacon ismuch the more attractive writer although less thorough than Mill, differenceswhich reflect the very different backgrounds of the two writers.What the sciences stand in need of (wrote Bacon) is a form of inductionwhich shall analyse experience and take it to pieces, and by a due process ofexclusion and rejection lead to an inevitable conclusion.6Theobald 5Induction (wrote Mill) may be defined as the operation of discovering andproving general propositions.7Note the emphasis here in both Bacon and Mill on proof and discovery as oneand the same operation. We shall come to dispute this in due course.Between the seventeenth century arguments of Bacon and the nineteenthcentury arguments of Mill on behalf of induction, come the eighteenth centurycriticisms of Hume in the Treatise (1738)s and the Enquiries (1777).9 I shallconsider Hume’s point of view fairly briefly here because I think that it isperhaps of more direct interest to the philosopher than the scientist. Humewas concerned at a very fundamental level with how we could form simplegeneralizations, with how we could pass from statements about our experienceof a limited number of examples of say, rusting, to the assertion of a generaliza-tion about it; from ‘This piece of iron x, y, z, and that piece of iron x, y, z,and .. .’ to ‘All pieces of iron x, y, z’.Hume was therefore concerned with thefirst stage of induction as a search for causes, namely how we get our observa-tional generalizations in the first place. He regarded any form of argumentwhich was not deductive as specious, and since no inductive argument fromparticular to general is deductive, he would not allow it as a legitimate sourceof knowledge. It should be said here in passing that Hume did not make it atall clear in what sense he regarded a deduction as a source of knowledge. Butwhatever his thoughts on this, he would not consent to induction as a methodof discovering true generalizations, nor therefore as a sound method of dis-covering the causes of phenomena. In recent times Russell expressed much thesame point of view.What is called induction (he wrote in The principles of mathematics) appearsto me to be either disguised deduction, or a mere method of making plausibleguesses.In deduction, discovery and proof of the truth of that discovery are one andthe same thing, whereas in induction they are not.It was this inter aka thatHume was concerned to point out.But let us be clear exactly what it was that Hume was saying. Hume wassaying that induction provides no proof of the correctness of its conclusions,although of course these conclusions may in fact be correct. Induction wasneither valid nor invalid argument, because strictly speaking it was no argu-ment at all. Hume would not allow that in an argument a conclusion can besupported by certain premises without following from them, and that argu-ments of this form may be sound or unsound although logically invalid.Forsoundness and validity are not at all the same thing. A good and persuasiveargument can be sound without having deductive validity, although it cannothave deductive validity without being sound.10Moreover as I hinted above, it is arguable whether a valid deductiveargument gives us fresh information, although an inductive argument claims toand often does. All new information is subject to correction by further empiricalevidence, and so therefore is any inductive conclusion. A valid deductive con-clusion is not corrigible in this way. But the fact that inductive conclusions arecorrigible does not mean that induction can never be a source of knowledge.52 R.I.C.ReviewAll it means is that we have to be prepared to look to other than induction for aproof of the correctness of an inductive conclusion. To this we shall returnshortly.I think we need say no more here about Hume’s criticisms of the startingpoint of inductive method in science, the formation of generalizations. For theproblem we are faced with is this. When we have got some inductive generaliza-tions about say, rusting, which we believe to be true, can we get any furtheralong the road to understanding and explaining the incidence of rusting fromthese alone? It is not difficult to see that the answer to this is that we cannot.Thus suppose pace Hume we do manage to formulate some generalizationsabout the conditions under which rust seems to occur.A conviction that theseare in fact causal conditions is defensible only if there is a theoretical, explana-tory background into which the generalizations fit. What I mean here is that ifan inductive generalization can be interpreted within some theoretical frame-work, then the connection it asserts is likely to be causally significant. We nolonger have to consider the generalization as merely the expression of a re-markable set of observational coincidences as Hume contended was all wewere ever entitled to do. For example the causal implications of generaliza-tions about rusting can be understood in terms of the ionic and moleculartheories of chemistry in a way which enables us to see why x, y, z are necessaryand sufficient conditions for rusting.Ionic and molecular theory explainsbecause inter alia it asserts that there are such things as molecules and ionswhich obey the general laws of matter and electricity, and which can be relatedto the fundamental substratum of chemistry, the elements and their recogniz-able powers of valency. We shall have to bear in mind this question of theoreti-cal backing in what follows, for without it we are thrown back upon Hume’sremarkable series of coincidences.And now for a closer look at Bacon and Mill. Here we shall see just howimportant theoretical preconceptions actually are in framing generalizations,and why therefore Bacon’s and Mill’s suggestions as to how scientists shouldproceed are insufficient. Both Bacon and Mill believed that we could acquirethe knowledge sufficient to control our environment only if we had no pre-conceptions whatsoever about what we might find.There are good reasonswhy Bacon and Mill thought like this. Bacon lived at a time when mediaevalhabits of mind were slowly being replaced by what we would regard as moremodern habits of mind. Thus knowledge from authority was being replaced byknowledge from observation, what Bacon himself called the ‘Presentation ofinstances to the understanding’.ll Bacon was inclined to distrust theorizing onthe grounds that whereas nearly everyone could make observations, by nomeans everyone could invent theories. Theories and hypotheses, ‘the fume ofsubtile, sublime or delectable speculation’, was open to all the old mediaevalabuses.Moreover, in Bacon’s day there were considerable practical difficultiesin following up investigations to any depth, and Bacon no doubt thought thatit was better to concentrate in the circumstances upon what could be doneeasily and effectively, namely observing and reflecting inductively upon thesimple surface aspects of things. There is however in Bacon a detectable ambival-ence on this point, but then this is perhaps no more than a reflection of the totalambivalence of his age. Thus he writes apparently in support of induction:Theobald 5The human understanding is of its own nature prone to abstractions . . . butto resolve nature into abstractions is less to our purpose than to dissect herinto parts.12whereas elsewhere he writes what could be taken to be support for hypothetico-deduction :For like as a man’s disposition is never well-known till he be crossed, norProteus ever changed shapes till he was straitened and held fast; so thepassages and variations of nature cannot appear so fully in the liberty ofnature, as in the trials and vexations of art.13Mill of course was educated in the philosophical tradition of Englishempiricism which had not been developed when Bacon was writing.Thisphilosophy supposes that all our knowledge has its source in experience, butmore controversially that the content of knowledge can never be logicallyricher than the content of experience. So it is not surprising that Mill had littletime for truths supposedly known by intuition.And one might guess that hehad a similar attitude towards the imagination. Like Locke before him, hehad no belief in innate ideas or in innate differences between men. It wasalmost inevitable then that he should decide upon a scientific method whichleft little room for that in which men clearly do differ, imagination. Besidesthis, Mill was interested in devising an acceptable scientific method for thedeveloping social sciences,14 and this could only be done, he thought, by con-centrating upon empirical generalization, for it was far too dangerous andprovocative to theorize about matters such as these. Mill was not unopposedin his advocacy of inductive method. Among nineteenth century controversial-ists on scientific method, Jevons, Pierce and Whewell all disagreed with him.Whewell for example in his History of the inductive sciences (1 837) formulateda primitive version of hypothetico-deduction later to be elaborated by Popperin his important Logic ofscientijic discovery (1935, 1959).Pierce and Jevonstook issue with Mill when he suggested that induction was a method of dis-covering causal connection and at the same time a rigorous method of provingit. But Mill found some support in Pearson who in his Grammar of science(1892) urged that the collection of facts into general bundles was the properaim of science, that facts ‘were the one thing needful to form the minds ofreasoning animals’ .I5Pearson’s claim raises the question as to what facts are, whether facts arefree from all theoretical or conceptual colouring, whether facts are untreatedreports of direct experience.Is the assumption of Mill and Pearson followingLocke that there is such a thing as pure, naive observation valid? I think thatmost philosophers nowadays would argue that all observation is theory-loaded at some point;16 and that if we have no theoretical background againstwhich to observe, then our observations will be mechanical acts ofsentience, and so meaningless. Kant made this point in his criticisms of empiri-cal philosophy at the end of the eighteenth century. He wrote in theProlegomena. . . although all judgements of experience are empirical, i.e. have theirground in immediate sense-perception, yet all empirical judgments are notR.I. C. Re views 5conversely for that reason judgments of experience, but in addition. . . ,special concepts which have their origin wholly apriori in pure understand-ing must still be added.. . and it is these which make the judgement ofexperience objectively valid.17Let us now look at Mill’s discussion of induction. Mill, like Bacon, discussesfour inductive methods in his System of logic (1843). I shall consider only twoof them, since the criticisms I shall level at these can be levelled similarly at theothers. One method Mill calls ‘the method of agreement’, and it works asfollows. We survey all the examples of the phenomenon P we can, and try tosee what factor (or factors) F they seem to have in common. This factor Fcanthen be taken as the cause or part of the cause of the phenomenon P, asreflected in generalizations of the form ‘For any F, P’ (sufficient condition) or‘For any P, I;’ (necessary condition). (From exercising the method of agree-ment alone we cannot tell which.As we shall see later-p. 58-we can decideonly by attempting to falsify the generalizations.) This seems clear enough,even to conforming with common practice, but upon reflection there areseveral evident difficulties with it. For example, how in fact shall we hit uponthe common factor F, when as far as Mill’s instructions are concerned, weought to approach the matter with open minds. If we are true to the philosophyof the method we ought to pay attention to every possible causal factor F.Suppose we are trying to find out why iron rusts. According to this Millianmethod, we ought to examine every aspect of those samples of rusty ironwhich we have before us-their size, shape, origin, position, purity, history,environment, owner and so on.Clearly this will not do. The only way we canmake the method work is to decide what factors are relevant to outbreaks ofrusting and what are not. We must have some idea what we are looking for, orwe cannot begin to look. We must have some idea about what might be thecauses of rusting before we begin; our search in other words, has to be guidedby some tentative theory of rust. By itself therefore, the method constitutes anineffective means of discovery. (The same sorts of criticism can be made ofMill’s complementary ‘method of difference’.)lgEven if we do succeed in discovering some F which seems to be universallycorrelated with rusting, we cannot rule out the possibility that both P and Fmay be caused by something else.For example suppose we found that water isalways present when iron rusts. We could not say that water was a cause ofrust, for as far as we can tell while using the method, the rust might be thecause of the water, or both the rust and the water might be the effects of somethird and unidentified influence. This no doubt seems absurd to us, but onlybecause we already have some theory of rusting in mind. If we approach thematter in true Millian manner, it is not at all absurd.As I have already said, we might be able to tell whether F is sufficient,necessary or necessary and sufficient condition of P by testing our generaliza-tions, but such testing according to Mill is not necessary if we have followedthe inductive method properly. But, testing apart, a causal analysis becomespossible only when we have some theory or hypothesis with which we can assessthe significance of the correlations we observe, with which we can separatereal causal connection from fortuitous conjunction.Mill we must suppose,Theobald 5like Hume before him, read no more into the notion of cause than that ofinvariable suc~ession.~~Mill calls the second method I consider ‘the method of concomitant varia-tion’. This at first sight seems close to aspects of contemporary scientificpractice, and was indeed close to the aspirations of nineteenth century philo-sophers of science like Mach who wished to limit science to establishing func-tional relationships without further enquiry into their physical significance.The method requires us to find some Fwhich varies in an orderly manner whenthe phenomenon P in which we are interested varies.This can be formulated inquantitative terms, though of course Bacon in the seventeenth century wouldhave found it difficult to exploit this possibility. But what, we may ask, is meantby ‘orderly’ here? What is an orderly variation? Since there is no such thing asabsolute or complete disorder, any set of data will exhibit some order. Thequestion is: is it significant order? Moreover the observation of simultaneousvariations in P and F does not of itself establish any causal connection, forboth variations may be governed by the variation of a third, unidentifiedfactor.Nor can we tell whether F i s a necessary, sufficient or necessary andsufficient condition of P. Only some theoretical preconceptions can help us toanswer these questions. For example we might observe that the more a piece ofiron rusts, the less oxygen there is in the immediate surroundings. Yet wecannot infer from this alone that the oxygen causes the iron to rust, thoughsuch an inference might be justified if we took our generalization in the light ofsome hypothesis about rusting.We must conclude then that although Mill’s methods may discover causesto us, they do not prove their discoveries.Mill’s claim that his inductivemethods constituted methods of discovery and proof at one and the same timeis false. For proof at least, we shall have to turn to the logically more rigoroushypothetico-deduction.The limitation of inductive science then, as I have described it, is that even ifwe do discover true correlations which may be useful predictively, we shall stillbe unable to prop up these predictions with satisfactory explanations. Here oneis led to think of one of Mill’s primary concerns, the social sciences, whereinductive generalization still plays an important part. There are of coursedifficulties in getting even as far as reliable generalizations in the socialsciences for various reasons, generally well known; the fact that human beingsmay be predisposed to behave in a certain way by external conditions, but theyare not usually constrained so to behave; the fact that how they behave willdepend upon their evaluation of those conditions; the fact that control experi-ments in the social sciences are not possible as they are in the physical sciences;the fact that society is arguably not a system in the way that a physical systemis a system; and so on.It may be that these difficulties rule out the formationof sensible hypotheses in the social sciences, thus confining social scientists tomaking inductive generalizations with limited predictive usefulness and littlereal explanatory power. Unless society is a system, it will not be possible toform clear ideas as to what universal forces may be at work within it.It mighteven be argued that the failure to find such forces suggests increasingly thatsociety is not a system. Ironically therefore Mill seems to have developed hisinductive philosophy, which in the event turns out to be inadequate, with56 R.I.C. Reviewperhaps the most intractable subject matter he could find at the back of hismind.Induction by itself then will not do as a source of explanation and under-standing. The laws of chemical composition do not by themselves help us tounderstand chemical changes. Hypothetico-deduction on the other hand,according to which we form relevant hypotheses of cause and structure andthen attempt to reject them seriatim by appealing to their experimentalimplications is in some ways nearer the mark.I say in some ways becausealthough it gives us a clear criterion of validity by its emphasis on falsification,it does not help us to understand the generation of hypotheses. And here wemay find ourselves back with all the informality of induction. I am inclined tothink that in chemical practice for example, scientists work in a variety ofways, which may now be inductive and now hypothetico-deductive. But I shallreturn to this point later (p. 65).TESTING IN SCIENCEI want now to turn to the important question of how we test scientific claims,because this rather nicely illustrates some of the important differencesbetween induction and hypothetico-deduction.20 It hardly needs saying thatsome rigorous testing of any empirical claim is a vital part of scientific method.Now there would seem to be two ways of testing a scientific statement; one isto try to confirm or verify it, the other is to try to falsify it.Both are used,though which depends quite a lot upon the science concerned and more thanthat, upon the context of the testing. Thus physics in contrast perhaps tobotany is a full-time matter of looking for trouble.We ought to remember here that testing, whether confirmation or falsifica-tion, has no proper place in inductive method because this method is supposedby its advocates, incorrectly, to supply its own validation. Nevertheless let ussuppose we are going to test both an hypothesis and a generalization. As aninductive generalization we may consider (a) ‘All damp iron rusts’; and as anhypothesis (b) ‘If iron is damp, then it rusts’.If we think about rust in terms ofthe hypothesis (b), then it is, I maintain, more natural to consider testing byfalsification rather than by confirmation. On the other hand, if we think of rustin terms of the general statement (a), then I would say that confirmation seemsthe more appropriate procedure for testing. For if our claim is a properlyformed inductive generalization, then we should not be expecting to find anyfalsifying cases, but merely confirming examples. And if it is an hypothesis, itstands accepted until shown to be false. ‘All damp iron rusts’ suggests then,that we look for further examples of water with rust. ‘If iron is damp, then itrusts’ suggests rather that we look for cases where damp iron does not rust.I want to take this a little further. The general statement (a) ‘All damp ironrusts’ can be converted into a logical equivalent (al) ‘All rust-free iron is dry’.LogicaZly (al) is equivalent to (a).And so we might think that what confirms(a) ought to confirm (al). But (a) is confirmed by damp, rusty iron, and (al) bydry, rust-free iron. And it must strike us as odd that in order to confirm (a) ‘Alldamp iron rusts’, it seems that all we need to do is observe dry, rust-free iron(which confirms (al) the logical equivalent of (a)). It strikes us as odd becauseTheobald 5such an observation does not seem to bear at all critically upon the originalgeneral statement (a).This peculiar situation shows us two important things.One, the fact that two statements such as (a) and (al) are logically equivalentdoes not mean that they are methodologically equivalent. And two, as long aswe think of testing a statement in terms of corroboration, we shall be depen-dent upon the logical form our statement takes. This can be very uneconomi-cal. If we are interested in the properties of a small class of Xs, then it will beextravagant, other things being equal, to divert our interest to what will thenbe a very large class of non-Xs.Suppose we now try to falsify (a) and (al). In the case of (a) we shall need toobserve damp, rust-free iron, and in the case of (al) rust-free, damp iron. Inother words, to falsify either (a) or (al) requires the same observation.Thefalsification of a generalization then does not depend upon the logical form ofthe generalization. Moreover the observation which falsifies (a) and (al) alsofalsifies (b). It seems then that as far as the methodology of testing is con-cerned we should concentrate upon the falsification of hypotheses and generalstatements rather than upon their confirmation. Now we had occasion to noteabove that falsification is more appropriate to (b), while confirmation is moreappropriate to (a) and (al). It seems then that when we are concerned withchecking the validity of general statements of inductive origin they are betterconstrued as hypotheticals open to falsification. So although we may acquiregeneral knowledge by some inductive generalizing process, Millian orBaconian, its validity is best established by attempting to falsify related hypo-theses. As far as testing is concerned, we must desert induction for hypothetico-deduction.It might be worth digressing here for a moment to point out that somephilosophers of science have argued that general statements and hypothesesare not real statements at all, but merely licences for forming such.Forexample, ‘All damp iron rusts’ or ‘If iron is damp, then it rusts’ licenses us toassert ‘This sample of iron is damp and so will rust’. The claim in effect is thatthe meaning of hypotheses and general statements is properly to be found inthe meaning of the singular observation statements whose truth or falsitywould confirm or falsify them.This is the philosophy of logical positivism,whose faults I cannot go into here,21 except to say that the identification of themeaning of a statement with the procedure for testing it has never been persua-sively established.In particular, for the positivist the meaning of an hypothesis which mentionsan entity such as an atom which is not directly observable, is to be found in themeaning of the observation statements which confirm or falsify it. The scene isthen set for the positivist to say that such a physical object is nothing more thana collection of observable properties. It is the sort of thing it is because it hasthe particular properties it has. But it could equally well be argued that anobject has the properties it does have because it is the sort of thing it is.Nowthis is an important move, because it leaves the nature of the object open to beinvestigated. If an object is nothing but a collection of observable properties,if it has no nature, indeed if there is really no such thing as ‘it’, what reason canwe have to investigate it beyond what we have already observed? Indeed whatreason can we have even to start investigation? And besides, if an object has58 R.I.C. Reviewno nature apart from its properties, what sort of properties are we to considerlooking for? Any? If so we are back with the problems of inductive methoddiscussed earlier, of not knowing how to start or when to stop. Positivism isnot a philosophy for research.In a later development of positivism, instrumentalism, the argument waschanged somewhat.A theory, it was contended, was simply a rule for inferringone observation statement from another. A theory asserted nothing whatso-ever about what the world was made of. But the same counter move can bemade here-it is because a theory asserts something in fact about the worldthat it is a successful rule for making inferences.Let us now return to testing. There is a simple reason why falsification is inlogic the proper way of bearing critically upon an hypothesis. I have so farmerely discussed some methodological reasons why this is so. Suppose ascientific argument is generally of the form ‘Given Tand A , then B’, where Tisa theory from which given A , B may be deduced.Now if B is true, the con-junction Tand A can be true or false. For any true proposition can be impliedby either a true proposition or a false proposition. On the other hand, if B isfalse, then Tand A can be only false. For no true proposition can imply a falseproposition. So if B is a deductive consequence of T and A , and if B is observedto be false, then T and A can be inferred to be false also. And if A areobserved initial conditions, the theory T must be false. We may concludetherefore that falsification is the proper way to test an hypothesis. ‘The aim ofscience is not to open a door to infinite wisdom, but to set a limit to infiniteerror’ as Brecht has Galileo say.22There is a further point worth making in connection with testing. In ordinarylife we are usually content to treat sufficient conditions as causes, whereas inscience we usually reserve this description for necessary and sufficient condi-tions. Now the confirmation of (a) tells us nothing about whether dampness isa necessary, sufficient or necessary and sufficient condition of rust.But thefalsification of (a) or (b) tells us at least that dampness is not a sufficient condi-tion of rust. If then we were to falsify (c) ‘All rusty iron is damp’, or ( d ) ‘If ironis rusty, it is damp’, this would tell us at least that rust was not a sufficientcondition of dampness and therefore that dampness was not a necessarycondition of rust. If attempts to falsify (a), (b), (c) and ( d ) fail (which we knowin actual fact they would not), then dampness can be taken to be a necessaryand sufficient condition of rust, at least until further evidence is forthcoming.PHILOSOPHICAL PROBLEMS FOR HYPOTHETICO-DEDUCTIONI have tried to show how, although knowledge may be obtained fortuitously byan inductive method, the critical evaluation of that knowledge proceeds bestby trying to falsify relevant hypotheses.I mentioned at the beginning of thisessay that induction remains content with the world at its face value, whereasthe invention and testing of hypotheses is not limited in this way. The contentof an hypothesis is not limited by our ordinary experience of the world. It is infact difficult to see how many of the most important principles and theories ofphysical science could have arisen by any generalizing operation alone.Howfor example could atomic theory have arisen in this way? And how can therole of conservation principles be interpreted inductively ?Theobald 5Since the content of an hypothesis is not limited by our everyday experi-ence, we soon have to face the question : are atoms, ions, electrons and so on,which are features of many physical hypotheses, simply fictions or are they realexistents? Are they merely grammatical ciphers in a scientific language, or dothey actually designate bits of the real physical world? Are atoms and ionsreal in the sense that we take the furniture of our laboratory to be real? Per-haps I should point out that in a logical sense it does not make any differenceto the properties of a thing whether it is a real existent or not.I mean by thatthat logically speaking, existence adds nothing to anything. If I describe a pieceof chemical apparatus in detail, and then add ‘But such a piece of apparatusdoes not exist’, all I do is to lose the attention of my audience. I do not changethe properties of the apparatus. So whether atoms or ions are real existents ornot makes no difference to the predictive power of any atomic hypothesis,though it must make a difference to its persuasive power of explanation, itspower to help us understand.Hypothetico-deduction then raises certain philosophical problems notraised by induction. It is often said that we never observe atoms in that sense of‘observe’ appropriate to ordinary common-sense observation.But is this so ?I am inclined to think not. I think that observing an atom or an ion or anelectron involves a perfectly proper sense of ‘observe’, a sense which I hope toexplain. The atomic theory is designed to explain all the visible aspects ofmacroscopic change, and therefore atoms themselves cannot be invested withordinary, visible properties. But this does not mean that they cannot beobserved.Imagine looking into the clear night sky and seeing a comet. You say toyourself ‘There’s a comet’ or perhaps ‘I can see a comet’. Rather more im-probably you might say ‘I can see the light-track of a comet’, even though in asense that is what you do see. You can quite properly say these things becauseyou undoubtedly know a little astronomical science and probably also some-thing about the appearance of moving luminous bodies, and because you areconfident that astronomers have some sort of astronomical tests up theirsleeves to confirm or falsify such claims.The argument ‘I saw an A ; A is B ; soI saw a B’ is generally valid only if you know that A is B. That is, if you saw A ,you are entitled to say you saw B, only if you know that A was B. From the factthat you saw a man, you are entitled to say that you saw a laughing man, onlyif you know the man was laughing. Of course you need not have observed hislaughing, someone might have told you. You can be entitled to say you saw alaughing man without being entitled to say that you saw a man laughing. Totake another example, if I saw a piece of apparatus, I am entitled to say I sawan ir spectrometer only if I know it is an ir spectrometer.But I need not haveseen that it was an ir spectrometer; someone, an expert, may have told me thatthat was what I was seeing. To return to comets: if we see a light-track in thesky, we are entitled to say we see the light-track of a comet because we knowfrom astronomers that such is in fact the light-track of a comet. And seeing thelight-track of a comet is seeing a comet making a light-track. It is in fact seeinga comet. What else could possibly count as seeing it? And no one on thisaccount would dream of saying that comets are not real.The same sort of considerations apply to seeing a-particles in cloud-60 R.Z. C. Reviewchambers or molecules in electron micro~copes.2~ One reason why thesehypothetical entities are bona fide candidates for the description ‘real’ is thatlike the comet, there is a particular conceptual background which makes certainclaims to see them legitimate and significant.There is an accepted reservoir ofknowledge which makes the second premise in the argument I outlined abovetrue. To take a chemical example, to see a chemical change occurring is to seean electronic rearrangement going on, once we know all about chemical theory.The important point is that what we say we can see depends upon our know-ledge. As does therefore what we can say is real.Against this some philosophers have argued that to say that we see anelectronic rearrangement going on, if taken literally, is to say something mis-leading if not false.It is argued that whereas seeing a chemical change going onin a flask is clear enough, to say that this is to see electrons rearrange is to hidethe fact that an inference has been made in accordance with some physicaltheory, an inference from what goes on in the flask to the fate of the electrons.This has been the view of many instrumentalist and positivist philosophers ofscience.24 But given the setting of atomic theory, seeing a chemical change inprogress just is seeing the electrons rearranging, and moreover that is all thereis to seeing the electrons rearranging. The confusion arises from failing tonotice the difference between seeing inside a theory and seeing outside it.Bothare legitimate seeings, and neither involves making inferences of any sort.Seeing is not just opening one’s eyes-seeing is an achievement. Note howfrequently ‘see’ is used with ‘can’ and how rarely if at all in the present tense,‘I see. . .’ or ‘I am seeing. . .’. For to see is to invest what you notice withsome significance. And this is what theories and hypotheses are for. Allobservation is dyed with theory to a greater or lesser extent, and it was acardinal and curiously romantic error of Mill, Pearson and others to supposethat there was such a thing as simple, naive observation, and therefore thatthere was such a thing as a fact which was not charged with some theoreticalpreconceptions.HYPOTHESESI have argued that although it may suggest correlations between observableproperties, induction cannot establish causal connection.And so it cannotsupply profound understanding. We need to devise hypotheses and test themcritically in order to explain and understand. The actual form any hypothesistakes and the particular suggestions it makes will depend a great deal uponour present knowledge. For any hypothesis which is to extend our presentknowledge will begin by suggesting analogies between new physical systemsand systems with whose behaviour we are already familiar. Rarely thereforewill an hypothesis depart completely from previous ideas. This might be takento imply that the refutation of an hypothesis will call the whole of the scienceconcerned into question. But this is not necessarily so. As has been said,falsification is the sign only that something has gone wrong, not that every-thing has gone wrong.On the occasions when an hypothesis does departradically from previous ideas the result is a conceptual revolution in science,examples of which will be very familiar. To the chemist the reorientation ofchemical thought by Lavoisier in the eighteenth century, by Dalton in theTheobald 6nineteenth, and by Bohr and later quantum theorists in the twentieth centuryare perhaps the most important.Sometimes, metaphysical conviction can lead to the retention of hypotheseseven when the evidence is against them, even when they have been technicallyfalsified. Witness the persistence of alchemical ideas despite their manyfailures,25 and the reluctance of chemists, for example Priestley, to abandonphlogiston theory even after the work of Cavendish and Lavoisier.As anotherexample we may recall that MendelCef persisted with his classification of theelements despite the fact that at the time his periodic arrangement failed inseveral places.Sometimes metaphysical conviction makes us continue with an hypothesiswhen there is no clear evidence for or against it. Dalton’s atomic hypothesiswas maintained in the face of a great deal of initial quantitative uncertainty,and an inability to see precisely how to put it to conclusive test. In fact therewas a gradual decline of interest in atomic theory during the middle of thenineteenth century until the work of Avogadro and Gay-Lussac was re-presented by Cannizzaro, and shown to be compatible with Dalton’s hypo-thesis after some modifications.These few historical points show that dis-covery, justification, judgement, imagination and metaphysical convictionbecome inextricably mixed during the evolution of scientific ideas. They alsoshow that hypothetico-deduction is not a scientific method which commandsthe scientist’s unswerving allegiance. As I pointed out in the introduction tothis essay, science like any other human activity cannot always be conductedsuccessfully by rule of logic alone.But the most interesting question to do with hypotheses has yet to be asked.How do we form hypotheses ? How does our guided imagination work ?4There are very difficult questions even to begin to answer.We can certainlymake a survey of the sorts of person who have had the necessary imaginativeframe of mind to devise hypotheses, and we can certainly make a survey of thephysical and intellectual conditions which predispose (but not determine) aman to be imaginative. But I doubt whether any statistical findings of this sortabout imaginative thinking will leave us any nearer either predicting orengineering its occurrence. And such a survey would in any case do little toclarify what imagination is or how it works in the business of discovery. And itis to aspects of the philosophy of the imagination I wish to turn now.All our attempts to come to terms with the experienced world depend uponour fitting this experience into an orderly, though flexible matrix of generalconcepts whose expression is usually verbal.As I have already remarked,scientific knowledge which is linguistic and propositional, requires more thana keen eye for phenomenal incident. To know is to create order and structure inthe world by means of language. Knowledge is not something absolute, just‘out there’ for quick capture, but is, as Coleridge realized, something to becreated and recreated because language is subject to constant, creative renewalby philosophers, poets and scientists. ‘The relation of language to thought andreality is not a passive ref? ection, but an active and tendentious reaction’ wrotethe literary critic and poet Christopher Caudwell.26 This I think is not such abad definition of scientific practice.We can see this more surely if we reflect upon how we come by the concepts62 R.I.C.Reviewwe have. Let us take the concept ‘chemist’ as an example. A common view ofthe origin of our concept ‘chemist’ would be this-all we do is to make asurvey of all chemical persons we know, and draw up an inventory of whatthey have in common. As a result we have a concept ‘chemist’ the meaning ofwhich is decided, that is defined, by a collection of common properties. Butclearly this will not do, attractively simple as it seems. And it will not do for thesame reasons that Mill’s method of agreement will not do. For until we have atleast some concept ‘chemist’ in our minds, we can have no idea who arechemical persons, and therefore no idea what common characteristics we areto look for. This is an important point.Suppose we take the concept ‘red’. It isoften argued that this concept is acquired by noticing and naming what iscommon to a set of red objects. But this can be done only if we are quite clearthat we are dealing with culuur as opposed to say shape, size or weight. Wemust be clear what sort of thing we are attending to before we start. Conceptsare not acquired independently, but in relation to other concepts. Another wayof putting this would be to say that language is a system of concepts, not acollection. We cannot and do not acquire concepts by diligent stock-taking.The concept ‘chemist’ like all other concepts is first and foremost a mentalinvention, but one whose meaning will change and develop as our experienceof the class of objects designated by the concept increases.It is not easy to sayprecisely why we invent concepts, nor how they and our experience developside by side. We may feel (inductively) that there is some uniformity in ourexperience which needs charting, or we may wish to discover whether there is.These are matters clearly for imaginative judgement, for hypothesis and testing,not simply for an inductive logic.What has been said here of the concepts ‘chemist’ and ‘red’ is, I think, trueof all concepts. They are first and foremost mental inventions and they designateopen classes. Whether or not some particular item of experience falls under agiven concept is a matter both for our perception and our judgement, but notfor our perception alone.It is a matter of weighing the evidence for or againstinclusion. This looseness in the application of language to the world is vitallyimportant, for it allows language to hover over our experience of the world,and settle upon it not only as an instrument of definition, but as an instrumentof promise, expectation and revaluation. Concepts are likeAxesAfter whose stroke the wood rings,And the echoes!Echoes travellingOff from the centre like horses.27In scientific contexts it is this looseness which allows us to overlook disagree-ment between hypothesis and observation without being guilty of logicalinconsistency.28 For, as we have seen, theories and hypotheses are not alwaysrefuted by such disagreements.Concepts then do not mirror reality; rather in an important way they createthe reality we talk about.Experience does not give our concepts meaning; wegive them meaning, and by so doing we endow experience with meaning. ThisTheobald 6is an extremely important point which explains why I head this essay with somewords from Eliot’s Dry Salvages, and which is at the heart of the conflictbetween induction and hypothetico-deduction. ‘Concepts lead us to make in-vestigations ; are the expression of our interest, and direct our interest’ wroteWittgenstein.29 This can be said equally of any hypothesis in science. Myremarks about concepts in the preceding paragraphs, although they need moreelaboration and defence than I can give them here, can be directly translatedinto the terms of the induction/hypothetico-deduction controversy.Forhypotheses are not elicited from experience, they are invented to make sense ofexperience and, like concepts-themselves in some ways hypotheses of persis-tence, identity, resemblance and uniformity-they are modified in the light of it.The object of ordinary language, like its scientific development, is to tiedown experience into labelled bundles, to reduce the apparent variety inexperience by collecting it into generalities. The scientific imagination tries tosimplify, though not, as I have tried to show, exclusively by induction. Thepoetic imagination as I understand it, tries to complicate. ‘What we havegotten by this revolution is a great deal of good sense.What we have lost is aworld of fine fabling’ wrote Richard Hurd, Bishop of Worcester, of the scien-tific enlightenment in the eighteenth century. The poet, the Blake who seesscientists as generalizing idiots, thinks of simplification as restricting the pos-sibilities for seeing the world, as numbing our sensitivity to small but impor-tant differences in things. ‘We have art in order not to die of the truth’ wroteNietz~che.~~ Scientific imagination is more controlled and has less room formanoeuvre than the artistic imagination, and yet in the long run it may carrymore universal conviction. For the fact that the scientific imagination workswithin the confines of previous knowledge and opinion guards scientifichypotheses against some of the subjective extravagancies which Bacon, Milland others feared.New ideas in science are either rejected or become acceptedas textbook truths, but new ideas from the artistic imagination are always freshto each generation. They are not assimilable to a body of knowledge. This isbecause in science the meaning of a term is designed to be independent of thecontext in which it is used and so becomes the expression of a timeless gener-ality in nature whereas in poetry, say, the meaning depends very much uponthat context and requires revaluation when that context changes.I have already mentioned the importance of analogy in science. The scien-tist’s imagination works by seeing in the apparently new much that is like thealready familiar.The artist works by seeing in the apparently familiar muchthat is actually new. The effectiveness of all analogy as Coleridge pointed out31is a complex and subtle matter of ‘the balance or reconciliation of opposite ordiscordant qualities; of sameness with difference; of the general with theconcrete, the individual with the representative; the sense of novelty andfreshness, with old and familiar objects; a more than usual state of order . . .’.Once we have accepted an analogy, once it convinces us, we are necessarilyconverted to a reconstructed reality. ‘The expression of a change of aspect is theexpression of a new perception and zt the same time of the perception’s being~ n c h a n g e d . ’ ~ ~ We cannot look at gases in the same way once we havesup-posed that they consist of bits of matter in Newtonian motion.We cannot lookat human beings and chairs in the same way once we accept that they both64 R.I.C. Reviewhave legs, nor at clouds in the same way once we have seen ‘a cloud that’sdragonish. . .’.33In science this sort of imaginative move is part and parcel of forming andtesting hypotheses. We start by seeing A as B and we then act on this sugges-tion to find out whether in fact A is B, whether the analogy stands up toexamination. Imagining is always preliminary to knowing. Imagining a gas asa collection of Newtonian particles is preliminary to knowing that a gas is infact more or less just that; and also to seeing why it is onZy more or less justthat.We can see then that hypothetico-deduction is an extension of aspects of thephilosophy of concept formation into the methodology of science.I have triedto show that induction is by itself methodologically inadequate, and in somerespects philosophically naive. This does not mean to say however that a Mill-ian approach to a problem is always inappropriate, as some critics seem tosuppose. I return to a point I made earlier which was that it is not possible tolegislate about how scientists should go about the business of discovery. It ispossible to legislate only about how they should check their discoveries, howthey should check their claims to know And here the verdict is clearly infavour of hypothetico-deduction.Hypothetico-deduction will evidently find favour with logicians andmathematically-minded physical scientists on account of its logical rigour,but it is not so clear that it will always find favour with the more experimentalscientist for whom the chance observation, the remarkable coincidence is ofgreat moment.One is sometimes tempted to think that those scientists whoargue often overbearingly for hypothetico-deduction have themselves lostcontact with the uncertainties and chances of day to day work in thelaboratory.CHEMISTRY AND SCIENTIFIC METHODI want now to say a few words about the position of the chemist in all this.Many important discoveries have been made in chemistry (as indeed they havein the other physical sciences) from the odd observation, the peculiar coinci-dence, so that although chemists may need the logic of hypothetico-deductionto put their ideas to the test, they often rely upon the informal consequences ofinduction to generate their ideas in the first place.The chemist is traditionallya scientist who often simply wants to know what occurs when A is mixed with B,besides wanting to produce a theoretical explanation for it. Although admittedlywe may wish to explain the reaction once we have mixed A with B, it isinductive curiosity which leads us to the mixing in the first place. In chemistrymany more things have been discovered than have been explained, and there isno sign that discovery is dependent for its impetus upon theory and explana-tion as it undoubtedly is in physics. Chemistry is still in many respects aninformal science. And as I have already remarked, important discoveriescome from such informality. I think that chemical curiosity is much less dis-ciplined than physical curiosity, and it is probably this element of ‘idlecuriosity’ which makes chemistry so successful in its relations with technology.For it is the chance rather than the systematic discovery which is ofteninteresting to the technologist.Theobald 65It is a notorious fact that in theoretical physics one experimental result isoften all that is necessary, indeed in practice is often all that is available toevaluate an hypothesis and if need be reject it. On the other hand in chemistrythere is more often than not a vast array of experimental results whichcan be brought to bear upon an hypothesis. Usually no single result bearsupon a chemical hypothesis in the way a single result can bear upon anhypothesis in physics. This is reflected in the fact that theories and models inchemistry are generally much looser formulations than those in physics. Thechemist is concerned with modifying details of a well-authenticated molecularmodel, not with rejecting it. On the other hand the physicist at his level ofphysical enquiry is concerned much more with making a conclusive choicebetween rival models. Accordingly there is not such a tight relationshipbetween hypothesis and observation in chemistry as there seems to be inphysics. These things are all a reflection of the fact that chemistry is still insome measure an inductive science.Chemistry then stands between physics on the one hand and the fieldbiological sciences on the other in being an area of inquiry peculiarly appro-priate for the exercise of both inductive and hypothetico-deductive method.However chemists will continue to enjoy this stimulating position only if theyresist the temptation to suppose that they must model their science either onphysics or biology. This, as I have argued elsewhere,34 is seriously to suppressthe imagination besides, as I believe, being logically and methodologicallyindefensible. There is more I believe in heaven and earth than is comprehendedin the philosophy of physics. But fortunately there is no reason to suppose thatchemistry must succumb to relations with either of its suitors. Those whowould have it do so will I think find relations become very strained, andI suggest they would do well to remember Tristram’s remarks about his father:he was systematical, and, like all systematic reasoners he would moveheaven and earth and twist and torture everything in nature to support hishypothesis. In a word he was seriousand be content to remain chemists.ACKNOWLEDGMENTThe excerpt from T. S . Eliot’s The Dry Salvages is reproduced by permissionof Faber and Faber.REFERENCES1 Novum organum (1 620) ; The advancement of learning (1 605).2 System of logic (1843).3 Logic of scientific discovery (1935, 1959).4 Patterns of discovery (1958); The concept of the positron (1963).5 Induction and intuition in scientific thought (1969).6 Thegreat instauration (1607), Plan of (trans. by J. Spedding, R. L. Ellis and D. D. Heath).7 System of logic, book 3, chapter 1.8 Treatise of human nature (1738).9 Enquiry concerning human understanding (1 777).10 See my Introduction to the philosophy of science (1968) for further discussion and11 Novum organum, Aphorisms, book 2, 15 (trans. by J. Spedding, R. L. Ellis and D. D.12 Ibidem, book 1, 51.66 R . I. C. Reviewsreferences.Heath) 13 Advancement of learning, book 2.14 System of logic, book 6.15 C. Dickens, Hard times, book 1, chapter 1.16 See, for example, Hanson, op. cit.17 Prolcgomena to any future metaphysics (1783), para. 18 (trans. by P. G. Lucas).18 These methods are discussed more fully in most books on scientific method, e.g. E. Nagel,19 System of logic, book 3, chapter 5.20 See my Introduction to the philosophy of science, chapter 4.21 See J. 0. Urmson, Philosophical analysis (1956), and G. J. Warnock, English philosophy22 Scene 9.23 See my ‘Observation and reality’, Mind, 1967, 76, 198.24 See E. Nagel, The structure of science (1961).25 See my ‘Alchemy; a philosophical reappraisal’, Technology and Society, 1965, 2, 135.26 Illusion and reality (1 937).27 Sylvia Plath, ‘Words’ in Ariel(l948).28 T. S. Kuhn, The structure of scientific revolutions (1962).29 Philosophical investigations (1 958), part 1, 570.30 Goethe is an excellent example of a man torn in the two directions. See E. Heller, The31 Biographia literaria 1817, 2, 12.32 Philosophical investigations, part 2, xi.33 Antony and Cleopatra, act 4, sc. 12.34 See my Introduction to the philosophy of science, chapter 6.An introduction to logic and scientific method (1934).since 1900 (1958, 1969) for useful discussions.disinherited mind (1952).Theobald 6

ISSN:0035-8940    

DOI:10.1039/RR9710400049

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

DIELECTRIC RELAXATION AND MOLECULAR STRUCTURE IN LIQUIDS Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada John Crossley, B.Sc., Ph.D., A.R.I.C. Introduction and basic theory . . . . .. . . . . . . Experimental methods . . .. .. . . . . . . . . Heterodyne beat method, 74 Very low frequencies, 75 Audio and radio frequency bridges, 75 Resonant circuit, 75 Transmission-line methods, 75 Microwave bridges, 75 Free-space methods, 76 Very high frequency methods, 76 Non-rigid polar molecules Aliphatic compounds, 82 Aromatic compounds, 83 Rigid polar molecules . . . . . . . . . . . . . . . . . . . . . . .. . . Non-polar liquids. . . . .. . . .. . . . . . . Polymer solutions. . . . . . . . .. . . . . . . Associated liquids.. . . ,. . . . . . . . . . . Water, 84 Alcohols, 85 Intramolecular hydrogen bonds, 86 Solute-solvent interactions, 88 Charge- transfer interactions, 8 8 Interaction between polar molecules, 90 Mixtures of polar compounds . . . . . . . . . . . . Summary . . . . . . . . .. . . . . . . . . References . . . . . . . . .. . . . . . . .. For a dipolar compound the so-called dielectric constant is by no means a constant but varies with the frequency of an applied electromagnetic field. This frequency dependence arises from the inability of molecular dipole orientation to keep pace with the changes in direction of the applied field at high fre- quencies, and as a result the permittivity (dielectric constant) decreases with increasing frequency in the region of anomalous dispersion.The time lag between the response of a dipole to the behaviour of the applied field is the phenomenon of dielectric relaxation. Experimental investigations of this effect provide appreciable information concerning molecular and intra- molecular motions. The purpose of this article is to present an introduction to the principles of dielectric absorption and a review of some selected applica- tions in the liquid state. Crossley 70 74 76 81 84 90 92 93 95 95 69 INTRODUCTION AND BASIC THEORY Consider the application of a static electric field to a liquid composed of ran- domly orientated electric dipoles, between the plates of a condenser. The permit- tivity of the liquid (€0) is given by the ratio of the capacity of this condenser to the capacity of the same condenser with a vacuum between the plates.It is then a simple matter to calculate the total polarizability (a) of the molecules using the Clausius-Mosotti relationship €0 + 2' - d - ~ 3 €0 - 1 M - 47rNa 01 = O ~ D + p2/3kT 1 in which A4 is the molecular weight, d the density and N the number of mole- cules per mole. The polarizability is also given by: 2 here CUD is the distortion polarizability which may be estimated from refractive index measurements. The last term on the right hand side of equation 2 is the orientation polarizability, and p is the electric dipole moment, k the Boltz- mann constant and T the absolute temperature. Combination of equations 1 and 2 gives the Debye equation which has been the basis for numerous dipole moment determination^;^^^,^ it is only strictly applicable to gases or very dilute solutions of a polar solute in a non-polar solvent.Onsager's re-examination of the Clausius-Mosotti-Debye treatments led to a relationship more applicable to polar liquids4 3 In this equation €0 and E , are the permittivities at static or low frequencies and very high frequencies respectively. For a polar liquid €0 may be consider- ably greater than E , . This is due to the inability of the dipoles to follow the field at high frequencies and thus there is no contribution from dipole orienta- tion to the capacitance and permittivity. For a dipolar substance there is a frequency range in between EO and E , , where the permittivity is not constant but decreases as the frequency of the applied electric field is increased, due to a time lag between the response of the dipoles to the voltage oscillations.This lag is a relaxation; the term may be applied to any system in which there is a time delay in the response of a system to changes in the forces which are applied to it. Dielectric relaxation is the exponential decay with time of the polarization in a dielectric when an exter- nally applied field is removed. At the molecular level we can picture a liquid composed of dipolar mole- cules, such as chlorobenzene, between the plates of a condenser, to which a low frequency electric field has been applied. In such a system there is a ten- dency for the dipoles to align themselves with respect to the field.Thus, the molecular dipoles may rotate in phase with the voltage oscillations of the applied field. If the frequency of the applied field is increased then the dipolar molecules must rotate faster in order to keep pace with the field. Eventually the R. I. C. Reviews 70 Fig. I. The frequency dependence of the real (E’) and imaginary (E”) parts of the permittivity in a relaxation region. rate of dipole orientation lags behind the frequency of the applied field, and ultimately molecular rotation can no longer maintain pace with the applied field. The existence of dielectric relaxation becomes apparent when its rate is close to the same order of magnitude as that of the frequency of the applied field.The dielectric relaxation time may be defined as the time after the removal of the applied field in which the polarization in a dielectric is reduced to l/e times its original value. Figure I shows the frequency dependence of the permittivity for an anoma- lous dispersion or relaxation region, in which the permittivity ( E * ) is a mathe- matically complex quantity. E* = E’ - iE” r=--- urnax 1 The Debye-Pellat equations (see ref. 2, p. 55 for derivation) which provide the basis for dielectric absorption studies give the frequency dependence of E’ 4 The real permittivity is E’ and E” is the imaginary part or loss factor. The latter is a measure of the conductance of the medium and its ability to dissipate energy.As the frequency of the applied field approaches zero, E” approaches zero, and E’ approaches the static permittivity €0; and, as the frequency ap- proaches infinity, E” again approaches zero, and E’ approaches em, the very high frequency or optical permittivity. The frequency at which E” is a maxi- mum (@,a,) gives the relaxation time (7) of the dipole orientation process in seconds. 5 71 Crossley and E” for a single relaxation process T , 6 separation of real and imaginary parts gives : and From equation 8 it is evident that E” is a maximum for OT = 1 and I t (€0 - Em) E m a x = 2 Thus from measurements of E’ and E” at frequencies in the absorption region it is possible to evaluate the relaxation time.Elimination of OT between equations 7 and 8 gives the equation of a circle 9 10 A plot of E’ against E” in the complex plane gives a semicircle (E” may only have positive values) the so-called Cole-Cole plot (Fig. 2a). So far the discussion has been limited to the Debye-Pellat equations and the case of a single discrete relaxation time. For many systems this may be satis- factory but for many more the theory must be modified to account for ( i ) a distribution of relaxation times for one relaxation process, or (ii) more than one discrete relaxation process. Cole and Cole5 assumed a continuous distribution of relaxation times about a most probable value TO and their general dispersion equation for the com- plex dielectric constant is 1 1 here 01 is the distribution parameter, an empirical constant which measures the width of the distribution (Fig.2b) and may have values between 0 and 1. When a = 0 the Debye equations are obtained. There are other distribution func- tions6 but the Cole-Cole distribution is the most widely employed. For systems which have contributions from n independent relaxation pro- cesses each of which shows Debye behaviour, Bud6’ has shown that the di- electric absorption can be obtained as the sum of Debye terms 12 T k is the relaxation time characteristic of the kth mode of relaxation and Ck is a factor representing the proportion by which the kth mode contributes to the R. I. C. Reviews 72 total dispersion : n k= 1 For two relaxation processes (71 and 72) we have: ( € 0 - Em) (€0 - E" Em) - - 1 + CW71 (w71)2 + 1 + c2w72 (w72)2 (E' - Em> - - c1+ c2 = 1 1 + ( c ~ W 2 ) ~ of a semicircle.Cl 1+(w.1Y2 16 The form of the complex plane plot for such systems will depend on the relative magnitudes of 7 1 , ~ ~ and C1 but may often be approximated by a sector a b Fig. 2. Cole-Cole plots for systems showing (a) a single Debye relaxation, (b) a distribution of relaxation times and (c) a separation into two relaxa- tion regions. Crossley 13 c k = 1 14 + _ _ ~ c2 15 73 Dipole reorientation may be considered as a rate process,s r being the reciprocal of a rate constant kl which follows an Arrhenius-type equation with temperature changes 1 kT r h and thus 1 AH? kl = - = - exp (- hGt/RT) l n T = - ~ - h T + EXPERIMENTAL METHODS T R where AGt, A H i , and AS? are the free energy, enthalpy and entropy of activation respectively.AH+ is the energy molecules require in order to rotate and may be readily obtained from a plot of In TT against 1/T. A common procedure when considering experimental E’ and E” data is to initially examine them as a Cole-Cole plot. The procedure then is to analyse the data, starting from initial estimates deduced from the complex plane plot, by a computer fit to the Cole-Cole equations, for TO, a and E , . The latter, un- like €0, is not an experimentally measured quantity. If a is non-zero and especially if the Cole-Cole plot indicates a separation (Fig.2c) it is usual to attempt a further computer analysis based on equations 14-16 for physically significant values of 71, 7 2 and C1. Equation 3 may be used to calculate dipole moments. It is possible to perform such analyses by graphical methods,g but these are often tedious and time-consuming, especially for systems with non- zero distribution parameters. The inadequacy of the Debye theory in relating the permittivity and dipole moment for polar liquids arises from the use of an expression for the internal field which is only strictly applicable to gases. Because of the internal field the relaxation times calculated as described above are macroscopic quantities, longer than those for the individual polar molecules. This effect will be discussed further when dealing with the relaxation of rigid polar molecules.To a large extent the nature of the apparatus depends on the frequency range and magnitude of the dielectric absorption to be measured. In view of the temperature dependence of r (equation 17), however, it is possible to bring an absorption into a desired frequency range by suitable choice of temperature. Many of the experimental methods involve measuring the capacitance of an empty condenser CO and the capacitance C and resistance R of the condenser filled with the dielectric under investigation. Essentially the real and imaginary parts of the permittivity result from Heterodyne beat methodl ~2 This type of apparatus is frequently used to measure static dielectric constants at 1 MHz. It is based upon the frequency control of an oscillator by adjusting the capacitance of its circuits which contain a dielectric cell.A precision 74 17 18 R . I . C. Reviews variable condenser is used to tune the signal of the variable oscillator to give a null when mixed with the signal from a fixed frequency oscillator. The null may be detected readily using a cathode ray oscilloscope. Introduction of a di- electric into the cell increases the capacitance in the tuning circuit; the balance is reachieved by decreasing the capacitance of the precision condenser. Calibra- tion of the instrument is achieved by using liquids of known permittivity. Very low frequencies Scheiberlo and Harris11 have described capacitance-resistance bridges which allow measurement of E’ and E” in the frequency range 0.008 Hz to 200 Hz.Audio and radio frequency bridges Capacitance-resistance bridges such as the General Radio Co. type 161 5-A (100 Hz-10 kHz), Schering bridge (20 Hz-1 MHz), WTW multidekameter (100 kHz-12 MHz) and Hewlett Packard 250 A RX meter (500 kHz-250 MHz) are based upon a Wheatstone bridge principle. A wide range of permittivity may be covered by suitable choice of dielectric cell capacitance. Vaughanl2 has given a description of dielectric cells and their calibration. Resonant circuit The Hartshorn and Ward apparatus13 has been used for liquids over the frequency range 50 kHz-100 MHz. A circuit containing capacitance and inductance is loosely coupled to a primary circuit in which a high frequency alternating current has been set up.The current flowing in the secondary circuit may be controlled by changes of capacitance and inductance. The real permittivity is obtained from the difference in the capacitance readings of a precision condenser required to give maximum current with and without the dielectric cell, which is in parallel with this precision condenser. The width of the resonance curves for the empty and filled cell and the known capacitance of the system give the dielectric loss. Transmission-line methods29 12¶ l4 This type of apparatus employs either co-axial lines (100 MHz-5 GHz) or waveguides (3 GHz-50 GHz). Both methods involve measuring the charac- teristics of a voltage standing wave produced in a liquid compared with that for the air filled system.The experimental arrangement and method of calcula- tion depend to a large extent on the loss of the dielectric. Microwave bridgesl5 This type of apparatus has been successfully used for dielectric absorption measurements of medium and low loss liquids and solutions in the frequency range 6-70 GHz. The liquid is contained between mica windows in one arm of the waveguide bridge; a variable attenuator and a phase shifter in the other arm are used to balance the bridge for increasing lengths of liquid. The attenuation and phase shift introduced by known lengths of liquid are used to calculate attenuation and phase constants and E’ and E”. CrossZey 75 20 21 22 23 Free-space methods At high frequencies the dimensions of waveguide apparatus are such that difficulties arise in the construction of transmission lines and microwave bridges.A typical free-space interferometric method propagates the 2 mm harmonic from a 4 mm Klystron.lG Very high frequency methods Dielectric measurements in the frequency region 5-500 cm-l have recently been made possible by the development of apparatus at the NPL. The methods have been fully described17 and a Grubb-Parsons instrument is now com- mercially available. RIGID POLAR MOLECULES Several relationships have been developed to deal with the effect of the internal field on dielectric relaxation, and to relate the macroscopic relaxation time ( T M ) given by equations 6-15 and the microscopic relaxation time ( T ~ ) of the individual polar molecules.The first treatment was attempted by Debye and gave : Powlesl8 considered the internal field required to give an exponential decay of the macroscopic polarization and obtained A more exhaustive treatment by O’Dwyer and Sack19 led to The effect of the internal field in a liquid, and a test of equations 20-22, was examined experimentally by comparing the macroscopic relaxation time of a highly polar liquid with that of a liquid composed of molecules of the same shape and size but with only a small dipole moment.20 For the latter types of liquid it is assumed that T M and T~ are indistinguishable. Alternatively, the comparison is made between the pure polar liquid and its dilute solution in a non-polar solvent.An additional assumption is that the relaxation times vary as the ratio of the macroscopic viscosities 71 and 72, i.e. where 71 is the observed macroscopic relaxation time for a weakly polar liquid I , or for a dilute solution of a highly polar substance in a non-polar solvent (71 = T M I = T ~ I ) and T M Z is the observed macroscopic relaxation time for a highly polar liquid 2. Figure 3 shows the relationship between /3 and (€0 - em) for a wide range of compounds. Curves D, P, and O’D & S are the relation- ships predicted on the basis of equations 20, 21 and 22 respectively. 76 R.I. C. Reviews / / 4.0 - 3.0 / I I I I , I 0 ,30 1 , 2 0 Fig. 3. Plot of /3 against EO - em. The circles represent experimental values.Curve D represents values calculated from eqn 20; curve O’D & S, values calculated from eqn 22; curve P, values calculated from eqn 21 (data from ref 20). For a majority of cases TM is greater than rP due to the influence of the internal field; the scatter of points is not unexpected in view of the assumptions involved. As a rough approximation equation 21 seems the most successful in relating TM and T ~ . Since EO - E , is usually less than 0.2 for the low-loss liquids and solutions used in many dielectric studies TM and rP are often considered as the same. Many fundamental attempts have been made to understand dielectric relaxation at the molecular level and obtain a relationship which will predict the relaxation time of a rigid molecule in a non-interacting environment.In general, comparisons between calculated and observed relaxation times have only had limited success. The majority of such investigations stem from Debye’s treatment which assumes the dipolar molecule to be a sphere of radius r surrounded by a continuous viscous fluid of internal friction coefficient 7 and gives the relaxation time T as: 24 Crossley 77 Few inolecules are completely spherical and the equation has been modified to include ellipsoidal molecules.21 In this case if the molecular dipole has components along each principal molecular axis, molecular rotation about each axis involves the sweeping out of different volumes and could then lead to three relaxation times for molecular relaxation.22 Though these equations may often fail quantitatively they do provide the variables which influence the relaxation time for a rigid dipolar molecule, i.e.molecular size and shape, direction of the molecular dipole, viscosity and temperature. An in-depth discussion concerning the theories of dielectric relaxation is not needed here. The following sub-sections may serve to illustrate the factors which determine the magnitude of a dielectric relaxation time for a rigid polar molecule. Temperature. The temperature dependence of T has been discussed ; equation 18 is found to hold true, within the limits of experimental error, in all cases, and the enthalpies of activation so obtained provide valuable information. Thus, relaxation studies may be carried out over a range of frequencies at a single temperature and also by using a limited number of frequencies and a range of temperatures. Fig.4. Plot of relaxation time in p-xylene solution against relative volume for fluorobenzene e, chlorobenzene A, bromobenzene 0, and iodobenzene (data from ref 23). 10’2T(S) -20 -1 5 relative relative volume volume I 1.2 1.2 ,1.5 ,1.5 I 1.1 1.1 11.4 11.4 1 .o .o , 1.3 1.3 R. I. C. Reviews 78 Molecular size. For rigid polar molecules of similar shape, having their molecular dipole moments in the same direction, measured in the same solvent at a constant temperature there is a linear relationship between molecular volume and relaxation time (Fig. 4).23 This correlation will not strictly hold amongst an assortment of molecular types or for pure liquids due to the dif- ferent molecular shapes, dipole moments (internal field effects), dipole moment directions and viscosities which would be involved.These variables must always be taken into consideration when attempting to predict the relaxation time for a rigid molecule. Direction of d@ole moment. In an applied electric field there is the possibility of molecular dipole reorientation about any axis which has a component of the molecular dipole perpendicular to it. For non-spherical molecules the volumes swept out by rotation about these different axes may vary considerably. Quinoline, isoquinoline and phthalazine show very similar relaxation times in the non-interacting solvent cy~lohexane.~* Since there is only one common Phthalazine Qu inol i ne lsoquinoline axis of rotation for these three molecules, it is probable that dipole re- orientation is predominantly by rotation about the axis perpendicular to the plane of the rings.Any appreciable amount of rotation about the carbon- carbon bond held in common by the two rings, which is forbidden for quinoline, would lead to longer relaxation times for isoquinoline and phthal- azine due to the increased volume swept out in this mechanism compared with that common to all three molecules. There are systems, however, where the direction of the dipole moment plays an important role. The relaxation time of 4-iodobiphenyl is over six times longer than that of 2-iodobiphenyl in the viscous Nujol at 20 0C.25 The dipole 4-lodobiphenyl 2-lodobiphenyl moment in 4-iodobiphenyl is directed along its long axis, and relaxation occurs by rotation about the short axis which involves far more displacement of solvent molecules than rotation about the long axis, which is the main orientation mechanism in the relaxation of 2-iodobiphenyl whose principle moment lies along a short axis of the molecule.Viscosity. Relaxation times calculated on the basis of equation 24 are often many times smaller than the experimental values (Table 1),26 because the macroscopic viscosity is far greater than the effective viscosity which concerns CrossZey 19 -1.0 - 0.5 Table 1. Ratios of relaxation times and viscosities in n-heptane (H), decalin (D) and nujol (N) at 20 OC.26 2.97 3.22 6.16 6.16 a-Chloronaphthalene I .98 a-Bromonaphthalene I .65 TN = 21 I , VD = 2.61 and TH = 0.42 Hz.the individual polar molecules. The effect of the macroscopic viscosity depends to a large extent on the shape of the relaxing polar molecule. For the almost spherical t-butyl chloride the relaxation time is only slightly lengthened by an increase in solvent viscosity of over 200-fold because of the relatively small solvent displacement incurred by the molecular rotation of the solute. The continuous environment assumed by Debye may be approached by increasing the size of the solute molecules relative to the size of the solvent molecules. Experimental studies27 (Fig. 5 ) have indicated that when the solute molecules are at least three times as large as the solvent molecules there is Fig. 5.Plot of ratio of observed t o calculated relaxation time against ratio of solute t o solvent mdecular volume for binary systems of rigid molecules (data from ref 27). a a a a a a 0 a 5.89 5.32 80.6 80.6 50 I 50 I 0 a 80 0 a a e a R.I. C. Reviews good agreement between experimental and calculated relaxation times, neglecting differences due to molecular shape and dipole location and direction in the molecule. It follows from equation 24 that for any given molecule in any given solvent the activation energies for dielectric relaxation (AH:) and viscous flow ( AHT) should be equal, and in many cases that is found experimentally.NON-RIGID POLAR MOLECULES For an aromatic molecule which contains a rotatable polar group there is the possibility of two relaxation processes i.e. molecular and group relaxations, the latter having the shortest relaxation time. In more flexible molecules a range of intramolecular relaxation processes may be possible. However, the usually inadequate number of data points in the absorption region often limits analyses to two relaxation time systems. - 0.08 E" Fig. 6. Cole-Cole plots for some aliphatic compounds in n-heptane solutions at 25 "C. The circles represent experimental points measured at 0.2, I .2,3.2,9.9,24.8,50.8, and 3 x 106 cm .2.00 &' .2.04 \2.od (data from ref 28). n-octyl bromide bo8 10.04 lo.04 A -0.1 6 -0.08 Crossley 81 6 Table2.Relaxation times (TO, 7 1 , 7 2 ) , distribution parameters (a) and relative contributions for some n-alkyl compounds in n-heptane solution at 25 'C.28 n-Dodecyl methyl ether - Di-n-butyl ether n-Octan-2-one 0.23 n-Octylaldehyde n-Octylamine n-Octyl bromide Aliphatic compounds Cole-Cole plots for several aliphatic compounds in dilute solution are shown in Fig. 6 and the relaxation analyses are presented in Table 2.28 Thorough and systematic investigations of the n-bromoalkanes both as pure liquids and in solution have been made at microwave frequen~ies~~ and the data for n- bromooctane (Fig. 6) showing a symmetrical distribution is typical. The distribution, which increases with increasing chain length, has been discussed in terms of a distribution of relaxation times due to segmental rotations, the limits being given by two extreme values corresponding to -CH2Br and molecular end-over-end rotations.In contrast, the data for some long chain ethers,30 sulphides,30 and alcohols31 show a separation into two distinct absorption regions. Furthermore, it is evident from Fig. 6 that di-n-butyl ether, =x cos 8 - 0.19 - - =X sin 6 82 3.5 15.4 34.5 0.75 0.62 4.2 - I .9 - 0.65 0.83 - 17.5 15.4 - - - 4.0 - 12.7 - - - I .8 Fig. 7. Calculation of the relative contributions Ct and C2 of the molecular and group relaxation times from group moments for an aromatic molecule with a rotatable polar group.R.I.C. Reviews n-dodecyl methyl ether, n-octylaldehyde and n-aminooctane have two disper- sion regions and the analyses (Table 2) are consistent with contributions from an intramolecular and a molecular relaxation time. The form of the Cole-Cole plots for n-bromooctane and n-octan-Zone suggest a distribution of relaxation times and the short TO values, especially for the ketone, indicate that rotations of the polar end groups are the dominant processes. It has been suggested28 that aliphatic molecules in which the polar group rotation occurs about a bond other than the C-C bond, as well as around the C--C bond, tend to show two relaxation processes, and in dilute solution the intramolecular process is dominant. Those molecules in which the polar group rotates only about a C-C bond tend to show a distribution of relaxation times.However, this is a simplification of a complex situation which is further complicated with increasing chain length and the number and location of polar substituent groups. log (frequency) Aromatic compounds For an aromatic molecule containing a freely rotating polar group it is pos- sible to calculate the contributions from molecular and group relaxations using group moments (Fig. 7). Thus analyses are given a useful starting point since the molecular relaxation time may be estimated by analogy with rigid molecules of similar shape and size. The results of Davies and Meakins (Fig. 8) give an excellent illustration of the two dielectric dispersion regions for two substituted phen0ls.3~ Such absorption curves have been the exception, due to the often relatively small 71/72 ratios, small C values and limited number of experimental points; more usually a symmetrical depressed centre Cole-Cole plot is obtained.The intra- molecular rotation of such groups as -CH2Cl, -CH&N, -0CH3, -CHO, Fig. 8. The dielectric absorption of (a) 0.76 M 2,4,6-tri-t-butylphenol and (b) 0.28 M 2,6-di-t-butyl-4-bromophenol in decalin solution at 20°C; subscripts, (i) molecular relaxation, (ii) group relaxation (data from ref 32). O i Crossley 83 1012TO (s) a - - 0. I 0.3 0.2 0.2 Table 3. Relaxation times (TO) and distribution parameters (a) for molecules similar in structure to diphenyl ether at 20 OC.33 4 21.2 25 Compound Diphenyl ether Diphenyl methane n-Butyl phenyl ether Diphenyl sulphide Bis(diphenylmethy1) ether 130 n-Decyl ether ASSOCIATED LIQUIDS 13 39 -COCH3,-OH, -NH2 etc., have been the subject of numerous investiga- tions,33 and the enthalpy of activation for group rotation has been used as a measure of the potential barrier. For aromatic amines the short relaxation time may be associated with NH2 group rotation or alternatively an inversion mechanism.Several studies34 have been made in order to establish which process is responsible and though there is evidence in support of the inversion mechanism it is by no means conclusive. The abnormally short relaxation times of diphenyl ether and several similar molecules (Table 3) have generated considerable interest35 and several mechanisms have been considered to account for their rapid relaxation processes. 'Double internal rotation', the coupled rotation of the phenyl groups about the C-0 bonds, seems to provide a more acceptable explanation than the mesomeric shift, inversion, atomic dipole and phenoxy group rotation mechanisms. Dielectric relaxation provides a very sensitive means of detecting molecular interaction.The formation of polar complexes, which reorientate under the influence of the applied field, leads to relaxation times considerably longer than those for the uncomplexed species. In many instances the interaction may not be sufficiently strong to allow the formation of a stable complex; however, the relaxation times of the molecules will be lengthened by the increased resistance to rotation brought about by the attractive influence of neighbour- ing molecules.Low frequency permittivity measurements and dipole moment data have been extensively applied to structural studies of associated systems.36 Di- electric relaxation is capable of providing additional information for such systems.37 Water38 The dielectric absorption of water may be characterized by a single Debye relaxation: at 20 "C T 'V 10 x 10-l2 s, the Cole-Cole distribution parameter is almost indistinguishable from zero, and the dipole moment calculated from equation 3 is the same as for the vapour. Thus the dielectric absorption of water is similar to that anticipated for a simple liquid.In view of the molecular complexity of liquid water, this behaviour is remarkable. The many experi- R. I. C. Reviews 84 mental and theoretical discussions which have arisen from these results and the abnormally large E , - n D values have been summarized by Davies.39 Measurements on very dilute solutions of water in non-polar solvents again reveal single relaxation times and dipole moments similar to the gas phase ~alue.~O At higher concentrations in p-dioxane solutions there are two relaxation times and the contribution from the shortest relaxation time de- creases with increased water c~ncentration.~~ A plausible interpretation of these results is that in the pure liquid during the extremely short time required for relaxation the molecules are in the same environment, in the dioxane solutions a second environment is provided and an additional relaxation time is observed. Alcohols Although alcohols have probably been studied more extensively than any other group of compounds and their dielectric absorption is well established, a complete molecular model capable of explaining the results is as yet unavail- able.Pure liquid primary aliphatic alcohols show a large low frequency absorption (71) and two much smaller high frequency absorptions (72 and 4 . 4 2 The 71 process gradually disappears on dilution with a non-polar solvent and in very dilute solution the absorption is dominated by the highest frequency pr0cess.3~9~3 For alcohols such as 2-methylheptan-3-01, which predominantly associate into small multimers (dimers and possibly trimers) due to the steric environment of the hydroxyl group, only the two high frequency processes (72 and 73) are observed and again 7 3 is dominant at high dilutions (Fig.9).44 Some isomers of octyl alcohol have provided an attractive and useful series of compounds to study. The molar volumes and vapour phase dipole moments of these compounds are very similar, thus any differences in their dielectric behaviour may be due to differences in association as a result of the wide variety of steric effects which can be obtained. The temperature and pressure dependence of the dielectric absorption and Kirkwood g values, where g = p2 (liquid)/p2 (vapour), infer that for n-alcohols association is predomi- nantly into linear chains whereas the more hindered isomers predominantly associate into closed dimers.45 It now seems reasonable to explain 73 in terms of the rotation of -OH groups about their C-0 bonds, and 7 2 is probably due to monomer or -OR group rotations.The Debye-like process (TI) which lengthens regularly with increasing number of carbon atoms for the n-alcohols is less readily explained. It is undoubtedly due to a mechanism which is sensitive to the steric environ- ment of the hydroxyl group since it is not evident for the more hindered alcohols. The molecular size dependence of 71 suggests that rotation of molecular units might be responsible, but if association is into linear chains the great distribution of sizes for such species would give rise to a distribution of relaxation times. Association into highly polar cyclic tetramers has been proposed to account for the zero distribution parameters and the molecular size dependence shown by 71 for the n-alcohols.46 A less specific but more generally acceptable interpretation considers the low frequency dielectric relaxation of alcohols in terms of hydrogen bond rupture followed by the rotation of monomers where the former is the rate determining step.Crossley 85 I I I I I @\ 50 0.0 I E" I 0.0 I EN €0 - E m - 0.4 I I I I ' 0.0 I EN EO - E m - 0.4 0.230 ,0.2 ,0.4- ,0.6 10.8 ) l . O Fig. 9. Normalized Cole-Cole plots for (a) 2-methylheptan-3-01 and (b) n-hexanol at the mole fractions indi- cated in n-heptane solution at 25 "C.The circles represent experimental points at the wavelengths (cm) indicated (data from ref 44). Intramolecular hydrogen bonds EQ - E m 0.4 3.0 86 EQ - E m -0.4 There have been some dielectric relaxation studies of intermolecularly hydro- gen bonded phenols and amines,47 but the majority of work has been con- cerned with intramolecular association in these compounds. It was mentioned earlier that the dielectric absorption of a molecule with a rotatable polar group may show two relaxation times. If a second group is introduced ortho to the first then the group rotation may be restricted by steric hindrance, if the groups are large, or, in certain cases, by intramolecular hydrogen bonding.The extreme effect of intramolecular hydrogen bonding is shown by molecules such as o-hydroxyacetophenone48 and ~alicylaldehyde~g which have R. I. C. Reviews 101270 (s) a 0.13 0.03 0.24 Table 4. Mean relaxation times (TO) and Cole- Cole distribution parameters ( a ) for some phenols and anilines in benzene solution at 20 OC.50 Compound 2.6-Di bromophenol 23 2,6-Dibromo-p-nitrophenol 56 23 2,6-Dichloro-p-nitrophenol 69 2.4-Di bromophenol 2,6-Dich loro-p-n itroan i I i n e 6 I Cole-Cole distribution parameters indistinguishable from zero and relaxation times similar to those for rigid molecules of the same shape and size. In contrast a molecule such as m-methoxyacetophenone, in which hydrogen bond- ing barriers to group rotation are minimized, has a shorter relaxation time and a non-zero distribution parameter4* indicating a contribution from group rotation.Sal icylaldeh yde o-Hyd roxyacetophenone cis 0 0 m- Methoxyacetophenone Table 4 shows the relaxation data for some halophenols and anilines in benzene solution.50 The relatively long mean relaxation times and small distribution parameters for the p-nitro compounds suggest that there is little contribution from -OH or -NH2 group rotation. Conversely, the relaxation data for 2,6-dibromophenol and 2,4-dibromophenol indicate contributions from -OH group rotation. These results infer that the nitro group aids the delocalization of r-electrons between the oxygen (or nitrogen) atom of the OH (or NH2) group and the ring, which results in a stronger intramolecular hydrogen bond.For 2,4-dibromophenol there is the possibility of the following equilibrium : trans The effect of such an equilibrium on the dielectric absorption, which depends on the strength of the intramolecular bond, has been investigated.49 It is possible to calculate the fraction of molecules (x) in the trans form from the observed dipole moment (pObs) and the calculated moments of the cis (pC) and trans (pT) forms using the expression: 25 d b s = w; + (1 - 4r-L: Crossley 87 The study of rigid polar molecules, capable of proton donation, in non-polar solvents of varying basicity and in an inert reference solvent, provides a means of evaluating the relative acidities and basicities of the solutes and solvents respectively.The strength of the interaction may be estimated from51 I170 I250 590 I340 I300 I550 I630 Table 5. Free energy of activation differences AAGt (J mol-l) for chloroethanes in several solvents (relative to cyclohexane) at 25 0C.54 Solvent Compound I, I -Dichloroethane I ,2-Dichloroethane I, I, I-Trichloroethane I, I ,ZTrichloroethane I, I, I ,ZTetrachloroethane I, I ,2,2-Tetrachloroethane I , I, I ,2,2-Pentachloroethane Solute-solven t interact ions Charge-transfer interactions Benzene p-Xylene Mesitylene p-Dioxane I300 2050 710 I800 I960 2970 1710 2090 2760 I420 2470 I250 2090 2050 3140 3010 3556 2970 I550 2170 2090 where 71 is the relaxation time in the inert solvent, 72 is the relaxation time in the potentially basic solvent and AG] and AG; are the free energies of activa- tion for dipole rotation under conditions 1 and 2 respectively.Such studies have been carried out for chloroform in a variety of environments52 and some aromatic amines in benzene and dioxane solution.53 Table 5 summarizes a detailed study of some chloroethanes, using cyclohexane as the inert reference solvent, in a variety of s0lvents.5~ These results show that in each of the basic solvents the ethane interaction increases with increasing protonic character of the ethane hydrogen atom, and for all the ethanes the interaction with the solvent increases with increasing solvent basicity.The results for 1,2-dichloro- ethane are somewhat anomalous and have been disc~ssed;5~,5* the degree of interaction is also reflected by the dipole moment changes but the inaccuracies in this parameter and its solvent dependence even for non-interacting mole- cules often limit its usefulness as an indication of solute-solvent interaction strengths. Dielectric measurements have been made on dilute solutions of the non-polar iodine in the non-polar solvents benzene and p-dioxane.55 Both systems have a dielectric loss in the microwave region and a relaxation time, approximately 3 x 10-l2 s, much shorter than would be expected for a stable polar complex. The relaxation has been discussed in terms of a rapid exchange mechanism between the polar molecular charge-transfer complex and its non-polar constituents. This work also included mixtures of the non-polar electron acceptor 1,3,S-trinitrobenzene and the polar potential electron donors tri- ethylamine, tributylamine and triphenylamine in dilute p-dioxane solution.Trinitrobenzene and triethylamine are shown to form a stable complex, the 26 88 R. I.C. Reviews system shows a single relaxation time which indicates little or no contribution from the uncomplexed amine. The data for trinitrobenzene-tributylamine shows a separation into two dispersion regions which are due to the re- orientation of the charge-transfer complex and the uncomplexed amine. It was not possible to detect any molecular complex for the trinitrobenzene- triphenylamine mixtures in p-dioxane solution.Dielectric relaxation data for systems involving non-polar electron acceptor molecules, tetracyanoethylene, tetrachloro-p-benzoquinone, 2,5-dichloro-p- benzoquinone and p-benzoquinone and the non-polar electron donor solvents mesitylene and p-dioxane have been reported (Table 6).56 0 Tetrachloro-p-benzoq u inone Tetracyanoethylene p-Benzoqu i none Acceptor or \.., Tetracyanoethylene Tet rac h loro-p- benzoqui none 0.78 0.75 2,5-Dichloro-p-Benzoquinone Molecular volume considerations predict a relaxation time of 20 x 10-l2 s for the p-benzoquinone-mesitylene system, which is much longer than the observed relaxation time. Similarly, the relaxation time of a tetracyano- ethylene-mesitylene complex would be close to that for p-benzoquinone- mesitylene, whereas the observed values are vastly different.These results suggest that the relaxation times are not due to the reorientation of the com- plexes, and it is probable that the lifetimes of these complexes are shorter than the time required for the complex to rotate. The relaxation times (Table 6) Table 6. Relaxation times (TO) and dipole moments ( p ) for some non-polar electron acceptor compounds in non-polar electron donor solvents at 20 OC.56 \ \ e o n p-Dioxone 10'27" (s) p (D) Mesitylene 101270 (5) p (D) 13.3 I 0.5 0.80 0.82 0.42 15.0 10.7 7.0 0.4 0.3 5.7 2,5-Dichloro-p-benzoquinone 9.3 7.7 0.38 3 .O 0.4 p-Benzoquinone I ,3,5-Trinitrobenzene - - Crosstey 89 Carbon tetrachloride Benzene 101270 (S) p (D) 14.0 18.4 22.0 5.87 4.71 4.75 28.I Table 7. Relaxation times (TO) and dipole moments ( p ) for maleic and phthalic anhydride in several solvents at 25 0C.57 Phthalic anhydride Maleic anhydride Solvent 10'270 (S) p (D) 3.86 3.55 3.69 3.59 3.93 5.8 9.4 I I .3 12.6 15.9 4.85 - - p-Xy I en e Mesi ty len e p- Dioxane show that the lifetimes of the complexes formed with either donor molecule increase in the order of increased acceptor strength i.e. p-benzoquinone < 2,5-dichloro-p-benzoquinone < tetrachloro-p-benzoquinone < tetrachloroethylene. This type of study has been extended to include polar electron acceptors, maleic and phthalic anhydrides, with non-polar electron donors solvents, mesitylene, p-xylene, benzene, and p-dioxane, and an inert reference solvent carbon tetrachloride.57 In all cases the dielectric absorptions are close to a single Debye relaxation and the results (Table 7) are interpreted on the basis of a relaxation rate which is a weighted average of the relaxation rates of the un- complexed anhydride and the complex.The fact that the dipole moments in carbon tetrachloride are greater than those in the aromatic solvents and less than those inp-dioxane is due to the broadside and head-to-tail alignments of the respective complexes with the result that the induced moments oppose and reinforce the parent dipole moment respectively.Interaction between polar molecules The dielectric absorption of chloroform, acetone, diethyl ether and triethyl- amine each in cyclohexane and binary polar mixtures of chloroform with acetone, diethyl ether and triethylamine in cyclohexane has been examined.58 For each of the polar molecules the dielectric absorption may be characterized by a single relaxation time. All the mixtures show considerable distribution parameters, much larger than anticipated in view of the similar relaxation times for the components, and have relaxation times longer than those of either constituent. These results (Table 8) were interpreted in terms of a contribution from a stable polar complex (71) formed between chloroform and the electron donor in addition to the uncomplexed polar components (72).If the dipole moment of the complex can be estimated it is possible to calculate the equili- brium constants for these interactions, from the C values. MIXTURES OF POLAR COMPOUNDS Measurements on mixtures of rigid non-interacting polar molecules whose individual dielectric properties are well characterized, provides an excellent means of examining the validity of analysing dielectric data for more than one relaxation time, and they should show whether or not the components retain R. I. C. Reviews 90 Table 8. Relaxation times (TO, 71, TZ), relative contributions (C&) and distribution parameters (a) for some electron donor and acceptor compounds in cyclohexane solution at 25 ‘C.58 Acetone Ether Triethylamine Chloroform Acetone and chloroform Ether and chloroform Triethylamine and chloroform 0 0 0 0 2.6 2.5 8.0 4. I 4.5 7.0 0.22 0.08 0.17 I I .3 fi = 0.108 fi = 0.235 fi = 0.492 Benzophenone in tetrahydrofuran in benzene Table 9.Relaxation times (71-benzophenone and 7-tetrahydrofuran), reduced relaxation time (./.I), calculated and experimental relative contributions (771p:/r]~p: and C1/Cz respectively) for some tertiary and binary mixtures at 20 “C; f1 gives the mole fraction of benzophenone in tetrahydrofuran.59 Benzo p h en o n e i n benzene Tetrahydrofu ran Benzophenone in tetrahydrofuran 21.2 2.96 2.97 3.13 3.0 3. I 3 . I 0.57 I .53 4.0 0.37 0.95 2.97 0.38 0.37 0.95 2.97 fi = 0.108 fi = 0.235 f .= 0.492 their individuality in the mixture. The data for the mixtures may be analysed using equations 14-1 6, and the resulting relaxation times compared with those for each component when measured alone obtained using equations 7 and 8. In addition, the weight factors (C values) obtained from the analyses may be compared with those calculated, for a binary polar mixture, using: - - - where C1, nl and p1 are the weight contributions, concentration and dipole moment of component 1 ; C2, nz, and p2 are the corresponding values for component 2. Some of the results from such a study for binary mixtures of polar compounds and solutions of binary polar mixtures in a non-polar solvent are presented in Table 9.59 For the solutions, the components retain their individuality and the calculated and observed C values are in good agreement.In general, especially when viscosity factors are considered, the relaxation times obtained for the binary mixtures are in good agreement with those for the components. However, for polar liquid mixtures the experimental Cl/Cz ratios are about 1.3 to 2.3 times longer than calculated. This is probably 27 91 Crossley - 25.5 35.0 67.9 20.5 21 .o 20.0 - - - - - - - - 0.29 0.43 0.39 - 3.5 4.6 6.8 - - 21 13 37 30.5 37.6 29.6 I .o 2.9 because the assumption Canp2 is insufficient for the binary mixtures and any accurate calculation must take into account the influence of the internal field.NON-POLAR LIQUIDS Whiffen60 found small dielectric losses for benzene, carbon tetrachloride, cyclohexane and decalin, all of which have no permanent electric dipole moment, in the frequency range 0.3 to 1.2 cm-1. Plots of loss tangent against frequency suggested Debye behaviour and relaxation times (- 1 x 10-l2 s) of the order of the time between molecular collisions. Other similar measurements at microwave frequencies61 served to confirm these findings and to show that the absorptions are not the result of impurities such as water.G2 It seemed probable that these absorptions result from dipole moments (< 0.1 D) induced in molecular collisions. The induced dipole is then regarded as changing direc- tion not by molecular rotation, the relaxation times are far too short, but because of a new collision at another instant with another neighbour.Conven- tional dielectric measuring techniques have not been well suited to precise studies for such systems since the absorption is not wholly described in the microwave region and the losses are often within the experimental error. The advent of interferometric methods has permitted accurate measurements of the absorption coefficient 01 in the sub-millimetre region and the general form of the absorptions has been established. The dielectric loss is related to the absorption coefficient 2 7rTTE‘IV a = ___- N 28 n where n is the refractive index and ‘v the frequency in wave numbers. Fig. 10.The microwave and far infrared absorption of pure liquid benzene, solid line represents the 01 values, dashed line the E” values (data from ref 63). 92 R.I.C. Reviews The absorption of liquid benzene is shown in terms of 01 and E” in Fig. 10.63 Similar curves have been obtained for carbon tetrachloride, carbon disulphide, p-dioxane and other non-polar liquids.63 These findings and the means of investigating the sub-millimetre region (10- 100 cm-1) have prompted several studies of rigid polar molecules,63~64 all of which absorb in this region which had long been a subject of conjecture in view of the anomalously large E , - n j values shown by many rigid polar liquids.65 The subject is still in its infancy, and although a substantial amount of experimental data now exists, no completely satisfactory quantitative inter- pretation is available.For the non-polar liquids the qualitative interpretations of Whiffen have not been significantly improved upon, the absorption for the polar liquids is also thought to arise from induced moments and the relaxation has been considered in terms of liquid lattice vibrations.66 POLYMER SOLUTIONS The vast majority of dielectric studies of polymers have been concerned with the solid phase and have provided valuable information. However, some attention has been given to polymers in solution, and an excellent review article has recently been published on this subject.67 Some data for poly-n- butyl isocyanate and polymethyl methacrylate are shown in Table 10 to illustrate’ this type of work.39 For the latter polymer-in-toluene solution the relaxation times show only small variations over a 120-fold increase in molecular weight.This indicates that when a sufficiently large multimer is formed, the relaxation involves the motion of small segmental units of the polymer chains. Conversely, the relaxation time for poly-n-butyl isocyanate in benzene solution increases with increased molecular weight and this polymer is classified as a rigid rod, the relaxation involving molecular rotation. Dielectric data have been used to distinguish between molecular configura- tions for polymers which may exist either as a rigid helix or in a random coil depending on solvent and temperature c~nditions.~g Considerable information has been obtained from dielectric absorption measurements of proteins and their aqueous solutions.T (s) Polymethyl methacrylate Poly-n-butyl isocyanate Table 10. Relaxation data for some polymer solutions.39 Solvent Degree of polymerization T (“C) Polymer Toluene -9.0 0.67 x 10-8 0.77 x 10-8 0.71 x 10-8 0.52 x 10-8 Benzene 22.5 I .58 x 10-6 25 x 10-6 224 x 10-6 5000 x 10-6 140 950 5000 I7500 10-5 mo/ecu/ar weight I .4 3 -84 7.34 23 ~~ Crossley 93 10-6 molecular wt Concn % (C) p (D) 10% (s) 0 . 2 44000 4.4 8400 0.84 7 5 Table 11. Dielectric parameters for DNA in aqueous solution, 15 OC.69 0.38 2 0.8 640 120 3800 I250 0.125 0.064 0.012 0.4 0. I - - 0.2 0.02 0.02 For proteins, e.g.a body tissue, the permittivity falls from possibly several millions to about five over 11 decades of frequency (Fig. 11). The a-dispersion arises from a Maxwell-Wagner relaxation effect, which is not peculiar to proteins and is due to electrostatic effects between particles of different permit- tivity and conductivity. Two factors may contribute to the 13-dispersion: (i) a lower permittivity resulting from the breakdown of material into small units, (ii) the reorientation of the polar protein molecules. The y-dispersion is due to the aqueous nature of body tissue and is very similar to the dispersion of water. An additional dispersion (8 region) has been detected, between the /3 and y regions, for some proteins in aqueous solution and is probably due to water bound to the protein.G8 Finally the dielectric parameters obtained for deoxyribose nucleic acid (DNA) measured between 30 Hz and 5 MHz are presented in Table 11.Both Fig. I I. The frequency variation of the permittivity of muscular tissue. I 4 2 10 , 6 R.I.C. Reviews 94 the relaxation time and dipole moment values increase with increased molecular weight indicating that DNA behaves as a rigid rod with its resultant electric dipole moment acting along the long axis.69 SUMMARY It is hoped that the previous pages will have served to introduce the great potential of dielectric relaxation studies. Those wishing to actively pursue the subject will require a more in-depth treatment than that given here and should find the recently published text, Dielectric properties and molecular behaviour,70 an excellent starting point.REFERENCES 1 J. W. Smith, Electric dipoZe moments. London: Butterworths, 1955. 2 C. P. Smyth, Dielectric behaviour and structure. New York: McGraw-Hill, 1955. 3 P. Debye, Polar molecules. New York: Chemical Catalogue, 1929. 4 L. Onsager, J. Am. chem. SOC., 1936,58, 1486. 5 K. S. Cole and R. H. Cole, J. chem. Phys., 1941,9, 341. 6 D. W. 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Tucker and S. Walker, Tetrahedron, 1964, 20, 2137.16 S. K. Garg, H. Kilp and C. P. Smyth, J. chem. Phys., 1965, 43, 2341. 17 J. E. Chamberlain, E. B. C. Werner, H. A. Gebbie and W. Slough, Trans. Faraduy SOC., 1967,63,2605; J. E. Chamberlain, H. A. Gebbie, G. W. F. Pardoe and M. Davies, Chem. Phys. Lett., 1968, 1, 523. 18 J. G. Powles, J. chem. Phys., 1953, 21,633. 19 J. J. O’Dwyer and R. A. Sack, Aust. J. sci. Res., 1952, A5, 647. 20 R. C. Miller and C. P. Smyth, J. Am. chem. SOC., 1957,79, 3310. 21 E. Fischer, Phys. Z., 1939, 40, 645. 22 F. Perrin, J. phys. Radium, 1934,5,497. 23 W. F. Hassell and S. Walker, Trans. Faruduy Soc., 1966,62, 2695. 24 J. Crossley and S. Walker, Can. J. Chem., 1968,46, 2369. 25 E. N. DiCarlo and C. P. Smyth, J. phys. Chem., 1962,66, 1105.26 E. L. Grubb and C. P. Smyth, J. Am. chem. SOC., 1961, 83, 4122. 27 R. D. Nelson and C. P. Smyth, J. phys. Chem., 1964,68,2704. 28 G. P. Johari, J. Crossley and C. P. Smyth, J. Am. chem. SOC., 1969,91, 5197. 29 K. Higasi, K. Bergmann and C. P. Smyth, J. phys. Chem., 1960,64, 880. 30 S. DasGupta, K. N. Abd-El-Nour and C. P. Smyth, J. chem. Phys., 1969, 50, 4810. 31 G. P. Johari and C. P. Smyth, J. Am. chem. SUC., 1969, 91, 6215. 32 M. Davies and R. J. Meakins, J. chem. Phys., 1957, 26, 1584. 33 C. P. Smyth, Adv. mol. Relaxation Processes, 1967-68, 1, 1. 34 S. K. Garg and C. P. Smyth, J. chem. Phys., 1967, 46, 373. 35 F. K. Fong, J. chem. Phys., 1964,40,132; J. E. Anderson and C. P. Smyth, J. chem. Phys., 1965,42,473; R. D. Nelson and C.P. Smyth, J. phys. Chem., 1965,69, 1006; K. Higasi and C. P. Smyth, J. Am. chem. SOC., 1960, 82,4759. 36 G. C. Pimentel and A. L. McClellan, The hydrogen bond. San Francisco: Freeman, 1960. 37 M. Davies, J. chem. Educ., 1969, 46, 17. Crossley 95 38 J. R. Hasted, Prog. Dielectrics, 1961, 3, 101. 39 M. Davies, chapter 4 in ref. 12. 40 S. K. Garg, J. E. Bertie, H. Kilp and C. P. Smyth, J. chem. Phys., 1968, 49, 2551; J. 45 W. Dannhauser, J. chem. Phys., 1968, 48, 191 1. Crossley and C. P. Smyth, J. chem. Phys., 1969, 50, 2259. 41 S. K. Garg and C. P. Smyth, J. chem. Phys., 1965, 43,2959. 42 S . K. Garg and C. P. Smyth, J. phys. Chem., 1965, 69, 1294; C. Brot and M. Magat, J. chem. Phys., 1963,39, 841. 43 M. Moriamez and A. Lebrun, Archs Sci., Genkve, 1960, 13,40; D.J. Denney and J. W. Ring, J. chem. Phys., 1963, 39, 1268; Y. Leroy, Thesis, Universite de Lille, 1961. 44 J. Crossley, L. Glasser and C. P. Smyth, J. chem. Phys., 1970,52, 6203. 46 P. Bordewijk and C. J. F. Bottcher, J. phys. Chem., 1969, 73, 3255. 47 J. Crossley, Adv. mol. Relaxation Processes, 1970, 2, 69. 48 A. A. Antony and C. P. Smyth, J. Am. chem. Soc., 1964,86, 156. 49 M. D. Magee and S. Walker, Trans. Faraday SOC., 1966, 62, 1748. 50 A. A. Antony, F. K. Fong and C. P. Smyth, J. phys. Chem., 1964, 68, 2035. 51 K. Chitoku and K. Higasi, Bull. chem. SOC. Japan, 1967, 40, 773. 52 A. A. Antony and C. P. Smyth, J. Am. chem. SOC., 1964,86, 152. 53 K. Chitoku and K. Higasi, Bull. chem. SOC. Japan, 1966, 39, 2160.54 J. Crossley and C. P. Smyth, J. Am. chem. SOC., 1969, 91, 2482. 55 J. E. Anderson and C. P. Smyth, J. Am. chem. SOC., 1963,85, 2904. 56 R. A. Crump and A. H. Price, Trans. Faraday SOC., 1970, 66, 92. 57 R. A. Crump and A. M. Price, Trans. Faraday Soc., 1969, 65, 3195. 58 M. D. Magee and S. Walker, J. chem. Phys., 1969, 50, 1019. 59 H. Kilp, S. K. Garg and C. P. Smyth, J. chem. Phys., 1966,45, 2799. 60 D. H. Whiffen, Trans. Faraday Soc., 1950, 46, 124. 61 J. Crossley and S. Walker, Can. J. Chem., 1968,46,847; E. N . DiCarlo and C. P. Smyth, J. Am. chem. SOC., 1962, 84, 1 128. 62 S. K. Garg, J. E. Bertie, H. Kilp and C. P. Smyth, J. chem. Phys., 1968,49, 2551. 63 G. Chantry, H. A. Gebbie, B. Lassier and G. Wyllie, Nature, Lond., 1967, 214, 163.64 M. Davies, G. W. F. Pardoe, J. E. Chamberlain and H. A. Gebbie, Trans. Faraday SOC., 1968,64, 847. 65 J. P. Poley, J. appl. Sci. Res., 1955, B4, 337. 66 N. E. Hill, Proc. phys. SOC., 1963, 82, 723. 67 H. Block and A. M. North, Adv. mol. Relaxation Processes, 1970, 1, 309. 68 E. H. Grant, S. E. Keefe and S. Takashima, J. phys. Chem., 1968,72, 4373. 69 S. Takashima, J. molec. Biol., 1963, 7, 445. 70 N. E. Hill, W. E. Vaughan, A. H. Price and M. Davies, Dielectricproperties and niolecular behaviour. London : Van Nostrand-Reinhold, 1969. R.I.C. Reviews 96 DIELECTRIC RELAXATION AND MOLECULARSTRUCTURE IN LIQUIDSJohn Crossley, B.Sc., Ph.D., A.R.I.C.Department of Chemistry, Lakehead University, Thunder Bay, Ontario, CanadaIntroduction and basic theory .. . . .. . . . . . .Experimental methods . . .. .. . . . . . . . .Heterodyne beat method, 74Very low frequencies, 75Audio and radio frequency bridges, 75Resonant circuit, 75Transmission-line methods, 75Microwave bridges, 75Free-space methods, 76Very high frequency methods, 76Rigid polar molecules . . . . . . . . . . . . . .Non-rigid polar molecules . . . . . . . . .. . .Associated liquids. . . . ,. . . . . . . . . . .Aliphatic compounds, 82Aromatic compounds, 83Water, 84Alcohols, 85Intramolecular hydrogen bonds, 86Solute-solvent interactions, 88Charge- transfer interactions, 8 8Interaction between polar molecules, 90Mixtures of polar compounds . . . . . . . . . . . .Non-polar liquids. . . . .. . . ... . . . . .Polymer solutions. . . . . . . . .. . . . . . .Summary . . . . . . . . .. . . . . . . . .References . . . . . . . . .. . . . . . . ..70747681849092939595For a dipolar compound the so-called dielectric constant is by no means aconstant but varies with the frequency of an applied electromagnetic field. Thisfrequency dependence arises from the inability of molecular dipole orientationto keep pace with the changes in direction of the applied field at high fre-quencies, and as a result the permittivity (dielectric constant) decreases withincreasing frequency in the region of anomalous dispersion. The time lagbetween the response of a dipole to the behaviour of the applied field is thephenomenon of dielectric relaxation.Experimental investigations of thiseffect provide appreciable information concerning molecular and intra-molecular motions. The purpose of this article is to present an introduction tothe principles of dielectric absorption and a review of some selected applica-tions in the liquid state.Crossley 6INTRODUCTION AND BASIC THEORYConsider the application of a static electric field to a liquid composed of ran-domly orientated electric dipoles, between the plates of a condenser. The permit-tivity of the liquid (€0) is given by the ratio of the capacity of this condenser tothe capacity of the same condenser with a vacuum between the plates. It isthen a simple matter to calculate the total polarizability (a) of the moleculesusing the Clausius-Mosotti relationship1in which A4 is the molecular weight, d the density and N the number of mole-cules per mole.The polarizability is also given by:2here CUD is the distortion polarizability which may be estimated from refractiveindex measurements. The last term on the right hand side of equation 2 is theorientation polarizability, and p is the electric dipole moment, k the Boltz-mann constant and T the absolute temperature.Combination of equations 1 and 2 gives the Debye equation which has beenthe basis for numerous dipole moment determination^;^^^,^ it is only strictlyapplicable to gases or very dilute solutions of a polar solute in a non-polarsolvent.Onsager's re-examination of the Clausius-Mosotti-Debye treatments led toa relationship more applicable to polar liquids4€0 - 1 M - 47rNa€0 + 2' d 3- ~ -01 = O ~ D + p2/3kT3In this equation €0 and E , are the permittivities at static or low frequenciesand very high frequencies respectively.For a polar liquid €0 may be consider-ably greater than E , . This is due to the inability of the dipoles to follow thefield at high frequencies and thus there is no contribution from dipole orienta-tion to the capacitance and permittivity.For a dipolar substance there is a frequency range in between EO and E , ,where the permittivity is not constant but decreases as the frequency of theapplied electric field is increased, due to a time lag between the response of thedipoles to the voltage oscillations. This lag is a relaxation; the term may beapplied to any system in which there is a time delay in the response of a systemto changes in the forces which are applied to it.Dielectric relaxation is theexponential decay with time of the polarization in a dielectric when an exter-nally applied field is removed.At the molecular level we can picture a liquid composed of dipolar mole-cules, such as chlorobenzene, between the plates of a condenser, to which alow frequency electric field has been applied. In such a system there is a ten-dency for the dipoles to align themselves with respect to the field. Thus, themolecular dipoles may rotate in phase with the voltage oscillations of theapplied field. If the frequency of the applied field is increased then the dipolarmolecules must rotate faster in order to keep pace with the field.Eventually the70 R. I. C. ReviewFig. I. The frequency dependence of the real (E’) and imaginary (E”) parts of the permittivity ina relaxation region.rate of dipole orientation lags behind the frequency of the applied field, andultimately molecular rotation can no longer maintain pace with the appliedfield. The existence of dielectric relaxation becomes apparent when its rate isclose to the same order of magnitude as that of the frequency of the appliedfield. The dielectric relaxation time may be defined as the time after the removalof the applied field in which the polarization in a dielectric is reduced to l/etimes its original value.Figure I shows the frequency dependence of the permittivity for an anoma-lous dispersion or relaxation region, in which the permittivity ( E * ) is a mathe-matically complex quantity.E* = E’ - iE” 4The real permittivity is E’ and E” is the imaginary part or loss factor.Thelatter is a measure of the conductance of the medium and its ability to dissipateenergy. As the frequency of the applied field approaches zero, E” approacheszero, and E’ approaches the static permittivity €0; and, as the frequency ap-proaches infinity, E” again approaches zero, and E’ approaches em, the veryhigh frequency or optical permittivity. The frequency at which E” is a maxi-mum (@,a,) gives the relaxation time (7) of the dipole orientation process inseconds.5The Debye-Pellat equations (see ref. 2, p. 55 for derivation) which providethe basis for dielectric absorption studies give the frequency dependence of E’Crossley 711 r=---urnaand E” for a single relaxation process T ,separation of real and imaginary parts gives :andFrom equation 8 it is evident that E” is a maximum for OT = 1 andI t (€0 - Em)2 E m a x =69Thus from measurements of E’ and E” at frequencies in the absorption regionElimination of OT between equations 7 and 8 gives the equation of a circleit is possible to evaluate the relaxation time.10A plot of E’ against E” in the complex plane gives a semicircle (E” may onlyhave positive values) the so-called Cole-Cole plot (Fig.2a).So far the discussion has been limited to the Debye-Pellat equations and thecase of a single discrete relaxation time.For many systems this may be satis-factory but for many more the theory must be modified to account for ( i ) adistribution of relaxation times for one relaxation process, or (ii) more thanone discrete relaxation process.Cole and Cole5 assumed a continuous distribution of relaxation times abouta most probable value TO and their general dispersion equation for the com-plex dielectric constant is1 1here 01 is the distribution parameter, an empirical constant which measures thewidth of the distribution (Fig. 2b) and may have values between 0 and 1. Whena = 0 the Debye equations are obtained. There are other distribution func-tions6 but the Cole-Cole distribution is the most widely employed.For systems which have contributions from n independent relaxation pro-cesses each of which shows Debye behaviour, Bud6’ has shown that the di-electric absorption can be obtained as the sum of Debye terms12T k is the relaxation time characteristic of the kth mode of relaxation and Ck isa factor representing the proportion by which the kth mode contributes to the72 R.I. C. Reviewtotal dispersion :nc k = 1k= 1For two relaxation processes (71 and 72) we have:E" - CW71 c2w72(€0 - Em) - 1 + (w71)2 + 1 + (w72)2131415c1+ c2 = 1 16The form of the complex plane plot for such systems will depend on therelative magnitudes of 7 1 , ~ ~ and C1 but may often be approximated by a sectorof a semicircle.c2 + _ _ ~(E' - Em> - Cl -( € 0 - Em) 1+(w.1Y2 1 + ( c ~ W 2 ) ~ab73Fig.2. Cole-Cole plots for systemsshowing (a) a single Debye relaxation,(b) a distribution of relaxation timesand (c) a separation into two relaxa-tion regions.CrossleDipole reorientation may be considered as a rate process,s r being thereciprocal of a rate constant kl which follows an Arrhenius-type equation withtemperature changes17 1 kT kl = - = - exp (- hGt/RT) r hand thus- h T + 1 AH? l n T = - ~ T R 18where AGt, A H i , and AS? are the free energy, enthalpy and entropy ofactivation respectively. AH+ is the energy molecules require in order to rotateand may be readily obtained from a plot of In TT against 1/T.A common procedure when considering experimental E’ and E” data is toinitially examine them as a Cole-Cole plot.The procedure then is to analysethe data, starting from initial estimates deduced from the complex plane plot,by a computer fit to the Cole-Cole equations, for TO, a and E , . The latter, un-like €0, is not an experimentally measured quantity. If a is non-zero andespecially if the Cole-Cole plot indicates a separation (Fig. 2c) it is usual toattempt a further computer analysis based on equations 14-16 for physicallysignificant values of 71, 7 2 and C1. Equation 3 may be used to calculate dipolemoments. It is possible to perform such analyses by graphical methods,g butthese are often tedious and time-consuming, especially for systems with non-zero distribution parameters.The inadequacy of the Debye theory in relating the permittivity and dipolemoment for polar liquids arises from the use of an expression for the internalfield which is only strictly applicable to gases. Because of the internal field therelaxation times calculated as described above are macroscopic quantities,longer than those for the individual polar molecules.This effect will bediscussed further when dealing with the relaxation of rigid polar molecules.EXPERIMENTAL METHODSTo a large extent the nature of the apparatus depends on the frequency rangeand magnitude of the dielectric absorption to be measured. In view of thetemperature dependence of r (equation 17), however, it is possible to bring anabsorption into a desired frequency range by suitable choice of temperature.Many of the experimental methods involve measuring the capacitance of anempty condenser CO and the capacitance C and resistance R of the condenserfilled with the dielectric under investigation.Essentially the real and imaginaryparts of the permittivity result fromHeterodyne beat methodl ~2This type of apparatus is frequently used to measure static dielectric constantsat 1 MHz. It is based upon the frequency control of an oscillator by adjustingthe capacitance of its circuits which contain a dielectric cell. A precision74 R . I . C. Reviewvariable condenser is used to tune the signal of the variable oscillator to give anull when mixed with the signal from a fixed frequency oscillator. The null maybe detected readily using a cathode ray oscilloscope. Introduction of a di-electric into the cell increases the capacitance in the tuning circuit; the balanceis reachieved by decreasing the capacitance of the precision condenser. Calibra-tion of the instrument is achieved by using liquids of known permittivity.Very low frequenciesScheiberlo and Harris11 have described capacitance-resistance bridges whichallow measurement of E’ and E” in the frequency range 0.008 Hz to 200 Hz.Audio and radio frequency bridgesCapacitance-resistance bridges such as the General Radio Co.type 161 5-A(100 Hz-10 kHz), Schering bridge (20 Hz-1 MHz), WTW multidekameter(100 kHz-12 MHz) and Hewlett Packard 250 A RX meter (500 kHz-250 MHz)are based upon a Wheatstone bridge principle. A wide range of permittivitymay be covered by suitable choice of dielectric cell capacitance.Vaughanl2has given a description of dielectric cells and their calibration.Resonant circuitThe Hartshorn and Ward apparatus13 has been used for liquids over thefrequency range 50 kHz-100 MHz. A circuit containing capacitance andinductance is loosely coupled to a primary circuit in which a high frequencyalternating current has been set up. The current flowing in the secondarycircuit may be controlled by changes of capacitance and inductance. The realpermittivity is obtained from the difference in the capacitance readings of aprecision condenser required to give maximum current with and without thedielectric cell, which is in parallel with this precision condenser. The width ofthe resonance curves for the empty and filled cell and the known capacitanceof the system give the dielectric loss.Transmission-line methods29 12¶ l4This type of apparatus employs either co-axial lines (100 MHz-5 GHz) orwaveguides (3 GHz-50 GHz).Both methods involve measuring the charac-teristics of a voltage standing wave produced in a liquid compared with thatfor the air filled system. The experimental arrangement and method of calcula-tion depend to a large extent on the loss of the dielectric.Microwave bridgesl5This type of apparatus has been successfully used for dielectric absorptionmeasurements of medium and low loss liquids and solutions in the frequencyrange 6-70 GHz. The liquid is contained between mica windows in one arm ofthe waveguide bridge; a variable attenuator and a phase shifter in the otherarm are used to balance the bridge for increasing lengths of liquid.Theattenuation and phase shift introduced by known lengths of liquid are used tocalculate attenuation and phase constants and E’ and E”.CrossZey 7Free-space methodsAt high frequencies the dimensions of waveguide apparatus are such thatdifficulties arise in the construction of transmission lines and microwavebridges. A typical free-space interferometric method propagates the 2 mmharmonic from a 4 mm Klystron.lGVery high frequency methodsDielectric measurements in the frequency region 5-500 cm-l have recentlybeen made possible by the development of apparatus at the NPL. The methodshave been fully described17 and a Grubb-Parsons instrument is now com-mercially available.RIGID POLAR MOLECULESSeveral relationships have been developed to deal with the effect of the internalfield on dielectric relaxation, and to relate the macroscopic relaxation time( T M ) given by equations 6-15 and the microscopic relaxation time ( T ~ ) of theindividual polar molecules.The first treatment was attempted by Debye andgave :20Powlesl8 considered the internal field required to give an exponential decayof the macroscopic polarization and obtainedA more exhaustive treatment by O’Dwyer and Sack19 led to2122The effect of the internal field in a liquid, and a test of equations 20-22, wasexamined experimentally by comparing the macroscopic relaxation time of ahighly polar liquid with that of a liquid composed of molecules of the sameshape and size but with only a small dipole moment.20 For the latter types ofliquid it is assumed that T M and T~ are indistinguishable.Alternatively, thecomparison is made between the pure polar liquid and its dilute solution in anon-polar solvent. An additional assumption is that the relaxation times varyas the ratio of the macroscopic viscosities 71 and 72, i.e.23where 71 is the observed macroscopic relaxation time for a weakly polar liquidI , or for a dilute solution of a highly polar substance in a non-polar solvent(71 = T M I = T ~ I ) and T M Z is the observed macroscopic relaxation time for ahighly polar liquid 2. Figure 3 shows the relationship between /3 and (€0 - em)for a wide range of compounds.Curves D, P, and O’D & S are the relation-ships predicted on the basis of equations 20, 21 and 22 respectively.76 R.I. C. Review4.0 -3.0 / //1 I , I 0 I , 2 0 I ,30 IFig. 3. Plot of /3 against EO - em. The circles represent experimental values. Curve D representsvalues calculated from eqn 20; curve O’D & S, values calculated from eqn 22; curve P, valuescalculated from eqn 21 (data from ref 20).For a majority of cases TM is greater than rP due to the influence of theinternal field; the scatter of points is not unexpected in view of the assumptionsinvolved. As a rough approximation equation 21 seems the most successful inrelating TM and T ~ . Since EO - E , is usually less than 0.2 for the low-loss liquidsand solutions used in many dielectric studies TM and rP are often considered asthe same.Many fundamental attempts have been made to understand dielectricrelaxation at the molecular level and obtain a relationship which will predictthe relaxation time of a rigid molecule in a non-interacting environment.Ingeneral, comparisons between calculated and observed relaxation times haveonly had limited success. The majority of such investigations stem fromDebye’s treatment which assumes the dipolar molecule to be a sphere ofradius r surrounded by a continuous viscous fluid of internal friction coefficient7 and gives the relaxation time T as:Crossley247Few inolecules are completely spherical and the equation has been modifiedto include ellipsoidal molecules.21 In this case if the molecular dipole hascomponents along each principal molecular axis, molecular rotation abouteach axis involves the sweeping out of different volumes and could then lead tothree relaxation times for molecular relaxation.22Though these equations may often fail quantitatively they do provide thevariables which influence the relaxation time for a rigid dipolar molecule, i.e.molecular size and shape, direction of the molecular dipole, viscosity andtemperature.An in-depth discussion concerning the theories of dielectricrelaxation is not needed here. The following sub-sections may serve to illustratethe factors which determine the magnitude of a dielectric relaxation time for arigid polar molecule.10’2T(S)-20Temperature. The temperature dependence of T has been discussed ; equation18 is found to hold true, within the limits of experimental error, in all cases,and the enthalpies of activation so obtained provide valuable information.Thus, relaxation studies may be carried out over a range of frequencies at asingle temperature and also by using a limited number of frequencies and arange of temperatures.Fig.4. Plot of relaxation time in p-xylene solution against relative volume for fluorobenzene e, chlorobenzene A, bromobenzene 0, and iodobenzene (data from ref 23).-1 5relative volume1 .o I 1.1 I 1.2 , 1.3 11.4 ,1.578 R. I. C. ReviewMolecular size. For rigid polar molecules of similar shape, having theirmolecular dipole moments in the same direction, measured in the same solventat a constant temperature there is a linear relationship between molecularvolume and relaxation time (Fig.4).23 This correlation will not strictly holdamongst an assortment of molecular types or for pure liquids due to the dif-ferent molecular shapes, dipole moments (internal field effects), dipolemoment directions and viscosities which would be involved. These variablesmust always be taken into consideration when attempting to predict therelaxation time for a rigid molecule.Direction of d@ole moment. In an applied electric field there is the possibility ofmolecular dipole reorientation about any axis which has a component of themolecular dipole perpendicular to it. For non-spherical molecules the volumesswept out by rotation about these different axes may vary considerably.Quinoline, isoquinoline and phthalazine show very similar relaxation times inthe non-interacting solvent cy~lohexane.~* Since there is only one commonPhthalazine Qu inol i ne lsoquinolineaxis of rotation for these three molecules, it is probable that dipole re-orientation is predominantly by rotation about the axis perpendicular to theplane of the rings.Any appreciable amount of rotation about the carbon-carbon bond held in common by the two rings, which is forbidden forquinoline, would lead to longer relaxation times for isoquinoline and phthal-azine due to the increased volume swept out in this mechanism compared withthat common to all three molecules.There are systems, however, where the direction of the dipole moment playsan important role.The relaxation time of 4-iodobiphenyl is over six timeslonger than that of 2-iodobiphenyl in the viscous Nujol at 20 0C.25 The dipole4-lodobiphenyl 2-lodobiphenylmoment in 4-iodobiphenyl is directed along its long axis, and relaxationoccurs by rotation about the short axis which involves far more displacementof solvent molecules than rotation about the long axis, which is the mainorientation mechanism in the relaxation of 2-iodobiphenyl whose principlemoment lies along a short axis of the molecule.Viscosity. Relaxation times calculated on the basis of equation 24 are oftenmany times smaller than the experimental values (Table 1),26 because themacroscopic viscosity is far greater than the effective viscosity which concernsCrossZey 1Table 1.Ratios of relaxation times and viscosities in n-heptane (H), decalin (D) andnujol (N) at 20 OC.26a-Chloronaphthalene I .98 6.16 2.97 80.6 5.89 50 Ia-Bromonaphthalene I .65 6.16 3.22 80.6 5.32 50 ITN = 21 I , VD = 2.61 and TH = 0.42 Hz.the individual polar molecules. The effect of the macroscopic viscosity dependsto a large extent on the shape of the relaxing polar molecule. For the almostspherical t-butyl chloride the relaxation time is only slightly lengthened by anincrease in solvent viscosity of over 200-fold because of the relatively smallsolvent displacement incurred by the molecular rotation of the solute.The continuous environment assumed by Debye may be approached byincreasing the size of the solute molecules relative to the size of the solventmolecules.Experimental studies27 (Fig. 5 ) have indicated that when the solutemolecules are at least three times as large as the solvent molecules there isFig. 5. Plot of ratio of observed t o calculated relaxation time against ratio of solute t o solventmdecular volume for binary systems of rigid molecules (data from ref 27).-1.0- 0.5aaaa aaa e0 aa0 a 0 a80 R.I. C. Reviewgood agreement between experimental and calculated relaxation times,neglecting differences due to molecular shape and dipole location and directionin the molecule.It follows from equation 24 that for any given molecule in any given solventthe activation energies for dielectric relaxation (AH:) and viscous flow ( AHT)should be equal, and in many cases that is found experimentally.NON-RIGID POLAR MOLECULESFor an aromatic molecule which contains a rotatable polar group there is thepossibility of two relaxation processes i.e. molecular and group relaxations, thelatter having the shortest relaxation time. In more flexible molecules a range ofintramolecular relaxation processes may be possible.However, the usuallyinadequate number of data points in the absorption region often limits analysesto two relaxation time systems.Fig. 6. Cole-Cole plots for some aliphatic compounds in n-heptane solutions at 25 "C. Thecircles represent experimental points measured at 0.2, I .2,3.2,9.9,24.8,50.8, and 3 x 106 cm(data from ref 28).- 0.08E" bo8-0.1 6 10.04-0.08 lo.04n-octyl bromide.2.00 &' .2.04 \2.od ACrossley68Table2. Relaxation times (TO, 7 1 , 7 2 ) , distribution parameters (a) and relativecontributions for some n-alkyl compounds in n-heptane solution at 25 'C.28Di-n-butyl ether - - 15.4 3.5 0.75n-Dodecyl methyl ether - - 34.5 4.2 0.62n-Octylaldehyde - - 17.5 I .9 0.65n-Octylamine - - 15.4 I .8 0.83n-Octyl bromide 0.19 12.7- - n-Octan-2-one 0.23 4.0 -- - -Aliphatic compoundsCole-Cole plots for several aliphatic compounds in dilute solution are shownin Fig.6 and the relaxation analyses are presented in Table 2.28 Thorough andsystematic investigations of the n-bromoalkanes both as pure liquids and insolution have been made at microwave frequen~ies~~ and the data for n-bromooctane (Fig.6) showing a symmetrical distribution is typical. Thedistribution, which increases with increasing chain length, has been discussedin terms of a distribution of relaxation times due to segmental rotations, thelimits being given by two extreme values corresponding to -CH2Br andmolecular end-over-end rotations. In contrast, the data for some long chainethers,30 sulphides,30 and alcohols31 show a separation into two distinctabsorption regions. Furthermore, it is evident from Fig. 6 that di-n-butyl ether,=x cos 8=X sin 682Fig. 7. Calculation of the relativecontributions Ct and C2 of themolecular and group relaxation timesfrom group moments for an aromaticmolecule with a rotatable polar group.R.I.C.Reviewn-dodecyl methyl ether, n-octylaldehyde and n-aminooctane have two disper-sion regions and the analyses (Table 2) are consistent with contributions froman intramolecular and a molecular relaxation time. The form of the Cole-Coleplots for n-bromooctane and n-octan-Zone suggest a distribution of relaxationtimes and the short TO values, especially for the ketone, indicate that rotationsof the polar end groups are the dominant processes.It has been suggested28 that aliphatic molecules in which the polar grouprotation occurs about a bond other than the C-C bond, as well as around theC--C bond, tend to show two relaxation processes, and in dilute solution theintramolecular process is dominant. Those molecules in which the polargroup rotates only about a C-C bond tend to show a distribution of relaxationtimes.However, this is a simplification of a complex situation which is furthercomplicated with increasing chain length and the number and location of polarsubstituent groups.Aromatic compoundsFor an aromatic molecule containing a freely rotating polar group it is pos-sible to calculate the contributions from molecular and group relaxationsusing group moments (Fig. 7). Thus analyses are given a useful starting pointsince the molecular relaxation time may be estimated by analogy with rigidmolecules of similar shape and size.The results of Davies and Meakins (Fig. 8) give an excellent illustration ofthe two dielectric dispersion regions for two substituted phen0ls.3~ Suchabsorption curves have been the exception, due to the often relatively small71/72 ratios, small C values and limited number of experimental points; moreusually a symmetrical depressed centre Cole-Cole plot is obtained.The intra-molecular rotation of such groups as -CH2Cl, -CH&N, -0CH3, -CHO,Fig. 8. The dielectric absorption of (a)0.76 M 2,4,6-tri-t-butylphenol and (b)0.28 M 2,6-di-t-butyl-4-bromophenolin decalin solution at 20°C; subscripts,(i) molecular relaxation, (ii) grouprelaxation (data from ref 32). log (frequency)O iCrossley 8Table 3. Relaxation times (TO) and distributionparameters (a) for molecules similar instructure to diphenyl ether at 20 OC.33Compound 1012TO (s) a- Diphenyl ether 4Diphenyl methane 21.2 -n-Butyl phenyl ether 25 0.IBis(diphenylmethy1) ether 130 0.2n-Decyl ether 39 0.2Diphenyl sulphide 13 0.3-COCH3,-OH, -NH2 etc., have been the subject of numerous investiga-tions,33 and the enthalpy of activation for group rotation has been used as ameasure of the potential barrier.For aromatic amines the short relaxation time may be associated with NH2group rotation or alternatively an inversion mechanism. Several studies34 havebeen made in order to establish which process is responsible and though thereis evidence in support of the inversion mechanism it is by no means conclusive.The abnormally short relaxation times of diphenyl ether and several similarmolecules (Table 3) have generated considerable interest35 and severalmechanisms have been considered to account for their rapid relaxationprocesses.'Double internal rotation', the coupled rotation of the phenyl groupsabout the C-0 bonds, seems to provide a more acceptable explanation thanthe mesomeric shift, inversion, atomic dipole and phenoxy group rotationmechanisms.ASSOCIATED LIQUIDSDielectric relaxation provides a very sensitive means of detecting molecularinteraction. The formation of polar complexes, which reorientate under theinfluence of the applied field, leads to relaxation times considerably longerthan those for the uncomplexed species. In many instances the interaction maynot be sufficiently strong to allow the formation of a stable complex; however,the relaxation times of the molecules will be lengthened by the increasedresistance to rotation brought about by the attractive influence of neighbour-ing molecules.Low frequency permittivity measurements and dipole moment data havebeen extensively applied to structural studies of associated systems.36 Di-electric relaxation is capable of providing additional information for suchsystems.37Water38The dielectric absorption of water may be characterized by a single Debyerelaxation: at 20 "C T 'V 10 x 10-l2 s, the Cole-Cole distribution parameter isalmost indistinguishable from zero, and the dipole moment calculated fromequation 3 is the same as for the vapour. Thus the dielectric absorption ofwater is similar to that anticipated for a simple liquid.In view of the molecularcomplexity of liquid water, this behaviour is remarkable.The many experi-84 R. I. C. Reviewmental and theoretical discussions which have arisen from these results and theabnormally large E , - n D values have been summarized by Davies.39Measurements on very dilute solutions of water in non-polar solvents againreveal single relaxation times and dipole moments similar to the gas phase~alue.~O At higher concentrations in p-dioxane solutions there are tworelaxation times and the contribution from the shortest relaxation time de-creases with increased water c~ncentration.~~ A plausible interpretation of theseresults is that in the pure liquid during the extremely short time required forrelaxation the molecules are in the same environment, in the dioxane solutionsa second environment is provided and an additional relaxation time isobserved.AlcoholsAlthough alcohols have probably been studied more extensively than any othergroup of compounds and their dielectric absorption is well established, acomplete molecular model capable of explaining the results is as yet unavail-able.Pure liquid primary aliphatic alcohols show a large low frequencyabsorption (71) and two much smaller high frequency absorptions (72 and 4 . 4 2The 71 process gradually disappears on dilution with a non-polar solvent and invery dilute solution the absorption is dominated by the highest frequencypr0cess.3~9~3 For alcohols such as 2-methylheptan-3-01, which predominantlyassociate into small multimers (dimers and possibly trimers) due to the stericenvironment of the hydroxyl group, only the two high frequency processes (72and 73) are observed and again 7 3 is dominant at high dilutions (Fig.9).44Some isomers of octyl alcohol have provided an attractive and useful seriesof compounds to study. The molar volumes and vapour phase dipole momentsof these compounds are very similar, thus any differences in their dielectricbehaviour may be due to differences in association as a result of the widevariety of steric effects which can be obtained. The temperature and pressuredependence of the dielectric absorption and Kirkwood g values, whereg = p2 (liquid)/p2 (vapour), infer that for n-alcohols association is predomi-nantly into linear chains whereas the more hindered isomers predominantlyassociate into closed dimers.45It now seems reasonable to explain 73 in terms of the rotation of -OHgroups about their C-0 bonds, and 7 2 is probably due to monomer or -ORgroup rotations.The Debye-like process (TI) which lengthens regularly withincreasing number of carbon atoms for the n-alcohols is less readily explained.It is undoubtedly due to a mechanism which is sensitive to the steric environ-ment of the hydroxyl group since it is not evident for the more hinderedalcohols. The molecular size dependence of 71 suggests that rotation ofmolecular units might be responsible, but if association is into linear chains thegreat distribution of sizes for such species would give rise to a distribution ofrelaxation times. Association into highly polar cyclic tetramers has beenproposed to account for the zero distribution parameters and the molecularsize dependence shown by 71 for the n-alcohols.46 A less specific but moregenerally acceptable interpretation considers the low frequency dielectricrelaxation of alcohols in terms of hydrogen bond rupture followed by therotation of monomers where the former is the rate determining step.Crossley 80.0 I I I I IE"EQ - E m0.43.0 ,0.2 ,0.4- ,0.6 10.8 ) l .OFig. 9. Normalized Cole-Cole plotsfor (a) 2-methylheptan-3-01 and (b)n-hexanol at the mole fractions indi-cated in n-heptane solution at 25 "C.The circles represent experimentalpoints at the wavelengths (cm)indicated (data from ref 44).0.0 I I I I @\ 50EN€0 - E m- 0.4' 0.0 I I I IENEO - E m- 0.40.230EQ - E m-0.4Intramolecular hydrogen bondsThere have been some dielectric relaxation studies of intermolecularly hydro-gen bonded phenols and amines,47 but the majority of work has been con-cerned with intramolecular association in these compounds.It was mentioned earlier that the dielectric absorption of a molecule with arotatable polar group may show two relaxation times. If a second group isintroduced ortho to the first then the group rotation may be restricted by sterichindrance, if the groups are large, or, in certain cases, by intramolecularhydrogen bonding.The extreme effect of intramolecular hydrogen bonding is shown bymolecules such as o-hydroxyacetophenone48 and ~alicylaldehyde~g which have86 R.I. C. ReviewTable 4. Mean relaxation times (TO) and Cole-Cole distribution parameters ( a ) for somephenols and anilines in benzene solution at20 OC.50Compound 101270 (s) a2.6-Di bromophenol 23 0.132,6-Dibromo-p-nitrophenol 56 0.032.4-Di bromophenol 23 0.242,6-Dichloro-p-nitrophenol 69 02,6-Dich loro-p-n itroan i I i n e 6 I 0Cole-Cole distribution parameters indistinguishable from zero and relaxationtimes similar to those for rigid molecules of the same shape and size. Incontrast a molecule such as m-methoxyacetophenone, in which hydrogen bond-ing barriers to group rotation are minimized, has a shorter relaxation time anda non-zero distribution parameter4* indicating a contribution from grouprotation.o-Hyd roxyacetophenone Sal icylaldeh yde m- MethoxyacetophenoneTable 4 shows the relaxation data for some halophenols and anilines inbenzene solution.50 The relatively long mean relaxation times and smalldistribution parameters for the p-nitro compounds suggest that there is littlecontribution from -OH or -NH2 group rotation.Conversely, the relaxationdata for 2,6-dibromophenol and 2,4-dibromophenol indicate contributionsfrom -OH group rotation. These results infer that the nitro group aids thedelocalization of r-electrons between the oxygen (or nitrogen) atom of the OH(or NH2) group and the ring, which results in a stronger intramolecularhydrogen bond. For 2,4-dibromophenol there is the possibility of the followingequilibrium :cis transThe effect of such an equilibrium on the dielectric absorption, whichdepends on the strength of the intramolecular bond, has been investigated.49 Itis possible to calculate the fraction of molecules (x) in the trans form from theobserved dipole moment (pObs) and the calculated moments of the cis (pC) andtrans (pT) forms using the expression:d b s = w; + (1 - 4r-L: 25Crossley 8Table 5.Free energy of activation differences AAGt (J mol-l) forchloroethanes in several solvents (relative to cyclohexane) at 25 0C.54SolventCompoundBenzene p-Xylene Mesitylene p-DioxaneI, I -Dichloroethane I170 I300 I420 I960I ,2-Dichloroethane I250 2050 2470 2970I, I, I-Trichloroethane 590 710 I250 1710I, I ,ZTrichloroethane I340 I800 2090 2090I, I, I ,ZTetrachloroethane I300 I550 2050 2760I, I ,2,2-Tetrachloroethane I550 2170 3140 3010I , I, I ,2,2-Pentachloroethane I630 2090 2970 3556Solute-solven t interact ionsThe study of rigid polar molecules, capable of proton donation, in non-polarsolvents of varying basicity and in an inert reference solvent, provides a meansof evaluating the relative acidities and basicities of the solutes and solventsrespectively.The strength of the interaction may be estimated from5126where 71 is the relaxation time in the inert solvent, 72 is the relaxation time inthe potentially basic solvent and AG] and AG; are the free energies of activa-tion for dipole rotation under conditions 1 and 2 respectively. Such studieshave been carried out for chloroform in a variety of environments52 and somearomatic amines in benzene and dioxane solution.53 Table 5 summarizes adetailed study of some chloroethanes, using cyclohexane as the inert referencesolvent, in a variety of s0lvents.5~ These results show that in each of the basicsolvents the ethane interaction increases with increasing protonic character ofthe ethane hydrogen atom, and for all the ethanes the interaction with thesolvent increases with increasing solvent basicity.The results for 1,2-dichloro-ethane are somewhat anomalous and have been disc~ssed;5~,5* the degree ofinteraction is also reflected by the dipole moment changes but the inaccuraciesin this parameter and its solvent dependence even for non-interacting mole-cules often limit its usefulness as an indication of solute-solvent interactionstrengths.Charge-transfer interactionsDielectric measurements have been made on dilute solutions of the non-polariodine in the non-polar solvents benzene and p-dioxane.55 Both systems have adielectric loss in the microwave region and a relaxation time, approximately3 x 10-l2 s, much shorter than would be expected for a stable polar complex.The relaxation has been discussed in terms of a rapid exchange mechanismbetween the polar molecular charge-transfer complex and its non-polarconstituents.This work also included mixtures of the non-polar electronacceptor 1,3,S-trinitrobenzene and the polar potential electron donors tri-ethylamine, tributylamine and triphenylamine in dilute p-dioxane solution.Trinitrobenzene and triethylamine are shown to form a stable complex, the88 R.I.C. Reviewsystem shows a single relaxation time which indicates little or no contributionfrom the uncomplexed amine. The data for trinitrobenzene-tributylamineshows a separation into two dispersion regions which are due to the re-orientation of the charge-transfer complex and the uncomplexed amine. It wasnot possible to detect any molecular complex for the trinitrobenzene-triphenylamine mixtures in p-dioxane solution.Dielectric relaxation data for systems involving non-polar electron acceptormolecules, tetracyanoethylene, tetrachloro-p-benzoquinone, 2,5-dichloro-p-benzoquinone and p-benzoquinone and the non-polar electron donor solventsmesitylene and p-dioxane have been reported (Table 6).560Tetracyanoethylene Tetrachloro-p-benzoq u inone2,5-Dichloro-p-Benzoquinone p-Benzoqu i noneMolecular volume considerations predict a relaxation time of 20 x 10-l2 sfor the p-benzoquinone-mesitylene system, which is much longer than theobserved relaxation time.Similarly, the relaxation time of a tetracyano-ethylene-mesitylene complex would be close to that for p-benzoquinone-mesitylene, whereas the observed values are vastly different. These resultssuggest that the relaxation times are not due to the reorientation of the com-plexes, and it is probable that the lifetimes of these complexes are shorter thanthe time required for the complex to rotate. The relaxation times (Table 6)Table 6. Relaxation times (TO) and dipole moments ( p ) for somenon-polar electron acceptor compounds in non-polar electrondonor solvents at 20 OC.56\ \ e o n or Mesitylene p-DioxoneAcceptor \.., 101270 (5) p (D) 10'27" (s) p (D)Tetracyanoethylene 13.3 0.80 15.0 0.78Tet rac h loro-p- benzoqui none I 0.5 0.82 10.7 0.752,5-Dichloro-p-benzoquinone 9.3 0.42 7.0 0.4p-Benzoquinone 7.7 0.38 5.7 0.3I ,3,5-Trinitrobenzene - - 3 .O 0.4Crosstey 8Table 7.Relaxation times (TO) and dipole moments ( p ) for maleicand phthalic anhydride in several solvents at 25 0C.57Maleic anhydride Phthalic anhydrideSolvent 10'270 (S) p (D) 101270 (S) p (D)Carbon tetrachloride 5.8 3.86 14.0 5.87Benzene 9.4 3.55 18.4 4.71Mesi ty len e 12.6 3.59 28. I 4.85p-Xy I en e I I .3 3.69 22.0 4.75p- Dioxane 15.9 3.93 - -show that the lifetimes of the complexes formed with either donor moleculeincrease in the order of increased acceptor strength i.e.p-benzoquinone < 2,5-dichloro-p-benzoquinone< tetrachloro-p-benzoquinone < tetrachloroethylene.This type of study has been extended to include polar electron acceptors,maleic and phthalic anhydrides, with non-polar electron donors solvents,mesitylene, p-xylene, benzene, and p-dioxane, and an inert reference solventcarbon tetrachloride.57 In all cases the dielectric absorptions are close to asingle Debye relaxation and the results (Table 7) are interpreted on the basis ofa relaxation rate which is a weighted average of the relaxation rates of the un-complexed anhydride and the complex.The fact that the dipole moments incarbon tetrachloride are greater than those in the aromatic solvents and lessthan those inp-dioxane is due to the broadside and head-to-tail alignments ofthe respective complexes with the result that the induced moments oppose andreinforce the parent dipole moment respectively.Interaction between polar moleculesThe dielectric absorption of chloroform, acetone, diethyl ether and triethyl-amine each in cyclohexane and binary polar mixtures of chloroform withacetone, diethyl ether and triethylamine in cyclohexane has been examined.58For each of the polar molecules the dielectric absorption may be characterizedby a single relaxation time.All the mixtures show considerable distributionparameters, much larger than anticipated in view of the similar relaxationtimes for the components, and have relaxation times longer than those of eitherconstituent. These results (Table 8) were interpreted in terms of a contributionfrom a stable polar complex (71) formed between chloroform and the electrondonor in addition to the uncomplexed polar components (72).If the dipolemoment of the complex can be estimated it is possible to calculate the equili-brium constants for these interactions, from the C values.MIXTURES OF POLAR COMPOUNDSMeasurements on mixtures of rigid non-interacting polar molecules whoseindividual dielectric properties are well characterized, provides an excellentmeans of examining the validity of analysing dielectric data for more than onerelaxation time, and they should show whether or not the components retain90 R.I. C. ReviewTable 8. Relaxation times (TO, 71, TZ), relative contributions (C&) and distributionparameters (a) for some electron donor and acceptor compounds in cyclohexanesolution at 25 ‘C.58- - - Acetone 2.6 0Ether 2.5 0Triethylamine 8.0 0 - - -Chloroform 4. I 0Acetone and chloroform 4.5 0.22 21 3.5 0.29Ether and chloroform 7.0 0.08 13 4.6 0.43Triethylamine and chloroform I I .3 0.17 37 6.8 0.39- - -- -Table 9. Relaxation times (71-benzophenone and 7-tetrahydrofuran), reducedrelaxation time (./.I), calculated and experimental relative contributions (771p:/r]~p:and C1/Cz respectively) for some tertiary and binary mixtures at 20 “C; f1 gives themole fraction of benzophenone in tetrahydrofuran.59Benzo p h en o n e i n benzeneTetrahydrofu ran - 21.2Benzophenone in tetrahydrofuranfi = 0.108 25.5fi = 0.235 35.0fi = 0.492 67.9Benzophenone in tetrahydrofuranin benzenefi = 0.108 20.5fi = 0.235 21 .of .= 0.492 20.02.96 30.5 0.57 0.372.97 37.6 I .53 0.953.13 29.6 4.0 2.973.0 - 0.38 0.373. I - I .o 0.953 . I - 2.9 2.97their individuality in the mixture. The data for the mixtures may be analysedusing equations 14-1 6, and the resulting relaxation times compared with thosefor each component when measured alone obtained using equations 7 and 8.In addition, the weight factors (C values) obtained from the analyses may becompared with those calculated, for a binary polar mixture, using:27where C1, nl and p1 are the weight contributions, concentration and dipolemoment of component 1 ; C2, nz, and p2 are the corresponding values forcomponent 2.Some of the results from such a study for binary mixtures ofpolar compounds and solutions of binary polar mixtures in a non-polarsolvent are presented in Table 9.59 For the solutions, the components retaintheir individuality and the calculated and observed C values are in goodagreement. In general, especially when viscosity factors are considered, therelaxation times obtained for the binary mixtures are in good agreement withthose for the components. However, for polar liquid mixtures the experimentalCl/Cz ratios are about 1.3 to 2.3 times longer than calculated. This is probablyCrossley 9because the assumption Canp2 is insufficient for the binary mixtures and anyaccurate calculation must take into account the influence of the internal field.NON-POLAR LIQUIDSWhiffen60 found small dielectric losses for benzene, carbon tetrachloride,cyclohexane and decalin, all of which have no permanent electric dipolemoment, in the frequency range 0.3 to 1.2 cm-1.Plots of loss tangent againstfrequency suggested Debye behaviour and relaxation times (- 1 x 10-l2 s) ofthe order of the time between molecular collisions. Other similar measurementsat microwave frequencies61 served to confirm these findings and to show thatthe absorptions are not the result of impurities such as water.G2 It seemedprobable that these absorptions result from dipole moments (< 0.1 D) inducedin molecular collisions.The induced dipole is then regarded as changing direc-tion not by molecular rotation, the relaxation times are far too short, butbecause of a new collision at another instant with another neighbour. Conven-tional dielectric measuring techniques have not been well suited to precisestudies for such systems since the absorption is not wholly described in themicrowave region and the losses are often within the experimental error. Theadvent of interferometric methods has permitted accurate measurements of theabsorption coefficient 01 in the sub-millimetre region and the general form ofthe absorptions has been established. The dielectric loss is related to theabsorption coefficient2 7rTTE‘IVna = ___- Nwhere n is the refractive index and ‘v the frequency in wave numbers.28Fig. 10.The microwave and far infrared absorption of pure liquid benzene, solid line representsthe 01 values, dashed line the E” values (data from ref 63).92 R.I.C. ReviewThe absorption of liquid benzene is shown in terms of 01 and E” in Fig. 10.63Similar curves have been obtained for carbon tetrachloride, carbon disulphide,p-dioxane and other non-polar liquids.63These findings and the means of investigating the sub-millimetre region (10-100 cm-1) have prompted several studies of rigid polar molecules,63~64 all ofwhich absorb in this region which had long been a subject of conjecture inview of the anomalously large E , - n j values shown by many rigid polarliquids.65 The subject is still in its infancy, and although a substantial amountof experimental data now exists, no completely satisfactory quantitative inter-pretation is available.For the non-polar liquids the qualitative interpretationsof Whiffen have not been significantly improved upon, the absorption for thepolar liquids is also thought to arise from induced moments and the relaxationhas been considered in terms of liquid lattice vibrations.66POLYMER SOLUTIONSThe vast majority of dielectric studies of polymers have been concerned withthe solid phase and have provided valuable information. However, someattention has been given to polymers in solution, and an excellent reviewarticle has recently been published on this subject.67 Some data for poly-n-butyl isocyanate and polymethyl methacrylate are shown in Table 10 toillustrate’ this type of work.39 For the latter polymer-in-toluene solution therelaxation times show only small variations over a 120-fold increase inmolecular weight.This indicates that when a sufficiently large multimer isformed, the relaxation involves the motion of small segmental units of thepolymer chains. Conversely, the relaxation time for poly-n-butyl isocyanate inbenzene solution increases with increased molecular weight and this polymeris classified as a rigid rod, the relaxation involving molecular rotation.Dielectric data have been used to distinguish between molecular configura-tions for polymers which may exist either as a rigid helix or in a random coildepending on solvent and temperature c~nditions.~gConsiderable information has been obtained from dielectric absorptionmeasurements of proteins and their aqueous solutions.Table 10.Relaxation data for some polymer solutions.39PolymerDegree ofpolymerization Solvent T (“C) T (s)Polymethyl methacrylate 1409505000I750010-5 mo/ecu/arweightPoly-n-butyl isocyanate I .43 -847.3423Toluene -9.0 0.67 x 10-80.77 x 10-80.71 x 10-80.52 x 10-8Benzene 22.5 I .58 x 10-625 x 10-6224 x 10-65000 x 10-6~~Crossley 9Table 11. Dielectric parameters for DNA inaqueous solution, 15 OC.6910-6 molecular wt Concn % (C) p (D) 10% (s)7520.80.40. I0 . 2 44000 4.4- 8400 0.84- 3800 0.380.2 I250 0.1250.02 640 0.0640.02 120 0.012For proteins, e.g. a body tissue, the permittivity falls from possibly severalmillions to about five over 11 decades of frequency (Fig.11). The a-dispersionarises from a Maxwell-Wagner relaxation effect, which is not peculiar toproteins and is due to electrostatic effects between particles of different permit-tivity and conductivity. Two factors may contribute to the 13-dispersion: (i) alower permittivity resulting from the breakdown of material into small units,(ii) the reorientation of the polar protein molecules. The y-dispersion is due tothe aqueous nature of body tissue and is very similar to the dispersion ofwater. An additional dispersion (8 region) has been detected, between the /3and y regions, for some proteins in aqueous solution and is probably due towater bound to the protein.G8Finally the dielectric parameters obtained for deoxyribose nucleic acid(DNA) measured between 30 Hz and 5 MHz are presented in Table 11.BothFig. I I. The frequency variation of the permittivity of muscular tissue.I2 4 , 6 1094 R.I.C. Reviewthe relaxation time and dipole moment values increase with increased molecularweight indicating that DNA behaves as a rigid rod with its resultant electricdipole moment acting along the long axis.69SUMMARYIt is hoped that the previous pages will have served to introduce the greatpotential of dielectric relaxation studies. Those wishing to actively pursue thesubject will require a more in-depth treatment than that given here and shouldfind the recently published text, Dielectric properties and molecular behaviour,70an excellent starting point.REFERENCES1 J.W. 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ISSN:0035-8940    

DOI:10.1039/RR9710400069

出版商:RSC    

年代:1971

数据来源: RSC