ANNUAL REPORTSON THEPROGRESS OF CHEMISTRY.GENERAL AND PHYSICAL CHEMISTRY.1. SURFACE CHEMISTRY.THIS section is restricted to certain aspects of monolayers at mobile interfaceswhich have evoked interest in recent years and have not been dealt with inearlier Reports.l.2The reduction of surface tension by the monolayer, termed the “ surfacepressure,’’ and previously denoted by P, is now denoted by IT in conformitywith the revised nomenclature. The change in phase boundary potential,or “ surface potential,” is denoted by A V and is usually related to the mole-cular density (n, in molecules/cm?) by the Helmholtz equation AT‘ = 4mp,where p is the vertical component of the apparent dipole moment; A =1016/n, is the area (in a.2) occupied by each molecule in the surface.Themeasurement of surface pressure, surface potential, and mechanical pro-perties such as elasticity and rigidity have been outlined before.l,The Str2ccture of Momlayers.-Of the principal two-dimensional phaseswhose existence is well established, viz., gaseous, expanded, and condensed,the structure in the first two is now generally accepted. Gaseous filmsapproach the ideal relationship IIA = kT a t very low surface pressures(areas generally of the order 1000-10,000 A.2/molecule), and obey anequation of the Amagat type IT(A - A,) = skT a t higher pressures. Lang-muir’s explanation for the ‘‘ expanded ” type,ll3 which have no three-dimen-sional analogy and which obey the equation of state (II - IT,)(A - A,) = C,has been generally accepted. In this equation TI, allows for the (negative)spreading pressure of the chains, A, is the ‘‘ co-area ” of the polar head-group, and C a constant equal to kT for un-ionised groups? As pointedout below, this explanation has been strengthened by work upon insolublemonolayers a t the oil-water interface.Considerable discussion still centres, however, upon the condensed films.The 1I-A curves of condensed films of simple long-chain hydrocarbonderivatives generally consist of two approximately linear portions, the firstand more compressible part extrapolating to about 22-30 A?, dependingupon the nature of the head-group, and the second showing a very lowcompressibility and extrapolating to 20-21 ~ .2 in all cases. N. K. Adam tihas ascribed these to close-packed heads and close-packed chains respect-N.K. Adam, Ann. Reports, 1936, 33, 103.J. H. Schulman, i&id., 1939, 36, 94.I. Langmuir, J . Chem. Physics, 1933, 1, 756.J. Marsden and E. K. Rideal, J . , 1938, 1163.“ Physics and Chemistry of Surfaces,” 3rd Edition, 1941 (Oxford Univ. Press)6 GENERAL AND PHYSIUAL CHEMISTRP.ively, whereas W. D. Harkins and G. E. Boyd class them as liquid andsolid, and D. E. Dervichian claims that breaks occur at areas prescribedby the packing in the various three-dimensional crystalline forms, e.g.,at 18.5, 19.5, 20.5, 22, and 23.5 A . ~ in the case of fatty acids. Dervichian’semphasis upon a close correspondence between the two-dimensional con-densed film and the three-dimensional crystal has been strongly criticised,For instance, Harkins and Boyd8 point out that accurate n-A curves offatty acids do not show breaks at his predicted areas, and that the liquidcondensed phase cannot be crystalline (as postulated by Dervichian), sincetheir viscosity measurements exactly fit the theory of liquid monolayersdeveloped by W.J. Moore and H. E~ring.~ In the Reporter’s opinion lothe available evidence is strongly opposed to any such general thesis,lla conclusion supported by. subsequent work. Although a powerful inter-action between polar group and the aqueous substrate is necessary to obtainspreading, it seems unlikely that the magnitude and direction of the forcesinvolved would exactly compensate in all cases for those in the crystallinestate.In the very incompressible region the recent alternative suggestionthat the long-chains are vertically orientated and close-packed l2 is basedupon the agreement between the compressibility of the monolayer and oflong- chain hydrocarbons in bulk, and between the cross-sectional areasof the monolayer and of paraffins near their melting points (both 19-20A .~ ) , i.e., in what is believed to be their most nearly oomparable states.12This formulation is supported by surface-potential measurements uponethyl stearate, the apparent surface moment of which can only be satis-factorily explained on the basis of vertically orientated chains.12The structure of the more compressible region of condensed films doesnot seem to be so definitely established, different factors appearing to beoperative with different head-groups.12 For instance, in some cases, e.g.,p-hexadecylphenyl derivatives, C,,H,,*C,H,X, the limiting area is deter-mined by the “ size ” of the phenyl nucleus, since with X = -OH, -NH2,or -OCH, the limiting area remains constant at ca.25 A . ~ ; in others (e.g.,cis- and trans-unsaturated compounds) , the hydrocarbon chain packingseems to be the decisive factor, and in certain others hydrogen-bond form-ation between the head-groups lo. l3 (e.g., long-chain amides, acetamides,and ureas).is also controvereial. In the more compressible “ liquid ” region, all filmsso far studied behave as Newtonian liquids, the surface viscosity 7 obeyingthe equation, log r) = log q, + kII, as predicted by Moore and Eyringgfrom Eyring’s theory of the liquid state.It seems reasonable thereforeThe suggested classification of condensed films as “ liquid ” or “ solid ”6 J . Physical Chem., 1911, 45, 20. J . Chem. Physics, 1939, 7, 931.Ibid., 1938, 6, 391. Ibid., 1940, 8, 129.10 A. E. Alexander, Trans. Paraduy Soc., 1941, 57, 426.11 See also A. Q. Nasini and Q. Mattei, Obzzelta, 1940, 70, 697.12 A. E. Alexander, Proc. Roy. Soo., 1942, A, 178, 486. Is Idem, aid., p. 470ALEXANDER SWRFAUP) UEIEMISTBY. 7to term this region “liquid condensed,” but the use of the t a m “solidcondensed” for aZE films in the other condensed region is less justifiable.Here some films are quite mobile (e.g., methyl ketones and esters), othersdefinitely very rigid (e.g., heavy-metal soaps, ureas, and amides).Thelong-chain alcohols, acids (un-ionised) ,I4 and esters,15 exhibit anomaloussurface viscosity, and this is possibly general for all mobile films in thisregion, but to term these “ solid ” seems unnecessarily confusing. Withthe more rigid films a definite elastic modulus can be obtained by applyinga torque to a diso on the surface,16 or by rotating a disc in the liquid andobserving the motion of dust particles on the film.17Spreading and Phase Changes of Monolayers.-By means of the Clapeyronequation in the form h = T(aH/aT)(Ap - AB) it is possible to determinethe latent heat of spreading (A) from the crystal to the monolayer.l* (AFand As are areas occupied by film and solid respeotively.) Some veryaccurate II-A and IT-T measurements have been carried out recently forthis purpose by Harkins and his co-workers, with myristic and pentadecylicacids, and latent heats and entropies of spreading determined.lgSuch measurements lead naturally to a consideration of the thermo-dynamics of monolayers, such as the energy, entropy, and heats of spreadingand expansion, and the order of phase change~.~*~OThere has been much discussion recently concerning the fundamentalstability of duplex or multi-layers, such as are formed initially by a mixtureof an .inert oil containing an amphipathic compound a8 “ spreader.” Thishas some practical bearing upon the use of oil films in antimalarial measures,and in preventing the evaporation of water, as discussed below.Withsimple long-chain acids, alcohols, etc., it seems that the equilibrium systemconsists of monolayer and lenses, but with polymerised spreaders, or withcertain dyes in the aqueous phase, stable (or metastable) thick films couldbe obtained.21 It is probably very significant that in all such cases theinterfacial films are rigid. W. A. Zisman22 has recently made a verythorough study of the spreading of drops of an inert oil containing polarcompounds, on the lines first initiated by I. Langmuir and K. B. BlodgettF8and as expected, maximum spreading is induced by compounds ionised a tthe interface (e.g., acids on alkaline solutions).I* L. Fourt and W. D. Harkins, J . Physical Chem., 1938, 42, 897; G. E. Boyd andW. D. Harkins, J.Amer. Chem. SOC., 1939, 61, 1188; L. E. Copeland, W. D. Harkins,and G. E. Boyd, J . Chern. Physics, 1942,10, 357.lS A. A. Trapeznikov, Acta Physicochim. U.R.S.S., 1038, 9, 273; 1939, 10, 65;Dokhdy Akad. Nazclc. U.R.S.S., 1941,30,319.l6 H. Mouquin and E. K. Rideal, Proc. Roy. Soo., 1027, A , 114, 690; A. A.Trapeznikov and P. Rebinder, Compt. r e d . (Doklady) U.R.S.S., 1938, 18, 185.l7 A. E. Bresler and S. E. Bresler, J . Physiccrl Chem. U.S.S.R., 1940, 14, 1604.l@ W. D. Harkins and G. C. Nutting, J. Amer. Chem. Soc., 1939,61, 1702.*O W. D. Harkins, T. F. Young and G. E. Boyd, J. Cheni. Physics, 1940, 8, 964;a1 E. Hepann and A. Yoffe, Tram. B’ar&y Soc., 1942, 38,408; 1943, 89, 217.22 J. Chern. Physics, 1941,9,634,729,789. 28 J. Pranklin Inat., 1934,218,143.A.Cary and E. K. Rideal, Proc. Roy. Soc., 1926, A , 109, 301.W. D. Harkins and L. E. Copeland, ibid., 1942, 10, 2728 GENERAL LND PHYSICAL CHEMISTRY.The kinetics of spreading, both from liquid droplets and from crystals,have also been further examined in some detail. In the usual method theedge of the advancing film is indicated by a suitable inert powder and photo-graphed by means of a cine ~amera.2~ Another technique, attractive inthat it avoids the possibility of contamination by powders, uses a series ofair electrodes, connected to an oscillograph with photographic registration,and arranged so that the spreading film passes radially under each in turn.25Spreading at interfaces can also be followed by the latter method.These experiments show that boundary spreading is in general veryrapid indeed compared with dissolution from the interface, even withcomparatively soluble substances (see also below).The rapidity of spreadingof insoluble oils such as oleic acid has indeed been used to investigate thestructure of adsorbed 26#27Evaporation through Monolayers.-The influence of boundary layersupon the movement of molecules from one phase to another has been littleinvestigated save for the study of evaporation from a water surface, andthis has derived much of its impetus from the very practical problem ofretarding evaporation from lakes and reservoirs in arid regions, such asSouth Africa, Australia, and parts of Russia. Although the practicalproblem does not appear to have been satisfactorily solved, yet some veryinteresting results have emerged.211 28Monolayers alone being used, it is generally agreed that only condensedfilms are appreciably effective, possibly not so surprising in view of the largeenergy barrier at the interface.With long-chain alcohols, such as octadecyl,as much as 95% reduction can be obtained at high film pressures. Markedreductions can also be obtained by the use of duplex oil films several pthick, by using suitable polymers as spreaders.21 These films, as pointedout above, appear to be reasonably stable in the laboratory, but deterior-ate when subjected to atmospheric conditions such as rain, wind, anddust.Reactions in Monolayers.-The study of reactions at the air-waterinterface by means of the monolayer technique is due principally to Ridealand his co-workers. Reactions can be followed by change of area or ofsurface potential, usually at constant IT.In almost all cases a very markeddependence upon the molecular orientation at the interface is observed-a true " steric '' factor which may modify the reaction velocity by a powerof 10 or more.24 E.g., F. Ford and D. A. Wilson, J. Physical Chem., 1938, 42, 1051 ; E. H. Mercer,Proc. Physical SOC., 1939, 51, 561 ; W. von Guttenberg, 2. Physik, 1941,118,22.25 A. E. Alexander and T. Teorell, unpublished.26 J. W. McBain and W. V. Spencer, J. Amer. Chem. SOC., 1940,62,239.2 7 A. E. Alexander, Trans. Faraday SOC., 1942, 38, 54.28 N. I. Glazov, J . Physical Chem. U.S.S.R., 1938, 11, 484; F.Sebba and H. V. A.Briscoe, J . , 1940, 106, 114, 128; F. Sebba and E. K. Rideal, Trans. Paraday SOC.,1941, 37, 273; A. S. Kheinman, J . Physical Chem. U.S.S.R., 1940, 14, 118; S. I.Sklyarenko, M. K. Baranaev, and K. I. Mezhueva, ibid., p. 839; I. Langmuir and V. J.Schaefer, J . Fmnklin In&. 1943, 235, 119ALEXANDER : SURFACE CHEMISTRY. 9The oxidation of unsaturated compounds by dilute permanganate,outlined in an earlier Report,l has been further studied with triolein 29 andwith cis- and trans-unsaturated acids.4 Erucic and brassidic acids showvery clearly the greater ease of packing of the hydrocarbon chains in thetrans-compound (and hence of removal of the double bond from the aqueousphase), since the oxidation of brassidic acid (trans) can be almost completelyinhibited by increasing the surface pressure, whereas the cis- form (erucic)is but little affected.Closely related to the above, and showing a common sensitivity tomolecular orientation, is the oxidation and subsequent polymerisation ofmonolayers of drying oils, by atmospheric oxygen, ozone, or ~ermanganate.~~Certain other types of polymerisation, vix., pepsin-formalin, cadaverine-pepsin, and tetra-aminobenzidine-stearaldehyde, have also been followedin mono layer^,^^ and it is clear that the method offers considerable scope inthis direction.The hydrolysis of lactones 32 and esters 33 and the lactonisation ofhydroxy-acids 34 also provide several points of interest.The alkalinehydrolysis of esters shows particularly well how the velocity of interfacialreactions can be controlled by means of a true steric factor.For example,with an expanded Hm of ethyl palmitate the short ethyl chain lies on thesurface, and the reaction can proceed quite rapidly (k = 0-04 min.-l). Oncompression to the condensed state the short chain is forced beneath thesurface, thus screening the ester group and reducing the velocity constantIc to 0-005 min.-l. The change can be depicted as below :Ethyl palmitate Ethyl palmitate Octadecyl acetate(expanded). (condensed). (condensed).I / /“0p&#.. milli-Debyes 536A , A.= ............ > 72k, min.-l ......... 0.04pcalc., 9 , 9 , 525\CH3193198200.005502525230.152s R. Mittelmann and R.C. Palmer, Trans. Furuduy SOC., 1942, 38, 506.30 G. Gee and E. K. Rideal, Proc. Roy. SOC., 1935, A , 153, 116, 129; J., 1937,772;P. H. Faucett, Drugs, OiZs and Paints, 1939, 54, 13; A. G. Nasini and P. Ghersa, AttiXO Cong. intern. Chim., 1939,4,236; C . Ockrent and W. H. Banks, Nature, 1940,145,861 ;A. G. Nasini and G. Mattei, Gazzetta, 1941, 71, 302, 422.31 S. E. Bresler, D. L. Talmud, and M. F. Yudin, J. Physical Chem. U.S.S.R.,1940, 14, 801; Actu Physicochirn. U.R.S.S., 1941, 14, 71.32 R. J. Fosbinder and E. K. Rideal, Proc. Roy. SOC., 1933, A , 143, 61.33 A. E. Alexander and J. H. Schulman, ibid., 1937, A , 161, 11.5; A. E. Alexander34 F. Kogl and E. Havinga, Rec. Truv. chim., 1940, 59, 601.and E. K. Rideal, ibid., 1937, A , 183, 70.A 10 QBIKEBAL AND PHYSICIAL OHEMISTRY.The validity of these configurations was confirmed by the agreement betweenthe observed surface moments (v) and those calculated by the Thomson-Eucken vectorial summation method.With a long-chain acetate, on theother hand, it is clear from the above diagram that condensation affordsno such protection, and the reaction is still rapid (k = 0.15 min.-1).The lactonisation of monolayers of hydroxy-fatty acids on acid sub-strates has shown in one case (@-hydroxyethyloctadecylmalonic acid) arate some lo3-lo4 times greater than in solution a t the same pH; inanother case (y-hydroxystearic acid) reaction was slower in the film.34 Thelatter effect has been ascribed to steric hindrance, whereas the former maybe due either to a particularly favourable proximity of hydroxyl andcarboxyl groups, or to a higher concentration of hydrogen ions in the surfacelayer than in the bulk medium.The latter explanation may well be thecorrect one, since various workers, from other considerations, have suggestedjust such a difference between surface and bulk It would be interest-ing to see this finally established by this rather direct method.The monolayer technique enabled J. S. Mitchell and E. K. Ridea136 toinvestigate the effect of ultra-violet radiation upon proteins, using as astarting point . monolayers of stearanilide and other simple compoundscontaining the -CO*NH- link. The presence of the aromatic nucleusinduced photochemical hydrolysia, leaving a monolayer of stearic acid, atwave-lengths of 2483 A.and less, the apparent quantum efficiency and rateof reaction being very sensitive to the orientation of the aromatic chromophorwith respect to the surface. The eame authors later 37 described the effectof irradiating protein monolayers, which causes liquefaction of the originallygel films and an increase in surface pressure. The above work suggeststhat peptide links adjacent to each chromophoric side chain undergophotolysis, the liberated chromQphoric residue dissolving into the substrate.The preliminary results of an essentially similar investigation have beenpublished by D. C. Carpenter.38Finally, mention may be made of the monolayer technique to studythe halogenation of phenols, where reaction is well known to proceedextremely rapidly in some cases (e.g., phenol with free bromine).Mono-layers of p-hexadecylphenol were employed, and the kinetics of reactionwith hypohalous acid, free halogen, and trihalide ions determined.39Reactions with a half-life of as little as 40 seconds could be followed satis-factorily. The halogenation of long-chain unsaturated compounds, par-ticularly by iodine chloride, which is closely related to t h e above, has beenfollowed in similar manner by A. G . Nasini and G. Mattei.40Insoluble Monolayers at the O i d Water Interface.-It is only comparativelyrecently that any marked success has been achieved in the study of insoluble36 J. F. Danielli, Proc. Roy. SOC., 1937, B, 122, 155; G. S. Hartley and J. W. Roe,Trans. Faraday Soc., 1940, 36, 101.36 Proc.Roy. SOC., 1937, A , 159, 206.3’ Ibid., 1938, A, 167,342.s9 A. E. Alexander, J., 1938, 729.s8 Science, 1939, 89, 251.O0 Qazzetta, 1940, 70, 635ALEXANDER: SURFAOI UHEMIS!FRY. 11monolayers at oil-water interfaces. This has been due to the difiicultieuencountered and not to the lack of interest, since such interfacial films areclearly of more direct relevance to the biologist and emulsion chemist thanare those at air-water interfaces,Some preliminary II-A measurements were obtained by F. A. Askewand J. I?. Danielli,41 using a slightly modified Langmuir-Adam surfacebalance, and bromobenzene as the oil phase. Various compounds weremeasured (egg-albumin, a-aminopalmitic acid, a long- chain amide, and amethyl cellulose), but very great difficulties with leaks and contaminationwere found.The various methods which appeared promising, oiz., the modifiedsurface balance, detachment methods such as the maximum pull on a ringor frame, and the Wilhelmy plate, were explored by A.E. Alexander andT. Te~rell,~Z using benzene as the oil phase, but only the ring method wasfound suitable. The interface is formed in a large Pyrex dish, and the inter-facial tension (’yo) determined. By means of a micrometer syringe, a smallbut accurately-known volume of a solution of the compound under examin-ation is then expelled into the interface, and the new interfacial tension(7) determined as before. Hence the spreading pressure (II) due to the filmcan be found, since II = yo - y.The molecular density at the interfaceis then increased, and the II-A curve constructed, not by reducing the areaas with air-water monolayers, but by further injections of spreading solution.This technique avoids errors due to leaks and varying contact angles, andgives an accuracy equal t o that of the usual film balance for air-watermonolayers.By this means accurate H-A curves for six compounds a t the benzene-water interface were determined, vix., for gliadin , serum albumin, lecithin,Iysolecithin, k e ~ h a l i n , ~ ~ and sodium cetyl sulphate. The results showinteresting similarities and differences when compared with those givenby the same compounds at the air-water interface. For example, theI-I-A curves are of the vapour expanded type in all cases, as might be expectedfrom the presence of the oil medium, but on compression the moleculesreach the same ultimate paoking as a t the air-water interface (see alsoHeymann and Yoffe),21indicating that the oil is eventually squeezed outcompletely from the interface.Films of the above phospholipoids at theair-water interface are of the liquid expanded type, obeying Langmuir’sequation of state (11 - rI,)(A - A,) = C , and this was also found to holdfor their interfacial films. The change from an air-water to a bcnzene-water interface had no effect on A , or C, but eliminated no almost completely(s.g., from 12.8 to 0-05 dyne/cm. with lysolecithin). As pointed out above,these results provide direct support for Langmuir’s theory of expandedfilms at air-water interfaces.The mechanical properties of interfacial films (e.g., elasticity and viscosity),O1 Proc.Roy. SOC., 1936, A , 15S, 695; g’rans. Paraday Soc., 1940, 36, 785.4 2 Ibid., 1939, 85, 727.43 A. E. Alexander, T. Teorell, and 0. G. Aborg, &bid., p. 120012 GENERAL AND PHYSIUAL CHEMISTRY.can be determined by an oscillating needlet4 or by the rotating themethods developed for air-water interfaces being followed closely, but thedetermination of the change in phase-boundary potential (AT) is not yetentirely satisfactory. The difliculty here arises from the high resistanceof even a thin film of benzene or other suitable non-polar oil. Some pre-liminary results for using a very thin film of benzene, a largepolonium air electrode, and a Lindemann or valve electrometer, indicatethat for a given molecular density the dipole orientation and change in phaseboundary potential are the same at both air-water and oil-water interfaces.When a suitable technique for interfacial potentials has been finallyevolved, a systematic examination of insoluble interfacial monolayers willbecome possible.The oil-water shows certain advantages over the air-water interface : spreading is facilitated (e.g., proteins spread readily andcompletely), and ionised substances, such as soaps and detergents, arestabilised sufficiently to make measurements feasible. A clear under-standing of interfacial monolayers should help considerably towards theelucidation of certain problems of adsorbed films.Some Applications of Monolayer Techniques to Other Fields.--No surveyof recent advances in surface chemistry would be complete without out-lining some of the ways in which the study of monolayers has assistedproblems in other branches of science.I n addition to those mentionedabove and certain physical aspects recently there are numerousapplications to organic and physical chemistry, to classical colloidal systemssuch as emulsions, and finally to biology, a field in which the obviousapplicability of surface studies is only just beginning to make itself manifest.(a) Structure of complex organic molecules. The use of monolayermeasurements as an aid towards elucidating the structure of complexorganic molecules is well shown by several recent publications.Thepossibilities inherent in such a method had been indicated by the earlywork of Adam and his co-workers on sterols,5 and in general terms it canbe said that the technique can indicate not only the general shape of themolecule but also the relative positions of the polar groups. (One polargroup at least is necessary to ensure suitable spreading.) For example,a molecule composed of a complex ring system with two polar groups inclose proximity would probably give a condensed film, thus allowing adirect comparison between the monolayer area and that calculated frommodels of likely structures. If, however, these polar groups are widelyseparated, then the molecule tends to lie flat on the surface, giving a gaseousor expanded film.Recent work on these lines has dealt with cerin, friedelin, and relatedcompounds,47 lupane derivatives:* the constitution of quillaic and oleanolic4 4 A.E. Alexander, Trans. Faraday Xoc., 1941, 37, 117.4 5 P. F. Pokhil, J . Physical Chem. U.S.S.R., 1939, 13, 301.46 A. E. Alexander, Reports Prog. Physics, 1943, 9, 158.4 7 N. L. Drake and J. K. Wolfe, J . Arner. Chem. SOC., 1940, 62, 3018.4 8 P. Bilham, E. R. H. Jones, and R. J. Meakins, J., 1941, 761ALEXANDER : SURFACE CHEMISTRY. 13acids,49 and the position of the carboxyl group in certain triterpene acids.60S. Stallberg and E. Stenhagen, in a series of papersY5l have dealt with mono-layers of compounds with branched hydrocarbon chains, with the objectof elucidating the structure of phthioic acid s2 and other complex biologicalcompounds.I n this connection it is important to emphasise the value of the multi-layer technique for inducing crystallisation of complex organic molecules,soften intractable to ordinary methods, so that additional information fromX-ray analysis might become available.(b) Emulsions.Two fundamental papers 54 upon the formation, stability,and structure of oil-in-water and water-in-oil emulsions were based uponmonolayer studies at the air-water interface as detailed in an earlier Report.2With some two- component monolayers, such as cholesterol and sodiumcetyl sulphate, a marked molecular association, or “ complex ” formation,is found, as evidenced by the reduction of the surface or interfacial tension,and owing to this extremely low oil-water tension very stable emulsionsare formed on shaking a mixture of cholesterol in oil and sodium cetylsulphate in water.A charged interfacial monolayer, as given by the abovesystem for example, was found to promote oil-in-water emulsions, whereason discharge, as by addition of calcium ion to an ordinary soap, rigid filmsare produced which tend to link the oil droplets together, enclosing theaqueous phase and so forming the invert water-in-oil type.One phenomenon, which has arousedconsiderable discussion during the past decade is the anomalously slowrate of transfer of molecules from bulk solution to a freshly formed interface,generally termed “ surface ageing.” 55 Soaps and synthetic detergentsoften require several days for equilibration if their concentration is belowthat for micelle formation; if above it, equilibration is very rapid.Un-ionised amphipathic compounds such as alcohols and acids also show thesame slow accumulation, the anomaly increasing with the chain length.A recent examination from a new angle shows that the time factor isreduced by the presence of simple capillary-active substances (e.g. , ethylacetate), and eliminated at an oil-water interface, even when the oil phaseis only of molecular dimensions (e.g., oleic acid or ethyl laurate monolayers) .56These results appear to rule out earlier theories, which postulated electro-static potential barriers at the surface 57 or the formation of surface pellicles(c) The surface ageing of solutions.49 P.Bilham and G. A. R. Kon, J., 1941, 552.60 P. Bilham, G. A. R. Kon, and W. C. J. Ross, J . , 1942, 35.61 Svensk Kern. Tidskr., 1940, 52, 223; 1941, 53, 335; J. Biol. Chem., 1941, 139,62 N. Polgar and S i r R. Robinson, J., 1943, 615.63 For recent reviews, see refs. (2) and (46).54 J. H. Schulman and E. G. Cockbain, Trans. Faraday SOC., 1940, 36, 651, 661;55 For early references, see ref. (5).ti6 A. E. Alexander, Trans. Paraday SOC., 1941, 37, 15.6 1 K. S. G. Doss, Gurr. Sci., 1935, 4, 405; 1937, 5, 645; Kolloid Z., 1939, 86, 205;345; 1942,143,171 ; 1943,148, 685.T. P. Hoar and J. H. Schulman, Nature, 1943, 152, 102.cf. G. S. Hartley and J. W. Roe, Trans. Faraday SOC., 1940, 86, 10114 GENERAL AND PHYSICAL CHEMISTRY.more than unimolecular in thi~kness,~8 and it seems that the slow, rate-determining step is the penetration, into the surface layer, of the hydrophobicportion of the molecule.A slow accumulation is also observed with polar compounds in non-aqueous media, both t o the surface59 and to an aqueous interface.6O Inthese organic media it would appear that the apparent retardation arisesprimarily from the low concentration of solute present in the monomericform, since media favouring dissociation, such as nitrobenzene, show muchless anomaly than benzene or cycbhexane, where association is almostcomplete.61Closely related to the above is the remarkable stability of interfacialmonolayers of relatively soluble substances (e.g., sodium cetyl sulphate),and the slow rate of re-solution of an adsorbed monolayer on compression.O2It would thus appear that with the amphipathic type of molecule bothentrance into, and escape from, an interface are markedly hindered processes.The question ofthe structure of protein monolayers, which, despite a large amount ofinvestigation is by no means settled, and may assist in unravelling thestructure of globular and fibrillar proteins, has been approached recentlyfrom two new angles.Since the protein molecule appears to function normally only in thepresence of water, it seemed natural to examine the properties of its charac-teristic -CO*NH- group, with its tendency for hydrogen bonding,m in thepresence of an aqueous medium.Accordingly, monolayers of long chaincompounds containing this group, such as amides, acetamides, and ureas,were compared with those given by acetates, esters, and methyl ketoneswhich clearly cannot associate by intermolecular hydrogen bonding of thetype >CtO- - - -H-N<.The differences were most striking, and it seemsthat cross hydrogen bonding plays a very important part in condensedmonolayers of compounds containing the -CO*NH- group, tending to bringabout condensation and ~olidification.1~ The molecular packing and themechanical properties were also very sensitive to the pH of the subrstrate,and the effects of other reagents such as urea and thiocyanates, also knownto have a marked influence on native proteins, are being studied.The other method of approach 64 has been the study of monolayers oflinear polymers such as polyacrylates, polymethacrylates, nylons, etc.,the last being of considerable interest in view of their close similarity tothe polypeptide chain.All these compounds have known structures andthis has led to a clearer appreciation of the various factors which contributetowards the overall behaviour of protein monolayers.58 J. W. McBain and L. H. Perry, Ind. Eng. Chern., 1939,31,35.59 Idem, J . Arner. Chem. SOC., 1940, 62, 989.6o A. F. H. Ward and N. Tordai, Nature, 1944,154, 146.61 A. E. Alexander and E. K. Rideal, ibid., 1945,156, 18.fi2 A. E. Alexander, Trans. Faraday Soc., 1942, 38, 64.63 W. T. Astbury, ibid., 1940,36, 871.c p D. Crisp and E. K. Rideal, PTOC.Roy. SOC., A (in the press).(d) Structure of proteins and of protein monolayersHIINSEELWOOD : PHYSICOCHEMIUAL ASPHCTS OF BAUTERIAL GROWTH. 15(e) Some biological applications. J. H. Schulman and E. K. Rideal 65h w e used the technique of penetration t o form mixed monolayers,2 inparticular cholesterol with saponin or various synthetic detergents, in aninvestigation of haemolysis and agglutination. They have also 66 examinedthe adsorption on to protein monolayers of homologous series of biologicallyactive compounds (e.g., estrogenic stilbene derivatives), and related theseto their known biological effects.The mechanism of fat abaorption in the intestine has been studied byA. C. Frazer, H. C. Stewart, and J. H. Schulman,67 using emulsion systemsbased upon the monolayer and emulsion work mentioned above.Finally, mention may be made of investigations into tho anthelmintioand anti-bacterial action of soap-phenol mixtures, in which the biologicalactivity is in general accelerated by low concentrations of soap, but inhibitedby high concentrations.Surface- studies 6* show that the accelerationarises from complex formation between soap and phenol, enhancing thesurface activity of the mixture. The acceleration is a maximum at thecritical concentration for micelles, since a t higher soap concentrationscompetition between soap micelles and biological interface brings aboutprogressive inhibition of biological activity.It is hoped that this necessarily brief outline has indicated some of thetypes of problem which can be assisted by surface studies.A. E. A.2. SOME PHYSICOCHEMIUAL AEIPEUTS OF BACTERIAL GROWTH,Introduction.Bacteria are unicellular, without differentiated organs. I n suitablemedia they grow and multiply by binary division. They perform a widevariety of chemical feats in virtue of enzymes which are sometimes isolablefrom the cell but are more often integral parts of the structure. The cellcontents are not homogeneous, but the internal differences are of a fine-grained order.fs2*3 The material of the cell is largely a network of macro-molecular substances, the most important of which are proteins, permeatedby an aqueous solution in which substances of lower molecular weight c9ndiffuse about. Such a system presents certain possibilities, suggestedby analogy with non-living systems, and one may well wonder to whatextent these possibilities in fact help to explain the mechanisms which thecell actually uses.We will begin by mentioning certain analogies, and then, in the light ofthem, consider some of the phenomena of bacterial growth.In the first place, i t is probable that the various enzyme functions of a cell0 5 Proc.Roy. SOC., 1937, B, 132, 29.6 G Nature, 1939, 144, 100.e8 A. E. Alexander and A. R. Trim, ibid., 1944,154, 177.67 Ibid., 1942, 149, 167.G. Knaysi and S. Mudd, J. Bact., 1943, 45, 349.G. Piekarski, 2. Bakt., 1939, 144, 140.C. F . Robinow, Proc, Roy. Sac., 1942, B, 180, 29916 GENERAL AND PHYSICAL CHEMISTRY.depend upon specific protein textures, and that these textures are built upduring growth by heterogeneous.polycondensation reactions not whollyunlike those with which chemical kinetics has already made us familiar.In the cell, moreover, there is a complex scheme of linked processes, in whichstarting materials, often of the simplest kind, are built up stepwise to givecell material. There must be a whole series of reactions so linked that theproducts of one enzyme process Muse from one region of the cell to another,there to participate in further enzyme processes.all individual enzymes must thereforeincrease in amount during growth-and, to a first approximation, in aconstant ratio. This suggests the scheme for the fundamental growthreaction :enzyme + substrate =expanded enzyme + products available for further cell reactionsI n other words the enzymes expand as they do their work.There areanalogies for this kind of reaction in other parts of chemistry. The de-composition of arsine is catalysed by arsenic according to the scheme:solid arsenic + arsine = more solid arsenic + hydrogen. The essentialbasis for the catalysis here is the tendency of the regular lattice of thesolid arsenic to expand by the accretion of like units.4 This principle israther important. Proteins and other macromolecular structures constituteordered arrays, and when they expand by the addition of fresh fragments-after the manner of crystal growth-there will be a release of free energy.This can compensate decreases elsewhere, so that other products of greatchemical activity, possibly free radicals, could be released, capable of takingpart in further polycondensation reactions in other parts of the cell.The concentration in which intermediate products reach the next enzymeof a sequence depends upon the spatial distribution of matter in the cell.This applies even if the intermediates are not labile, since, if they are of lowmolecular weight, the process of loss from the cell by diffusion will be incompetition with that of utilisation by other enzymes.Thus we have the picture of a sequence of consecutive reactions, withrelative rates determined by spatial, as well as temporal factors, occurringin different regions between which transit of intermediate products is governedby the establishment of definite concentration gradients.These reactionslead to the expansion of various ordered arrays of macromolecular com-pounds. One problem confronting us is this : which, if any, of the character-istics of the bacterial cell are understandable in terms of the chemicalkinetics of spatio- temporally linked reaction sequences, and, indeed, interms of relative reaction rates of different enzyme processes ? A few of theoutstanding phenomena of bacterial growth will be discussed from this pointof view.In asequence of reactions of the kind envisaged, if all raw materials are suppliedti Cf. Topley and Wilson, “ Bacteriology.”Cells reproduce themselves :The first group of facts relate to the so-called growthC. N. Hinshelwood, J., 1939, 1203HINSHELWOOD : PHYSICOUHEMICAL ASPECTS OF BACTERIAL GROWTH.17from a constant environment, a steady state will be established in which allthe regions of the cell material expand at rates such that their relativeproportions remain unchanged. For some reason, unspecified at present, thecell divides when it exceeds a certain size. Each cell can go on growing at thesame rate as its parent. Hence the total number increases with timein geometrical progression according to the law n = n,,ekl, where no is theoriginal number and n the number a t time t , E being a constant. This lawis, in fact, rather closely followed (with understandable deviations) over aquite wide range of growth known as the logarithmic growth p h s e . Ulti-mately the supply of raw material becomes exhausted, or substances areformed which inhibit some stage or stages of the reaction sequence, andgrowth ceases.The cells may go on living but they enter what is calledthe stationary phase. During this, some of the intermediate products of thesequence are lost by decomposition or diffusion : more profound changes,fi8 NITime.Fxa. 1.Time.FIG. 2.possibly involving structural alterations in the proteins, may also occur.The result is that even if the cells are transferred to fresh media there may bea delay, known as the lag phase, before the steady state necessary for growthand division is re-established. The phases of the growth cycle are shown inFig. 1. (If the stationary phase is prolonged the cells die : the death ratewill not be considered a t present.) In Section 1 we shall deal briefly withvarious factors governing the length of the lag phase, the rate of growth inthe logarithmic phase, and the onset of the stationary phase.Having considered the initiation and maintenance of growth, we shouldlogically proceed to discuss cell division.This can be done a t present onlyin an imperfect manner, and it is not clear what is the appropriate physico-chemical model. However, various interesting and suggestive facts havecome to light which will be dealt with in Section 2.The various enzymes concerned in the sequence of growth processesare the seat of reactions possessing many characters in common with surfacereactions, and which must be susceptible to inhibition by substances havingaffinity for them.Enzymes will also be inhibited by agents which damag18 QENHIRAL AND PHYBICIAL UHEIKTSTRY.them structurally or deprive them of their substrates. These factors f0rt.nthe basis of various forms of drug action, some of which are considered inSection 3.One of the most striking characteristics of bacteria is their power ofadaptation. When cells which have been grown for some time in a givenmedium are transferred to a new one in which the necessary carbon or nitrogenis supplied in the form of compounds not present in the original medium,growth may be slow during the first few growth cycles, but gradually in-orease in rate in the manner illustrated (in a slightly idealised way) in Fig, 2.Similarly, in the presence of certain drugs there may initially be a verymarked deceleration of growth, but after a few growth cycles the cells becomeimmune and grow quite unhindered by the drug.This remarkable behaviour,apparently so typical of a living organism, can, hypothetically, be explainedin terms of an automatic adjustment of the enzyme balance in the cell : itbecomes, in fact, a study in relative reaction rates. If we consider thesequence of reactions by which various enzymes are synthesised, it is clearthat the rate of each one can respond to changes in the local concentrationof the intermediate supplied by the preceding enzyme of the series. If therelative rates of formation of two enzymes change, then the relative propor-tions of two types of cell material will change also, until a new state of balanceis attained.With suitable auxiliary assumptions this principle can be madeto explain various facts about adaptation. Some aspects of the vast subjectof adaptation are discussed in Section 4.1. Phases of the Growth Cycle.(a) The Lag Phase.-The sequence of cell reactions may begin with verysimple materials; for example, some bacteria grow well in media where theonly source of nitrogen is ammonia and the only source of carbon a simplecoqpound like glycerol. The compounds built up are of much greatercomplexity, and some organisms will not start to grow unless they are sup-plied with molecules of various specific kinds ready made.6 Among neoessarygrowth substances for various baoteria are glutamine,’ tryptophan,* uracil>nicotinic acid, thiamin and various amino-acids.Similar compounds aretherefore likely intermediates in the chain of processes occurring in cellswhich can start with simpler materials. Indeed, certain bacteria whichnormally demand tryptophan, an essential constituent of the protein, canadapt themselves to build it up, fist from indole and then from am-monia.8.1Ot11 Hence the order, ammonia, indole, tryptophan is indicated,and it depends simply upon the state of the cell enzymes whetber the earlier6 €3. C. J. G. Knight, “Bacterial Nutrition,” 1936.7 H. McIlwain, P. Fildes, U. P. Gladstone, and B. C. J. Gc. Knight, Biochem. J.,8 P. Fildes, G. P. Gladstone, and B. C. J. G. Knight, Brit.J . Exp. Path., 1933, 14,1939, 33,223.189. *G . M. Richardson, Biochem. J., 1936, 30, 2184.10 P. Fildes and B. C. J. G. fcnight, Brit. J . Exp. Path., 1933, 14, 343.11 P. Fildes, as., 1040, 21, 67HINSHELWOOD : PHYSICOCHEIHICAL ASPEOTS OF BACJT'ERIAL GROWTH. 19stages operate or whether they have to be by-passed. Very simple sub-stances indeed may be necessary for growth. One of the most interesting iscarbon dioxide. When a stream of air freed from this is passed throughcultures in some media, growth is delayed indefinitely.l2# l3, l4 Many cellsalso require inorganic ions; for instance, when glucose is used as carbonsource in presence of inorganic phosphate, the length of the lag phase maydepend markedly upon small concentrations of magnesium ions.15 Somesubstances used in growth are probably broken down in one departmentof the cell and passed on to another department for further processing.When 8taphylowccus aureus adapts itself to grow without ready-madealanine, it can simultaneously dispense with various other amino-acidswhich it normally demands.16 This may mean that the mechanisms whichare mobilised deal, not specifically with the individual amino-acids, but withactive fragments common to them all.During the early stages of the lag phase there is no apparent increasein cell substance ; later the cell volume increases and this is usually heraldedby an increased production of metabolites such as carbon di0xide.l'.l8# l9One of the obvious conclusions is that during the lag there must be establishedthe necessary concentrations and concentration gradients of the less complexintermediates.This is confirmed by the fact that the lag in simple artificialmedia is longer than in media like meat extract which contain a varied stockof ready-synthesised compounds. The simpler intermediates are diffusiblesubstances which not only pass from one part of the cell to another, but maybe easily lost into the surrounding medium. This comes to light in thequantitative study of the lag phase. We may take the example of Bact.Zactis cerogenes, which grows easily with glucose as carbon source and am-monium sulphate or amino-acids as nitrogen source. The following experi-ment is made : cells from a growing culture are transferred to a fresh supplyof the same medium and the length of the lag is determined (cf.Fig. 1).The lag proves to be a function of the age of the parent cells, i.e., the timebetween start of growth of the parent and the transfer to the new medium.When amino-acids are the eource of nitrogen, the result is as shown in Fig. 3,but with ammonium sulphate as source the relation illustrated in Fig. 4 isfound.20 The explanation of the minimum is as follows. In the ammoniumsulphate medium a diffusible intermediate escapes into the solution.Ordinarily cells transferred to a new medium carry with them a certainvolume of the original medium. In Fig. 4 the initial fall of the lag to aminimum is due to the increasing amounts of the intermediate transferredle G. P.Gladstone, P. Fildes, LtndiG. M. Richardson, Brit. J . Exp. Path., 1935,16,335.l8 S. Dagley and C. N. Hinshelwood, J., 1938, 1930.l4 W. Kempner and C. Schlayer, J . Bact., 1942, 43, 387.l5 R. M. Lodge and C. N. Hinshelwood, J., 1939, 1692.l6 G . P. Gladstone, Brit. J . Exp. Path., 1937, 18, 322.l7 G. Mooney and C.-E. A. Winslow, J . Bact., 1935, 30, 427.l 8 E. Huntingdon and C.-E. A. Winslow, ibid., 1937, 33, 123.lD C.-E. A. Winslow and I. J. Walker, Bad. Rev., 1939, 8, 1472o R. M. Lodge and C. N. Hinshelwood, J . , 1943, 21320 GENERAL AND PHYSICAL CHEMISTRY.with the cells. Actual filtered medium from the old culture lowers the lag ofa new culture. The lag of freshly transferred cells which have been washedfree of the old medium is much increased.Moreover, with the washed cellsthe lag depends in a marked degree on the actual number transferred, sincethey all form the diffusible intermediate and pour it into the medium to forma common store, and the more there are to contribute their quota the sooneris the critical concentration built up. A fairly satisfactory quantitativetheory of the lag under such conditions can be constructed with the aid of theassumptions (i) that the lag ends when the concentration, c, of some activesubstance in each cell reaches a critical value, c' ; (ii) that c is made up inthree ways, being expressed by c = av + Pnot + yt, where v is the volume ofthe old medium transferred, no the number of cells transferred, and a, p,and y are constants. The first term &presents the active substance trans-ferred with the old medium accompanying the cells, the second that builtup by them in time t in the new medium, and the third that built up in a givenAgeFIa.3.AgeFIG. 4.cell without help from the medium or from the other cells. The resultingexpression for the lag gives a reasonable account of its variation with separatechanges in v and no (which can be varied independently with washed cells).The variation of lag with no is specially striking : i t is not found when amino-acids are used instead of ammonium sulphate, since under these conditionsloss of the intermediate into the medium plays a less important part.Various qualitative references to the influence of the inoculum size on thelag occur in the earlier literat~re.~ Diffusible substances which leave the cellplay a part in the functioning of various specific enzymes such as deaminases,and dehydrogenases under conditions where these are not directly involvedin growth.21*22s23 To what extent the increases in lag on continued ageingof the cells (Figs.3 and 4) are due to simple decay or loss of active inter-mediates, and to what extent due to actual structural changes in the protein,is imperfectly known.The lag is a function of the concentrations of medium constituents,but does not vary rapidly with that of the ordinary food materials. It is21 J. Yudkin, Biochem. J., 1937, 31, 865.23 E. F. Gale and M. Stephenson, Biochem. J., 1938, 32, 392.23 D. D. Woods and A. R. Trim, ibi&., 1942,36, 50HINSHELWOOD : PHYSICOCHEMICAL ASPECTS OF BACTERIAL GROWTH.21specifically influenced by various drugs, and this influence is easily modifiableby adaptive changes in the cells in a way which will be considered later.(b) The Logarithmic Growth Phase.--& the end of the lag phase thesteady state is established, and the rate of increase of all the cell substanceat any moment becomes proportional to the amount present, i.e., dm/dt =km. In so far as division occurs when a standard size is reached (which isonly roughly true), this means that the number of cells increases in geometricalproportion with time, the number doubling in equal increments of a periodusually called the mean generation time. The constancy of the meangeneration time is an approximation : small deviations arise from severalcauses-consumption of the food materials, accumulation of inhibitors,non-survival of some of the newly formed cells, and variation in the meansize of cells formed at different stages.24 Quantitatively, however, theseTime,FIG.5.0 A C BFIG. 6.effects are usually quite small until the logarithmic phase approaches itsend. One or more of them then rapidly becomes serious and growth becomesslower and stops,25 often with what appears as considerable abruptness.The actual growth rate of a given organism varies widely, at least over atenfold range, according to the nature of the medium. The mean generationtime, at a given temperature, depends upon the relative amounts of the variouscell enzymes and upon the nature and concentration of the substratesprovided.The fist factor seems to govern the extraordinary way in whichcells modify themselves to utilise a given new substrate, a process calledtraining or adaptation. The course of the training process is instructiveand is illustrated in Fig. 5 . Bact. Zactis cerogenes accustomed to use glucoseas a carbon source is grown in a medium where the glucose is replaced byglycerol. At first the growth rate is low, but increases suddenly a t a certainstage of the logarithmic phase as a t the point When the experimentis repeated with cells already grown once in the new medium, this pointoccurs successively earlier, at a2, a3 . . . and finally passes below the range24 A. T. Henrici, Proc.SOC. Exp. Biol. Med., 1922, 20, 179; 1923, 21, 215, 343, 345.z 6 C. N. Hinshelwood, Biol. Rev., 1944, 19, 135.26 R. M. Lodge and C. N. Hinshelwood, Trune. Furuduy Soc., 1944,Qo, 67122 GIINERAL AND PHYSIUAL UHHMISTRY.of observation. The simplest interpretation of this is given in Fig. 6. Thecells may be assumed to utilise the new medium by two mechanisms, onewith a slower growth rate but a shorter lag, the other with a higher growthrate but a longer lag (as shown by the lines AX and BY, respectively).The latter represents a mechanism which utilises glycerol more efficientlybut is not originally in a state of mobilisation. The training consists in theprogressive reduction of the lag phase of this competing mechanism, aa shownby the movement of the second part of the growth curve from the positionBY to that CZ.The existence of what appear to be alternative and com-peting mechanisms comes to light in other connexions, e.g., in training toutilise new nitrogen sources, or to resist the action of sulphonamide drugs.27Any mode of growth involves a complex sequence of reactions, and the varietyof possible links in any such chain is shown by the number of substrates whichthe cells can use and the variety of reactions which they can provoke. Froma given starting point to the final cell materials a whole network of routes canbe imagined. For example, an amino-acid might be deaminated to giveammonia-used then just as though it were supplied from ammoniumsalts-or the entire amino-acid might be first incorporated and subsequentlytransformed.Two alternative growth modes might differ only in one or twoparticular links, yet might be of very different efficiency in utilising a givensubstrate. They would involve different enzymes, and the readiness withwhich one or the other would come into play would depend upon the pro-portion of the various enzymes present. Only when the cells are fullytrained to a given medium will the proportions be adjusted to give optimumgrowth and a simple logarithmic growth curve. (In fact, the logarithmicform of the growth curve is a good criterion of the degree of adaptationand of the extent to which the various cell processes are in balance.)Although the rates of particular reactions and the combination of re-actions used in the total growth sequence are modifiable by training, thereare well-defined limits to the adjustments possible.When adaptationhas gone to its limit the slowest step determines the overall rate of growth.The fact that there is probably a single rate-determining step is relevant to thequestion of the relation between rate and nature of substrate. The meangeneration time of Bact. Zactis cerogenes varies little for a series of amino-acids as nitrogen sources, so that presumably the step involving their utilisa-tion is not a rate-determining one, though in some other respects the be-haviour of different amino-acids may be very divergent.2s*29 On the otherhand, the rate of growth may vary widely according to the nature of thecarbon source.For one representative bacterium, a t least, the meangeneration time has been shown to be almost independent of the mediumpH-which is rather remarkable in view of the profound influence which2T D. S. Davies and C. N. Hinshelwood, Trans. Faraday Soc., 1943,39,431.28 S. A. Koser and L. F. Rettger, J . Infect. Dis., 1919,24,301; M. Sahyun, P. Beard,E. W. Gchultz, J. Snow, and E. Cross, i6id., 1936, 58,28 ; J. Gordon and J. W. M’Leod,J . Path. Bact., 1926, 29, 13.R. M. Lodge w d C. N. Hinshelwood, J., 1943,208HINSHELWOOD : PHYST~O(3HEMIUA.L ASPBCTS OR’ BAOTERIAL GROWTH. 23pH has on cell activities in general.30 Experimental data on the relationof growth rate to concentration of medium constituents are not very abundant.On the whole, however, it appears that over wide ranges of concentrationthe growth rate varies little, and only shows a marked falling off when theconcentration of the foodstuff becomes very small indeed.30* 31* s2 This canbe understood.The cell reactions resemble heterogeneous reactions,and their rates are probably related to concentration of substrate by anequation like that of an adsorption isotherm, rate = k c / ( l + bc), where ois the concentration and lc and b are constants. As soon as c exceeds acertain value, this expression becomes nearly constant, in a way familiarin the study of surface reactions. Since the scale of cell processes is a minuteone, the saturation limit may very well be reached early. (This is one of thereasons why the mean generation time remains nearly oonstant over thelogarithmic phase despite the consumption of food material.)L imifing factor -toxic ,productsf-- L imiting f a c t o r - /- exhaustionIIniiia/ co:7cn.o f food materia/.mFIU. 7.Time.Ra. 8.(c) The Stationary Phase ; Total Cell Population which the Medium willSupport.-One or other of several factors may, according to circumstances,be the limiting one in determining the end of the growth phase and themagnitude of the final population. When exhaustion of foodstuff is thelimiting factor, the population is directly proportional to the initial concen-tration of one of the medium constituents. Sometimes this linear relationholds over a certain range and then breaks down a t concentrations whereaccumulation of toxic products beoomes limiting, as shown in Fig.7.33The effects on the total final popuIation of growth inhibitors added tothe medium are various. One might have expected the inhibitors tolengthen the mean generation t i q e without affecting the final state : this,however, is seldom found. Alcohols,34 for example, not only decrease rateof growth but reduce the final population in nearly the same ratio. AdversepH reduces the final population without much affecting the rate at which30 R. M. Lodge and C. N. Hinshelwood, J., 1939, 1683.31 W. J. Penfold and D. Norris, J. Hyg., 1912,12,627.32 S. Dagley and C. N. Hinshelwood, J., 1938, 1930,33 R. M. Lodge and C. N. Hinshelwood, J., 1939, 1683.34 E. A. Poole and C. N. Hinshelwood, J., 1940,166524 GENERAL AND PHYSICAL CHEMISTRY.it is attained.30 Lag may be specifically affected without change in totalpopulation.16 Aeration of the medium may affect total population but notnecessarily growth rate.35# 29(d) The Phase of Decline.-Unless they are renewed by division, cellsdie, the death rate being increased by antiseptics, high temperatures, andvarious radiations.According to some observers, the number of survivorsat time t out of an initial population of No is given by the exponential decaylaw, N = Noe-U, where 1 is a constant (curve I in Fig. 8). Others maintainthat the true form of curve is more like that of I1 in the figure, and that Iis found only as an approximation observed over a certain range, andespecially when some of the cells have already died before the first measure-ments are made (as would be found if observation began a t the point a oncurve 11).The experimental decision of the question whether the exponentialcurve is indeed the ideal form seems to be not quite easy.36 An importanttheoretical question lies at the basis of the matter. If the death of a cellexposed to an unfavourable environment occurred after a definite time, thenumber of survivors would vary with time as in curve I11 : if the populationwere non-homogeneous initially, I11 would be rounded off to give IVY which,by assuming the appropriate frequency distribution of resistance could beadjusted to reproduce 11. A very special distribution could even change thecurve into I, but such a distribution would be improbable, since it wouldinvolve a maximum proportion of cells, not with the average resistance, butwith the very lowest resistance.If we accept the (controversial) fact thatI may in certain examples be accurately followed, and if we reject as im-probable the assumption of the very special initial distribution necessaryto explain i t in terms of the varying resistance theory, then some very interest-ing conclusions follow. The differential equation of I is - l / N . W/dt =constant, i.e., the probability of death for any individual in any givenelement of time is independent of the previous history, as in radioactivedecay. If death is caused by the presence of an antiseptic (for example),yet is independent of the time of exposure, it can only mean that a t somemoment a chance conjunction of events occurs exposing the cells to lethalinfluences which have not so far affected them.For the analogous actionof X-rays on the cells of Bact. coli, it has been suggested that the chanceevent is an encounter of a quantum of radiation with some localised sensitivesite in the celL37 This is a special hypothesis which leads to a result of thecorrect form, but the possible implications of the exponential law are of amore general character.2. Cell Division and Cell Morphology.Another phenomenon which seems to depend upon conjunctions of eventsCells do not all attain the same sizes6 0. Rahn and G. L. Richardson, J . Bact., 1942,44,321.36 A. J. Clark, “General Pharmacology”; cf.It. C. Jordan and S. E. Jacobs, J .37 D. E. Lea, R. B. Haines, and C. A. Coulson, Proc. Roy. A‘oc., 1936, B, 120, 47;is the division of the growing cell.Hyg., 1944,43,275.1937, 123, 1 ; J. A. Crowther, ibid., 1926, B, 100, 390HINSHELWOOD : PHYSICOCHEMICAL ASPECTS OF BACTERIAL GROWTH. 25before dividing : nor are the successive intervals of time between divisionsof individual cells precisely equal. They are distributed about a mean in anapparently random fashion. In experiments on Bact. lactis cwogenes at30°, 30% of the divisions occurred after intervals between 30 and 35 minutes,and 91% of the total occurred within the range 20-50 minutes. The dis-tribution was not far from symmetrical about a mean of 30-35 minutes.Very few cells had a fission time of more than double the mean.The varia-tion was, however, striking : it was not due to inhomogeneity of the strain,for daughter cells from an early division were not consistently different fromthe average.38Under certain conditions, divisions may be so much delayed that verylong filamentous or snake-like cells are formed.39~ 40* 41 In these conditionsthe size distribution becomes much more scattered than normal. The factorsconducive to long-cell formation with scattered size distribution are ( a ) thepresence of certain drugs which inhibit division without inhibiting growthto the same extent, and ( b ) the transfer of the cells to an unaccustomedgrowth medium, to which the growth and division functions adapt themselvesat different rate~.~20 43* 44With one coliform organism the distribution of cell sizes at a givenmoment has been found to follow approximately the law n(1) = e-lI5,where n(Z) is the number of cells of length greater than 1, and l i s the meanlength.This suggests that the attainment of a given size greater than themean is governed by a conjunction of probabilities statistically analogous tothat which permits a molecule to traverse a free path greater by an assignedfactor than the mean. The view has been put forward, on the basis of variousexperiments on Bact. lactis cerogenes, that two independent factors controldivision and elongation of the cell (the latter being diffusible into the medium)and that these two factors are separately modifiable by a d a p t a t i ~ n .~ ~3. The Influence of Drugs on Bacterial Growth.The influence of drugs on bacteria is varied : some act as general proto-plasmic poisons, some by interfering with specific members of the series ofenzyme reactions upon which growth depends. Some have specific effectson lag, mean generation time, or on cell division probability. Accordingto the evidence brought forward by Fildes and 0thers,4~* 461 47* 48e 49n 60* 6138 G. D. Kelly and 0. Rahn, J . Bact., 1932, 23, 147.3D E. W. Ainley Walker and W. Murray, Brit. Med. J., 1904, 2, 16.40 R. TunniclifF, J . Infect. D k , 1939, 64, 59.41 A. D. Gardner, Nature, 1940, 146, 837.42 C. N. Hinshelwood and R. M. Lodge, Proc. Roy. SOC., 1944, B, 132, 47.43 R. M. Lodge and C.Hinshelwood, Trans. Faraday SOC., 1943,39,420.4 4 G. H. Spray and R. M. Lodge, ibid., p. 426.4 6 P. Fildes, Brit. J . Exp. Path., 1940, 21, 315.4 6 D. D. Woods, ibid., p. 74.4 8 H. McIlwain, ibid., p. 148.61 G. P. Gladstone, Brit. J . Ezp. Path., 1939,20,189.4' H. McIlwain, ibid., p. 136.49 P. Fildes, ibid., 1941, 22, 293.H. McIlwain, Biochem. J., 1942, 86, 417speoific inhibitory actions are often exerted by substances which areatructurally related to normal metabolites of the cell. Their structure makesthem liable to be taken up by the enzymes in competition with the normalsubstrate, for whioh, however, they are in other respects not adequatesubstitutes. Competing pairs of substances in this sense are thiol compoundsand r n e r ~ u r y , ~ ~ aminobenzoic acid and sulphonamides,46 nicotinic acid andpyridine-3-sulphonic acid:* amino-acids and their sulphonic acid analogues,48and EO on.Only a few aspects of the extensive subject of drug action can be dealtwith here.(a) Influence of Drug Concentration on Antibacterial Action.-Much of theearlier 36 consisted of measurement4 on the rate at which disinfectantskilled cells, the death rate being expressed in the form acn, where a is constantand 12 is frequently a quite high power of the concentrakion c of the drug.This power law could hardly have a real theoretical significance and wouldseem to be an approximation for a law of rather different form.Suppose a.certain concentration, G ~ , of the drug oould be tolerated by the cell, beingdealt with by neutralising mechanisms of some kind : the death rate mightwell be proportional to c - co.Then we have :andlog rate = log const. + log ( c - co)d(1og rate)/dc = l / ( c - co)If we also write rate = acn, then d(1og rate)/dc = n/c, and comparison of thetwo expressions gives n = c/(c - cJ. If the tolerance is appreciable,c - co will be small over the range in which the first serious action of the drugis exerted, i.e., n will be large, though it would not appear constant overmore than a limited range. The tolerance to small concentrations which thisview implies is in fact observed in some cases. In the action of bacteriostaticagents such as proflavine (2 : 8-diaminoacridine sulphate) on the lag of coliformorganisms, the initial tolerance is not only observable but is t o be expectedtheoretically.Fig. 9 shows results found for proflavine and Bact. lacbiso~ogenes.6~ Each curve of the family corresponds to a strain of cells whichhas been repeatedly grown in presence of a certain concentration of the drug(see later). The form of the curves is explained as follows : the proflavineinterferes with the working of an enzyme which yields an intermediateconstituting the substrate of a second enzyme. The rate of working of thissecond enzyme is related to the concentration of the intermediate by anadsorption isotherm (Fig. 10). Normafiy, the concentration of intermediateprevailing in the cell has a value such as A,; it can be reduced to B beforeany inhibition is manifest, i.e., there will be a tolerance proportional toA,B, after which further drug oauses a fall in rate to, say, C. The cellswhich have been trained in presence of drug at higher concentrations havegreater reserves of the intermediate-forming enzyme, so that the concentration62 D.S. Davies, C. N. Hinshelwood, and J. M. G. Pryce, l'rane. Paraday SOC., 1945,in pressKINSH~WOOD : PKPSICOCHIOMICAL ASPECTS OF BAOTERIAL GROWTH. 27of the intermediate itself starts at values A,, A , . . . . ., with correspondinglygreater tolerances. If the curves in Fig. 9 were expressed in terms of a lawmaking the lag depend upon the nth power of the drug concentration, thenthe values of n would have to be high. Actually, the curves can be representedby a formula derived rationally from the above theory.The influence of various inhibitors on the mean generation time oansometimes be expressed quite well by a simple linear relation of growthrate and concentration : R = R,(1 - bc), where R is the growth rate inpresence of a concentration c of drug, Ro that with none, and b is a con&ankW(b) Drug Action and Structwe.-This subject cannot be dealt with indetail but reference should be made to a few general matters.Two effectsrCConcn. o f ihtcrrnediate .Prof/a vine concn.FIG. 9. FIG. 10.of structure have to be distinguished. In the first place, the partition of thedrug between the medium and the part of the cell where it acts will dependupon the structure of the drug molecule. In particular, it will show regularvariations during the ascent of a homologous series.54 Many quite largequantitative differences in drug action are explained away when varyingphase distributions are allowed for, e.g., by comparing actions at equivalentchemical potential^.^^ The antibacterial action of various phenole seems tobe related to the solubility of the phenol in olive oil (a possible model forcertain regions of the interior of the cell) .56 The inhibitory action of straight-chain aliphatic alcohols increases by a constant factor from one member ofthe homologous series to the next higher : this shows that each CHa grouphas its own contribution to make to the total effect, and indicates that theaction occurs in a part of the cell capable of attaching alcohol to its substanceby each link of the hydrocarbon chain.530 g4* 57.58The differences in antibacterial action of such classes of compound63 S.Dagley and C. N. Hinshelwood, J., 1938,1942.G4 K. H. Meyer, Trane. Paraday Soc., 1937,33,1062.6 5 J. Ferguson, Proc. Roy. Soc., 1939,B, 12'4,387.G' A. H. Fogg and R. M. Lodge, Tram. Paraday Soc., 1946, in press. '' F. W. Tilley and J. M. SoMer, J. Baot., 1926,12,303.5 R W. S. Stiles, " Introduotion t o Plant Physiology," 1936, p. 8128 GENERAL AND PHYSICAL CHEMISTRY.as sulphonamides, acridine derivatives, triphenylmethane dyes, and so oncan hardly be explained in this way. The actions depend upon the inter-vention of the drug at specific stages of the reactions involved in growth.The antibacterial action of substances related to known metabolites, alreadyreferred to, supports this view, for which further evidence is provided by thephenomenon of " cross training ".For example, Buct. Zuctis cerogenestrained to resist proflavine has also increased resistance to methylene- blue,but not to sulphonamide. When thoroughly trained to sulphanilamide, itwill resist sulphaguanidine and vice versa : but sulphonamide training doesnot immunise to proflavine. This offers yet another method of classifyingspecific inhibitory actions. On the other hand, with Stuphylocaccus uureuSnicotinamide antagonises not only sulphapyridine but quite unrelateddrugs It is also of interest to note that some antiseptics are statedt o cause death of cells before causing significant inhibition of several typicalenzymes of those cells.604.Bacterial Adaptation.One of the most remarkable properties of bacteria is their capacity forsuffering changes in morphology, biochemical and other characters and intheir power to withstand the action of certain drugs. These changes,though important and sometimes profound, are limited in amplitude in thesense that the main species characteristics are always preserved.61 Strepto-cocci are never transformed into coliform bacteria, though within the in-dividual groups continuous ranges of forms exist, all possibly, and somedemonstrably, intercoqvertible.62-66 A great deal of attention has beenpaid to the changes in the forms of colony in which bacteria grow on solidmedia. One initial type often gives rise to variants, sometimes stable,sometimes unstable,67* 68 the changed colony form being linked in varyingdegrees with other changes 69-74 in character.This phenomenon is often,though not very happily, referred to as bacterial dissociation. The adapta-tion of bacteria to utilise new food sources has already been mentioned.There is no reason to suppose that the development of the power to resist theaction of certain drugs is other than a special example of the operation of thegeneral adaptive mechanism.To illustrate some of the principles involved in adaptation we will con-6o M. A. Bucca, J . Buct., 1943, 46, 151.61 A. I. Virtanen, {bid., 1934, 28,447.62 P. Fildes, Brit. J . Exp. Path., 1927, 8, 219.63 L. W. Parr, Buct.Rev., 1939, 3, 1.64 C. Nyberg, K. Bonsdorff, andK. Kauppi, 2. Bukt., 1937,139,13.6s 0. Sievers, ibid., p. 27.6 7 P. Hadley, J . Infect. Dis., 1937, 80, 129.6 8 F. Farag6, 2. Bukt., 1934, 133, 139.69 H. B. Gillespie and L. F. Rettger, J . Bmt., 1939, 38, 41.7O C. S. Flynn and L. F. Rettger, ibid., 1934,28,1.71 M. I. Bunting, ibid., 1940,40,57,69; 1942,43,593.78 I. M. Lewis, ibid., 1934, 28, 619.74 L. W. Parr and M. L. Robbins, ibid., 1942, 43, 66.W. B. Wood and R. Austrian, J . Exp. Med., 1942,75, 383.e6 F. Neri, ibid., 1940, 146, 166.73 M. W. Deskowitx, ibid., 1937, 33, 349HINSHELWOOD : PHYSICOCHEMICAL ASPECTS OF BACTERIAL GROWTH. 29sider a, particular example in more detail. Bact. hctis cerogenes grown inpresence of prof3 avine acquires an immunity to the drug, the relation betweenlag and test concentration for strains trained at a series of concentrationsbeing shown in Fig.9.75 The differences in the spacing of the curves corre-spond almost exactly to the training concentration,T6 showing that thetraining consists in the graded response of the cell to the actual inhibition,rather than in the selection of an inherently resistant strain from a mixedpopulation of resistant and non-resistant cells.(The Reporter 77 has discussed some of the literature bearing on thequestion of adaptation versus selection. There seems to be good evidencethat adaptation is usually initiated by actual changes in individual cells.Naturally, when some of the cells have become adapted they will outgrowany unadapted cells, and in this sense selection must always be super-posed on any other adaptive mechanism.But it is the initiation of theprocess which is of greater interest from the physicochemical point ofview. Very varied opinions have been expressed about the subject inUnder favourable conditions the adaptation is very rapid, being completeafter a few cell divisions, though it does not occur during the lag phase itself,i.e., in the absence of any actual increase of cell substance. For the systemunder consideration the following simple model accounts for many of the facts.We assume an enzyme I whose total substance expands according to theequation h1/dt = k1x1. It gives an active intermediate which is partlyused by another enzyme, 11, and partly lost by diffusion. The intermediateattains in the region between the two enzymes a concentration c, such thatdc/dt = k ' l ~ l - E,cn - k,cx,/(l + bc) = 0.The term k,cn is the totalrate of loss by diffusion and is thus proportional to the total area of cell wall,i.e., to n, the number of cells. The total substance of enzyme I1 expandsa t a rate related to c by a Langmuir isotherm, so that the consumption of cfrom this cause is given by the term k3cx2/(l + bc). Assuming that celldivision waits until the amount of enzyme I1 per cell attains to some standardamount, i.e., that n = px,, we can easily solve the equations. The ratioxr/x2 tends on growth of the cells to a definite limit, whatever its initial value.If now a drug impairs the production of the intermediate so as to slow downthe synthesis of enzyme 11, the overall multiplication rate will be lowered.The ratio xJx2 will, however, expand as growth occurs, and c will rise, thisin turn causing an increase in the rate of growth of enzyme I1 until themultiplication rate returns to normal.The whole process constitutes anadaptation to the drug. There is, in fact, evidence that the enzymic40,397.general. 6 7 a 7+*1 17 6 D. S. Davies, C. N. Hinshelwood, and J. M. G. Pryce, Trans. Faruday SOC., 1944,76 Idem, ibid., 1945, in press. '' See ref. (26).'* E. W. Todd, Brit. J . Exp. Path., 1930,11,368.70 R. R. Mellon, J. Bact., 1942, 44, 1.8o A. C. Giese, {bid., 1943, 46, 323.81 D. S. Davies and C. N. Hinshelwood, Trans.Faraday SOC., 1943, 39,43130 GENERAL AND PHYSICAL UHXCMTSTRY.oonstitution of drag-resistant bacteria has been changed in variousways.82--85In the light of the hypothesis that adaptation coiisists in a change ofenzyme balance resulting directly from the modification of the relativereaction rates of different cell processes, it is possible to discuss the spontaneousloss of training which sometimes occurs on subsequent growth in the drug-free medium, the marked retention of the training which occurs in othercases, the induced reversion of trained strains caused by growth in presenceof other antibacterial agents, and similar problems.The above-mentioned model provides a reasonable expression for thelag-conoentration relation. If we assume that the increase of lag caused bythe proflavine is essentially due to the lowered rate of working of an enzymesuch as enzyme 11, this lowered rate being caused by a reduced concentrationof the active intermediate from its normal value cI1 t o a new value c, thenwe have the following equations :G = C ’ ~ - +(m), +(m) being some function of the drug concentration.L = A/& where L is the lag and R the rate of working of the enzyme, and A aconstant :From these we obtainR = k c / ( l + bc).1/(L - Lo) = k/A[c’le/+(m) - C’JWith proflavine, the experimental results are well represented with + (m) =fm, where f is a constant, corresponding to st virtually quantitative titration.The value of cfl for the various trained strains proves to be related to thetraining concentration P by the linear equation c ’ ~ = const.(54 + P).The effect of training is thus to restore c from the reduced value it hasinitially in presence of the drug to the original value it would have had foruntrained cells in the absence of any drug at all. Cells trained to methylene-blue also show increased resistance t o proflavine. The lag-concentrationcurves for methylene-blue are of a very markedly different shape, indicatinga different form for the function +(m). If the form of the latter is calculatedfrom the experimental results for untrained cells in presence of methylene-blue, then it can be used to predict how methylene-blue-trained cells shouldbehave both at other methylene-blue concentrations and in presence ofproflavine.Certain outstanding qualitative differences are correctlyaccounted for, and there is even a measure of quantitative agreement.86No more is to be claimed for the views just outlined than that theyillustrate the general sort of way in which adaptation may be related to82 W. P. Wiggert and C. H. Werkman, Biochem. J., 1939,33,1061.83 R. A. McKinney and R. R. Mellon, J. Infect. Dis., 1941, 68,233.M. Landy, N. W. Larkum, E. J. Oswald, and F. Streightoff, Scknce, 1943, 87,295.8 s E. H. Rennebaum, J . Bact., 1935,30,625.86 J. M. G. Pryce, D. S. Davies, and C. N. Hinshelwood, Trans. Farday SOC.,1945, in pressBARRER : ZEOLITES AS ABSORBENTS AXD MOLMCULAR SIEVES. 31kinetic principles. For more general ctocounts of adaptive enzymes, andin particular for the distinution which may be drawn between adaptiveand constitutive enzymes, reference should be made to the discussions ofJ.Yudkin 8' and of R. Dubos.88In contrast to hypotheses about the progressive modifioation duringadaptive changes of the enzyme balance of cells stand those views whiohrefer adaptation and variation to alternative possibilities presented to thecell a t the moment of division.89*90 Such views would acoount naturallyfor cases where variant forms are found to be produced in a stableratio,'l* 72s 73 and also for completely irreversible variations (whioh arebelieved to occur, though the evidence for complete irreversibility must benegative only) .67* 90Not very much is known about the nuclear apparatus of bacteria, orabout the way in which the cell proteins are organised.If there is somesort of centre of organisation, and if an abnormal mode of division occurs,it is quite conceivable that appreciable modification of the actual proteinpatterns is produced in the new cells. This might well lead to rather im-portant changes in properties. Perhaps, therefore, we might imagine thatsimple changes in the quantitative efficiency with which a given carbon ornitrogen source is utilised or in the resistance to a given drug can be explainedby nothing more profound than the automatic shift in enzyme balance ofthe kind just discussed. On the other hand, to explain rather more profoundchanges in qualitative character we must perhaps invoke changes in theprotein pattern dependent upon nuclear changes.The equation on p. 16provides a basis for the expansion of precisely those enzymes which are neededin the utilisation of a given foodstuff, so that shifts in enzyme balance arethe natural explanation of adaptive changes to new media.86*Spontaneous variations in bacteria occur : more frequent and moreprofound changes are caused by exposure to radiations such asThese changes probably involve activation energies (or local entropy de-creases) very unlikely to be provided under the normal oonditions of He.Still more profound changes in the fundamental patterns, of the kind whichwould distinguish one species from another, would involve activationenergies so high that they would only be available under conditions whichwould usually kill the cells.8 7 ~ 88* 91C.N. H.3. ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES.Certain minerals show remarkable occlusive properties, and because ofthe way in which they have influenced current views of sorption a surveyof these properties is opportune. Of all mineral occlusives, none are ao well87 Biol. Rev., 1938, 13, 93.s9 G. R. Reed, J . B d . , 1933, 25, 645.ea Bad. Rev., 1940, 4, 1.Cf. discussion by A. Haddow, A& Int. Union against Cancer, 1937,2, 376.Cf. G. S. Miriok, J . ESP. Med., 1943.78, 256.g a Cf. the discussion in E. Schroedinger, " Whet is Life Y ", 194432 GENERAL AND PHYSICAL CHEMISTRY.known as the zeolites.1 These crystalline minerals (Table I) should bedistinguished from amorphous gel zeolites and permutites used in watersoftening.They are aluminosilicates (R,R2‘)0,A&03,nSi0,,mH20, whereR = Ca, Sr, and Ba and R’ = Na, K. The ratio of base to A1203 is always1 : 1 and the ratio (A1 + Si) : 0 = 1 : 2. Although the minerals are wide-spread, massive deposits are not usually found. They are formed undercomparatively alkaline and probably stagnant conditions by the hydro-thermal alteration of older rocks and lavas at temperatures believed to lieas a rule between 100’ and 350°.2 Under more acid conditions hydrothermalreactions would largely yield clays.3 Zeolites, like clays, are frequentlyZeolite groupand zeolite.*Mwdenite group.YordeniteP tiloli teHeulandite group.HeulanditeBrewsteriteEpis tFbitePhzlzpslte group.PhilipsiteHarmotomeStilbiteGismonditeLaumontiteChabuzitc ptoup.ChabavteGlIIdhiteLevyniteFauj asiteNalrolite group.NatroliteScoleciteMesoliteThomsoniteEdingtoniteA?MZW.TABLE I.Some Typical Zeolites.Crystal chemicalX - F Sstudes,Probable ideal formula.ref. no. nature.(Ca,E1,Na3AI,Silo0,.,69H,0 - Robust three-dimensional- network Ca,KlNa,)A1,Si1,O,,,4H,O -CaAl SiIOlI,5H10(Sr C i Ba)Al Si6Ol6,5HSOLide deulandite(K,,Ca)AI s4011 44H10 9(K2,Ba)Al:Si5Ol4~5HaO 10 Robust thrAe-dimensional(Nal,Ca)A1,SiIO,.,6H10CaAllSi,O8,4HXOCaAI,Si40,,,4BI,0(Ca,Na,)AI,Si,O ,,,6H,O 8 Robust three-dimensional(Na,,Ca)A1,Si,01,,6H,0 - Robust t9ee-dimensionalCaAI,Si,Ol,,SH,O(Na,,Ca)A1,Si5O1,,1OHaO8, 8a Laminar structure - - - --- network - - - - -networknetwork - - - -Approx.interstitialvolumeavailableto H,O(c.c./g.).0.1350.1020-1480.1360.1550.1650.1410.1720.2060-1530.2140.2110.1840.281Chain structures with a.0.095rather weak cross-link- 0,138chain8NaAlSiO 2HOCaAl &,6,,$!tH?b0.1400.11NaCalAI,Si~0,0,6H10BaAlSiO 3H 0NaAl!Si,b 6:?Ha 6 ’ 14 Robust three-dimensional 0.078Betwken scolecite and natro- 12a, 12b, 12c ing oflr aluminosilicate 0.095-0.138litenetworkThe classiEcation is a modiEcation of that given by C. Dana 4 but the grouping m y still need revisionas more X-ray and crystal chemical data accrue The crystal Lhemical classiEcation in col. 4 is basedin part on X-ray data and in part on studies of the stability of the zeolite as an absorbent and towardsheating.0 Chemical formulae are ideal values, and isomorphous replacements of the type Ca $2Na orCaAl G NaSi may cause considerable modification of these formulae.‘Earlier data on sorption equilibria in zeolites are discussed by J.W. McBain,“ Sorption of Gases by Solids,” Routledge, 1932, Chap. 6. Some kinetic aspects aresummarised by Barrer, “Diffusion in and through Solids,” Cambridge Univ. Press,1941, Chap. 3.a Cf. Lindgren, ‘* Mineral Deposits,” McGraw Hill Book Co., 1919, p. 427.a F. Norton, Amer. Min., 1939, 24, 1; 1941, 26, 1. A survey of work on hydro-thermal formation of minerals, including zeolites and clays, is available up to 1937(G. Morey and C. Ingerson, J.Econ. Geol., 1937, 32, 607-761).“ A System of Mineralogy,” 6th Edtn.; see also W. Bragg, ‘‘ Atomic Structureof Minerals,” Oxford Univ. Press, 1937, Chap. 16.W. Milligan and H. Weiser, J. Physical Chem., 1937, 41, 1029; R. M. Barrer,pro^. Roy. Xoc., 1938, A , 167, 392, 406BLUtRER: ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES. 33highly hydrated (Table I), and they have a rather low density and refractiveindex which reflect this high water content. They have a well-developedproperty of base-exchange, which is, however, shared by other mineralswhich are not zeolites such as the sodalite-hauyne group of minemh,15ultramarine, and some clays.Like all silicates, zeolites are built by the union of Si04'"' tetrahedraby sharing one or more oxygen atoms with neighbouring tetrahedra.SomeSiO,"" tetrahedra are replaced by A10,""' tetrahedra, thus imparting anegative charge to the framework, which is neutralised by an electro-chemical equivalent of interstitial cations. The framework is alwayssufficiently open to enmesh also interstitial water molecules, and upon thisproperty depends their behaviour as occlusives. AS indicated in Table I,three kinds of anionic framework exist : 14 (i) Robust three-dimensionalahminosilicate network structures. (ii) Structures with ahminosilicatesheets rather weakly bonded to one another. (iii) Structures with ratherweakly cross-linked aluminosilicate chains.The zeolites lose their crystal water on heating, with a variable degreeof lattice shrinkage which is a minimum with the first kind of zeolite, andmay be large, or even amount to irreversible lattice collapse, in the case ofthe laminar or fibrous zeolites.5 From the present point of view the non-collapsing zeolites are the most important, for these not only regain theircrystal water readily, but may sometimes occlude other gases and vapoursin place of water.* The water or other molecules are situated in the sameCf.M. Hey, Min. Mag., 1932, 23, 51 ; A. Winchell, Amer. Min., 1925, 10, 90;1926, 11, 82.R. M. Barrer, Trans. Faraday SOC., 1944, 40, 555.J. Wyart, Bull. SOC. franq. Min., 1933, 56, 81; see also 8a W. H. Taylor, Proc.Roy. Soc., 1934, A , 145, 80.' J. Wyart and P. Chatelain, Bull. SOC. franc. Min., 1938, 61, 121.lo J. Sekawina and J.Wyart, ibid., 1937, 60, 139.l1 R. M. Barrer, unpublished data on the sorptive properties of harmotome.lZa W. H. Taylor, C. Meek, and W. Jackson, 2. Krist., 1933, 84, 373; 12b W. H.Taylor and R. Jackson, ibid., 1933, 86, 53; 12c M. Hey and F. Bannister, Min. Mag.,1932, 23, 51, 243, 305, 421, 483.l3 W. H. Taylor, ref. (8a).* Chabazite,l gmelinite,7 mordenite 7 and " active " analcite,16 all robust networkstructures (Table I), absorb non-polar gases, although not all robust network structureswill do so, and the property depends on the relative dimensLon of molecule and inter-stitial zeolitic channel. Laminar and fibrous zeolites, if not over-heated during out-gassing, may occlude small polar molecules such as ammonia in place of water, buthave not yet been found to occlude non-polar gases.17 reported thatharmotome could absorb hydrogen sulphide and ethyl alcohol in place of water, butfor the latter at least this report requires reinvestigation.11 In view of recent work onsorption by fibrous zeolites,6 it is also most unlikely that the fibrous zeolite thomsonitecould occlude glycol and isopropyl alcohol, as reported by M.Hey.19 In these caseseffects noted with larger organic molecules could have been due to selective removalof water from the liquid. M. Grandjean20 stated that levynite, gismondite, andharmotome could occlude mercury vapour. This is possible even for harmotome inview of the small radius of the mercury atom (-1.5 A.).Molecules which may replace water as constituents of the lattices of a number ofzeolites are given in Table V.l4 W.H. Taylor, 2. Krist., 1930, 74, 1.G. FriedelREP.-VOL. XLT. 34 GENERAL AND PHYSICAL CHEMISTRY.interstices as the interstitial cations, with which they are in close associ-afion.l39' Like these cations they may diffuse from one site to another,but whereas cations cannot leave the lattice unless their place is taken byan electrochemical equivalent of other cations (base-exchange) the waterand other molecules may do so, and thus form vagabondising constituentsof the crystal lattice. In this they recall the behaviour of hydrogen insolution in palladium, cerium, thorium, and other metals.', 21 The gas-zeolite system is best considered as an interstitial solid solution in whichthe occluded molecule interacts physically only with its environment,through dispersion, polarisation, and ion-dipole forces.21 The term " per-sorption " applied to gas-zeolite systems is inadequate and should bereserved for the irregular dispersion of the sorbate in the sorbent, as is thecase in charcoal or silica gel.As should characterise binary interstitial solid solutions in which theoccluded component is a vapour, most zeolites give a continuous sorption-desorption isotherm (Fig.1). This property is also reported z2 for water ina number of other crystalline substances, including MgPt( CN),, strychninesulphate, basic zirconium oxalate, oxalates of cerium and some rare earths,some flavanol derivatives, glucosides, and crystalline proteins.In addition,certain clays and other silicates,23 such as ~iloxene,~* will take up wateror ammonia continuously, probably within the crystal lattice, or betweenindividual aluminosilicate sheets.26Isotherms, Isobars, and Isosteres.-Typical isotherms, isobars, and iso-steres in zeolites are illustrated in Figs. 1, 2, and 3. A number of equationshave been suggested to describe the isothermals.26 The most successful ofthese are based upon kinetic or statistical models 27 and have the formp = constant x 0 / ( l - 0) . . . . (1)See W. H. Bragg, ref. (a), p. 265, for structural details of the hauyne-sodalitegroup.l6 R. M. Barrer and D. A. Ibbitson, Trans. FaracEay SOC., 1944, 40, 195, 206.1' R. M. Barrer, ref. (5).a1 R. M. Barrer, Trans. Faruday SOC., 1944, 40, 374.22 A list of references to work on crystals with zeolitic components, and crystalsgiving continuous sorption isothermals is given by McBain, ref.(1).z3 J. Sameahima and N. Morita, Bull. Chem..Soc. Japan, 11935, 10, 485, 490;I. Cornet, J . Chem. Physics, 1943, 11, 1; L. Hansen, W. Samuel, and P. Forni, Ind.Eng. Chern., 1940, 32, 116 ; L. Spitze and L. Hansen, iW., 1942, 34, 506.24 Kautsky and his collaborators have studied the sorptive properties of siloxene.A list of references is given by McBain, ref. (1). See also H. Kautsky and F. Grieff,2. anorg. Chem., 1938, 236, 124.25 G. Nagelschmidt, 2. Krist., 1936, 95, 481 ; see also S. B. Hendricks, J . PhysicalChem., 1941, 45, 65.z6 0. Weigel, Sitz. Gea. Naturwiss. Marburg, 1924, 73; G.Huttig, Fmtschr. Chem.,1924, 18, 5 ; Kolloid-Z., 1924, 35, 337; 0. Schmidt, 2. physikal. Chem., 1928, A , 133,263; E. Rabinowitsch, ibid., 1932, B, 16, 43.27 M. Hey, Min. Mag., 1935, 24, 99; E. Rabinowitsch and W. C. Wood, Trans.Faraday SOC., 1936, 32, 947; R. M. Barrer, ref. (21).la Bull. SOC. franp. Min., 1896, 19, 94.2o Bull. Soc.franc. Min., 1910, 33, 5. Min. Mag., 1932, 23, 103BARRER : ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES. 35where 8 denotes the fraction of all the available interstices occupied bysolute molecules, and p is the equilibrium pressure. Values for the constant in750135ti?Q‘h: * 4rooa 75 2B-a96 50G273°K.384°K. 25c 20 Pressure (cm.). 300FIG. 1.Isotherms of N, in chabazite.’?“I-0 I400 500 600 7000 Temperature (OK).FIG.2.Isobars of typical gases in ~?tabazite.~0Curve 1 : He (400 mm.).Curve 2 : H, (400 mm.).Curve 3 : N, (400 mm.).Cuwe 4 : CO, (200 mm.).Curve 5 : NH, (100 mm.).Curve 6 : H,O ( 7 mm.).(1) have recently been obtained21 corresponding to the progressive conversionof translational and rotational degrees of freedom of the solute moleculeinto vibrational ones when the solute is transferred from the gas phase t36 GENERAL AND PHYSICAL CHEMISTRY.the zeolitic interstice. The value of this constant is a sensitive measure ofthe mobility of the solute within the crystal lattice, and actual isothermsare best reproduced with values of the constant in (1) appropriate for three-dimensional oscillators within the zeolite, so that no large proportion ofoccluded molecules is mobile.An extension of equation (1) has been based 22 upon the discovery l6that, contrary to earlier observations,28 chain molecules such as n-paraffinsmay be copiously absorbed in certain zeolites. This sorption has beenfollowed for the n-paraffins as far as n-heptane, and it is certain that such160 200 25il 300 350 400 450 500Temp.(OK).FIG. 3.12.18 c.c./g. of mineral.I6a molecule must; have a stretched configuration in the interstitial channelof the zeolite, which sheaths it closely. Many configurations possible inthe gas phase are thus precluded when the molecule enters the crystal,and this in turn introduces an additional negative entropy of occlusion.The configurational entropy has been calculated under certain conditions,and also the total entropy of occlusion.The negative value of the con-figurational entropy of occlusion increases with increasing chain length, anddecreases the affinity of occlusion (AA = AU - TAX). For very long-chain solutes the configurational effect would outweigh all others, and makeAA positive, so that occlusion would become negligible in spite of the greatheat of occlusion.Xaturation Values for Sorption in Zeolites.-Attempts have been madeto measure the quantity of gas required to saturate a zeolite. 0. Weigel *9sammarised all analyses of the water content of zeolites and plotted theamount of water, in order of increasing value, against the number ofanalyses. The curves obtained had lengthy central portions either nearlyflat or of gentle slope, suggesting a fairly definite if not quite sharp satur-ation value towards water, because at room temperature water is very28 0. Schmidt, ref.(26); T. Baba, Bull. Chem. Xoc. Japan, 1930, 5, 190; J. Same-shima, ibid., 1929, 4, 96.2s Centr. Min., 1922, 164-178, $01-208.Isosteres for some hydrocarbons occluded in chabazite. The amount sorbed iBARRER: ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES. 37strongly absorbed? and occlusion isotherms are nearly rectangular. Thewater contents in Table I may thus be taken as substantially saturationvalues towards this species. Reported approximate saturation values forother vapour-chabazite systems, in C.C. (at N.T.P.)/g., are :He ...............400 30 NH, ............... 270,30 170 l7H, CO, .................. 170 ............... 350,30 280 l7A .................. 250 l7 CH,*OH ............ 166 s1N, ............... 175,30 170,30 164 l7 C2H,*OH ......... 110 91These data are not always concordant, and the lack of uniformity is prob-ably in large degree a measure of the uncertainty of the extrapolation ofexperimental data, which has often been considerable. Another piocedurehas therefore been suggested 16 in which a well-established experimentalfigure is taken, and saturation values for other solutes derived from it.Satisfactory figures are 170 C.C. (at N.T.P.)/g. for nitrogen-chabazite, or266 C.C. (at N.T.P.)/g. for water-chabazite, the latter figure being derivedfrom the formula of Table I.Long-chain solutes are stretched out withinthe narrow zeolitic channel (see previous section), so thatLength (or diameter) of N, (or H,O) - Saturation value for chain moleculeLength of chain molecule - Saturation value for N, (or H,O)In this way the saturation values given in Table I1 were obtained. Satur-ation values towards water may be obtained from Table I for other zeolites,TABLE 11.Saturation Values in Chabaxite and Active Analcite.Saturation value, C.C. (at N.T.P.)/g.Diameter or v AGas. length (A,). Chabazite. Active analcite.H20 ........................... 2.76 266 * 97 *NH, ........................... 3.60 193 72 .............................. 3.74 186 69 .............................. 3-84 181 670, .............................. 3-83 181 67N, ..............................4.08 170 f- 63CH4 ........................... 4-00 173 64C2HS ........................... 5.54 125 46.4t~n-C,H,o ........................ 7.78 89 33.1~z-CGH~, ........................ 9.04 77 28-5n-CeH14 ........................ 10.34 67 25-0C,H, ........................... 6-52 106 39.5n-C,H,, ........................ 11.56 59 22.2* Experimental value from Table I.f- Value used to calculate all others save that for water.but molecular sieve effects (p. 44) or lattice collapse on dehydration (seenext section) may effectively prevent other gases from replacing the water.Phase Formation and Zeolitic Solid Solution.-A recent investigation hascombined X-ray measurements with dehydration isobars,32 and suggests30 E.Rabinowitsch, ref. (26); E. Rabinowitsch and W. Wood, ref. (27).31 0. Weigel and E. Steinhoff, 2. Krist., 1926, 61, 126.32 W. Mi- and H. Weiser, ref. (6)38 GENERAL AND PHYSICAL CHEMISTRY.that, among fibrous zeolites, scolecite and natrolite give evidence of lowerhydrates but that thomsonite and mesolite dehydrate without discontinuity,as do stilbite and heulandite (laminar structures) and chabazite and analcite(robust three-dimensional networks). The less robust chain and sheetstructures show lattice changes during heating which may extend to com-plete breakdown of the parent structure; but the chabazite and analcitelattices show little change in X-ray pattern on heating. The existence ofan amrnoniate of natrolite has also been demonstrated ; l7 above a thresholdpressure, sorption of ammonia occurred autocatalytically to give an approx-imately univariant system.It has been suggested that zeolitic solidma. 4.Gibbs free energy (G) plotted against Tfor the system Zeolite, xNH, + (1 - x)NH,. Fig.4~ shows the condition for a continuous sorption isobar, and Fig. 4B the condition for adiscontinuous sorption isobar, x changing at a certain temperature from 0.9 to O-1.2'solution occurs normally when the zeolitic lattice suffers no marked changeduring sorption or desorption, and'that new phases tend to form if con-comitant lattice changes are ~onsiderab1e.l~ Another criterion has beensuggested which shows how the alternatives of zeolitic solid solutions(ammonia-chabazite) or phase changes (ammonia-natrolite) may arise.Ina system Zeolite, zNH, + (1 - x)NH,, for example, when the free energy-temperature curves lie for different values of x as in Fig. 4A, the sorption-desorption isobar is continuous, but with a small displacement as in Fig. 4B,one or more discontinuities may arise.21 This is because the lower envelope,representing the most stable state of minimum free energy, curves conBARRER: ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES. 39tinuously and touches each G-T curve at one point only in Fig. 4A; but inFig. 4B this envelope consists of two intersecting lines, coinciding with theG-T curves for x = 0.9 and x = 0.1.Heats and Free Energies of ZeoZitic Solid 8oZution.-Heats of occlusionin zeolites have been measured directly,% and by the use of the Clapeyronequation.34 Although it is not certain that the calorimetric and thermo-dynamic methods measure exactly the same heat, these heats do not differsignificantly where both methods have been used on the sameFor non-polar gases these heats are within experimental accuraoy inde-pendent of temperature, but are characteristic functions of the amount ofgas occluded 179 16, 21 (Fig.5). A rather similar form of curve is alsoobtained for van der Waals adsorption on glass,36 charcoal,37 gra~hite,~Band simple ionic crystals,39 and various causes of it have been discussed.38, 1 7 9 3 9Sorption heats in chabazite, even of small permanent gases such as nitrogen,are unusually large,l7 and it has been shown that this can be due to theinterstitial position of the solute molecule.For equal diameters of soluteand interstice the van der Waals energy of interaction may amount toabout 8 times the value observed when the occluded molecule is transferred33 A. Lamb and E. Ohl, J . Amer. Chem. SOC., 1936, 67, 2154; M. G. Evans, Proc.Roy. SOC., 1931, A , 134, 97; M. Hey and F. Bannister, Min. Mag., 1934, 28, 483.34 Idem, ref. (12); M. Hey, ref. (27); A. Tiselius, 2. phyaikal. Chem., 1935, A , 174,401 ; E. Rabinowitsch, ref. (26);. R. M. Barrer, refs. (5) and (7).36 E.g., M. Hey, ref. (27); see also S. Brunsuer, “ The Adsorption of Gases andVapours,” Vol. 1, Oxford Univ. Press, 1944, Chap. 8.as W. Keesom and J. Schweers, Physicu, 1941, 8, 1007, 1020, 1032.37 A. van Itterbeek and W.van Dingenen, ibid., 1937, 4, 389.38 R. M. Barrer, Proc. Roy. SOC., 1937, A , 161, 476.39 W. J. Orr, ibid., 1939, A , 178, 349.J. de Boer and J. Custers, 2. phyeikal. Chem., 1934, B, 26, 22640 GENERAL AND PHYS1tJA.L CHEMISTRY.to a plane surface of the same sorbent. The sorption heat of n-paraffinsin chabazite is nearly a linear function of the number of CH, groups in themo1ecule,l6 and rises by about 3000 cab. per CH,. For high molecularweight n-paraffins the sorption heat could become greater than for manychemical reactions, although it is due only to dispersion and polarisationforces .I6Entropies (p. 36) and free energies of occlusion have also been obtainedfor sorption in chabazite and active analcite, as functions of the charge ofgas and a t different temperatures.16 Free energy-0 curves have the sameform as the corresponding heat of sorption-0 curves, and the standard freeenergy series for a given zeolite and group of solutes parallels the corre-sponding series of sorption heats; i.e., entropies of occlusion are all of thesame order (AA = AU - TAS).I I I I00 20 40 so 80 Id0 % DehydrationFIG. 6.Sorption at 0" and 1600 mm..of CO,, O,, and H, by chabazite at various degreesof dehydration .41Curve ( 2 ) : 0,. Cutwe (1) : C02. Curve (3) : H,.Factors Injluencing Sorption Equilibrium in Zeolites.-The power of azeolite to occlude depends in large measure upon the severity and durationof the heating and degassing treatment to which it has been subjected.These determine the extent of both lattice collapse or alteration and ofdehydration. Fig.6 shows the equilibrium sorption of carbon dioxide bychabazite as a function of the water content of the mineraL41 A maximumoccurs a t about 95% dehydration; conditions severe enough to removemore water probably lead simultaneously to lattice collapse. Parallel withthis striking result, the heat of occlusion was measured as a function of thedegree of hydration 42 (Fig. 7).A recentstudy using mordenites revealed that base-exchange may alter both mole-4l A. Lamb and J. Woodhouse, J . Amer. Chem. SOC., 1936, 58, 2637.A. Lamb and E. OH, ref. (33).Sorption equilibrium may also be modified by base-exchangeBARRER: ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES.41cular sieve properties of zeolites (p. 4.4) and sorption equilibria and heats.’Affinities and heats of sorption of nitrogen and oxygen increased, for theexchanging ions Na+ < Ba++ < Ca++, in the order of the relative polarisingpower of the interstitial cation. Similar results were deduced 7 from thereIative escaping tendency of water vapour from base-exchange stilbites,chabazites, and he~landites.~~Yo Debydraflon of chabazite.FIG. 7 .Heats of sorption of CH,*OH i n chubazite (at a charge of 0.000178 mol.Ig.) a8 afunction of the yo dehydration of the chubazite.gaKinetic Behaviour of Solutes in Zeolites.-One must distinguish betweenthe usual simple zeolitic solution and the less usual sorption in whicha new phase appears, with discontinuity in the isobar or isotherm (p.38).In the latter case the sorption rate was “autoca6alytic” and the processwas regarded as a growth and diffusion of phase boundaries.17 Whensimple zeolitic solutions are formed, occlusion has been interpreted as a purediffusion process not complicated by slow rates of transfer of solute acromthe ingoing surface. As a diffusion process, sorption rates follow Fick’alaws, and the ‘‘ parabolic ” diffusion equation has been most successfully73 16 Potential solutes may be divided into three groups (seeTable V) : ( a ) molecules occluded extremely rapidly, (b) those occludedslowly at room temperature or above, ( c ) those excluded from the zeolitelattice. These groups grade continuously into one another.Group (a)comprises small molecules, or molecules of small cross section (e.g., inchabazite, gmelinite, and active analcite, helium, hydrogen, oxygen, nitro-gen, argon, methane, and ethane; and in natural mordenite, helium,hydrogen, oxygen, and nitrogen). For the first three minerals, Group ( b )included n-paraffins, and Group (c) included iso-paraffins and aromatichydrocarbons, but in mordenite Group ( b ) included methane and ethane,and Group ( c ) n-paraffins. Group ( b ) solutea enter the lattice by a diffusion43 E. Lowenstein, 2. anorg. Chem., 1909, 83, 69.4 4 P. Emmett and T. DeWitt, J . Amer. Chem. Soc., 1943, 66, 1263.B 42 GENERAL AND PHYSICAL CHEMISTRY.process of which each unit act requires an energy of activation.l6Y 7, 44 Thisenergy is needed to move the solute molecule from one position of maximumsorption potential to another within the crystal, and in an appropriatelattice was observed even for the inert gas argon. Table I11 records anumber of apparent energies of activation, which for the n-paraffin seriesincrease somewhat with increasing charge of gas and chain length.TABLE 111.Apparent Activation Energies for Diffusion of Some Solutes into Ze~lites.~sZeolite.Diffusing molecule. Ed (cals. /g.-mol. of solute).C3H 8 4500, 6700Chabazi te n-C&,, 8900, 8600n-C 6H 12 7100, 6800n-C,H,, 11,100, 11,400, 9600C,H,.CN 5900H.CO,Et 7300CH,*CO,Me 40,000Active analcite C3H 8 6800, 7300Mordenite C2H6 4300CH3*CN 830035004900 ACa-Mordenite N, 1040Several measurements have been made of diffusion constants, D, ofammonia and water in analcite and h e ~ l a n d i t e .~ ~ In heulandite, diffusionanisotropy occurs, and at 20" normal to the (201) and (001) planes (eachnormal to the unit lamellae of the heulandite layer lattice), Dool : D,, =1 : 11-6-20. In one sample the activation energies for the diffusion across(001) and (201) planes were 5400 and 9140 cals./g.-mol. A kinetic theoryof the diffusion constant has been successfully applied to Tiselius's measure-ments of D.47 The diffiision constant is a function of the charge of solutealready within the lattice. At first, inhomogeneity of the zeolite lattice,chemical or physical, causes solute molecules to be anchored at sites ofhighest sorption potential, so restricting their mobility.Again, when nearlyall sites are occupied the chance of an adjacent vacant site into which thesolute might move is small and so mobility is again reduced. In a homo-geneous zeolite, D is related to 8, the fraction of the interstices occupied,and Do, its value when 8 = 0, by D = Do (1 - 8).48 A considerable volumeof evidence has been obtained that D does behave in the mannerpredicted.16, 7,Rates of Occlusion of Solutes as a Function of Water Content, HeatTreatment, and Grain Size.-The treatment accorded to a zeolite may greatlymodify the sorption rates of solutes. One important variable is the extentof dehydration, the mineral occluding gases more rapidly the more com-pletely the water is removed, providing conditions do not simultaneously4 6 R.M. Barrer, J . Soc. Chem. Ind., in press.46 A. Tiselius and S. Brohult, 2. physikal. Chem., 1934, A , 168, 248; A. Tiselius,ibid., 1934, A, 189, 425; idem, ref. (34).4 7 M. Hey, ref. (26); see also idem, Phil. Mag., 1936, 22, 492.48 R. M. Barrer, ref, ( 1 ) ; Trans. Faraday SOC., 1941, 37, 690.Ba-Mordenite NBARRER : ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES. 43give lattice collapse. Conditions for producing very active chabazite insmall amount have been established,l6 and the influence of degree ofdehydration on some sorption rates and equilibria measured44 (cf. p. 44)).Molecular sieve properties of chabazite in relation to its water content aresummarised in Table IV.These results were extended by the observationTABLE IV.Effect of Degree of Dehydration upon Sorption Rates and Equilibria inChabaxite.Dehydr-&ion, yo. Gas. Behaviour.50 C02 Considerable sorption at -78".67N,H,N2O2A96 NO:} All three gases copiously and rapidly occluded at - 183".Athat nitrogen entered the chabazite when 67% dehydrated by a process ofactivated diffusion,44 whereas there is no measurable energy of activationAppreciable sorption at -78"; occlusion at negligible rate at -195".Considerable sorption at - 195".Still only small sorption in finite time at -183".Copiously occluded at - 183".Copiously occluded at - 183".700 200 300Time. in minutes.FIG. 8.Influence of state of subdivision of chabazite upon the rate of occlusion of propanein it.In all experiments, the 11u;~ss of chabazite = 3.336 g., the initial pressure ofC,H, = 9.6 f 0.1 cm. H g and T = 200" C.16Curve 1 : Particles $-J$ inch diameter, outgassed 14 hrs. at 470-480" C.Curve 2 : Particles 20-30 mesh, outgassed 14 hrs. at 470-480" C.Curve 3 : Particles 80-100 msh, outgassed 14 hrs. at 470-480" C.Curve 4 : Particles < 200 mesh, outgassed 14 hrs. at 470-480" C.Curve 6 : Particles 180-200 mesh, outgassed 7 hrs. at 470-480" C.when this gas enters a well outgassed chabazite.17 It seems likely thatwater molecules immobilised in the zeolitic channel a t - 1 8 3 O impede theflow of nitrogen within the crystal. Other examples in which one substanc44 GENERAL AND PHYSICAL CHEMISTRY.impedes the sorption of a second substance by a zeolite have been reported,and may in some cases be due to the same cause.45An investigation of the influence of degree of subdivision upon sorptionrates of n-paraffins revealed that, in qualitative accord with the theory ofdiffusion, the smaller the particle dimensions the more rapid was thesorption l6 (Fig.8).Molecular Xieve Properties.-The sorptive properties of several zeoliteshave now been extensively investigated, and in Table V are summarisedTABLE V.Molecules Occluded or Excluded by the Three Classes of Molecular Sieve.45Typicalmoleculesrapidlyoccluded atroom temp.or below.Section (i).Class 1 minerals HeNeAH 2N2 co(302 cos, cs,H2OHCl, HBrNOZZ:*OHCH,*NH,CH,*CNHCNC1aCH,Cl, CH3BrCH,FCH3-SHSection (ii).Class 2 minerals HeNeNH3Ha0Section (iii).Class 3 minerals HeNeTypical moleculesmoderately rapidlyor slowly occludedat room temp.orabove in the thermalstability range.C,H, and simpleC,H ,*OHC,H ,*NH,higher n-paraffisCZHSFCaH5BrI,, HICH,BraCH,IC,H ,*CNC,H,*SHHCO,Me, H*C02EtCOMe,CH,*CO,MeNHMe,, NHEt,CaHbC1CHj*NH,CI.I,.CNCH3Cl, CH,FHCNCS,c1,AHClm3Typical moleculesoccluded at negligiblerates, or totallyexcluded withinthe thermalstability range.Branched-chain hydro-carbons. cyclo-Paraf-fins. Aromatic hydro-carbons. Derivativesof all these hydro-carbons. Heterocyclicmolecules (e.g., thio-phen, pyridine, pyr-role).CHCl,, CCl,,CHCl:CCl,, CH,*CHCl,,CHCl,.CCl,, C,C16 andanalogous bromo- andiodo-compounds. Se-condary straight-chainalcohols, thiols, ni-triles, and halides.Primary amines withNH, attached to asecondary C atom.Tertiary ammes.Branched-chain ethers,thioethers, and second-ary amines.All classes of moleculesin cols. 3 and 4 above.All molecules referred toin col. 4, section (ii).Also : CH,, C,H;,CH,*OH, CH,*SH,C H a * C N , CHa*NH,,CHSCl, CHaFBARRER : ZEOLITES AS ABSORBENTS AND MOLECULAR SIEVES. 45the sorptive and molecular sieve characteristics of three distinct categoriesof sieve.45 Class 1 is represented by well-outgassed chabazite, gmelinite,active analcite, and a synthetic crystalline zeolitic mineral; class 2 bywell-outgassed natural mordenite ; and class 3 by well-outgassed calciumand barium mordenites produced hydrothermally a t high temperatures.Convenient dehitions of the three classes of molecular sieve were given asfollows : Class 1 exclude iso-paraffins, occlude n-paraffins slowly, and methane,ethane, and molecules of smaller cross-section rapidly, a t room temperature.Class 2 exclude n-paraffins (and molecules of similar and larger cross-section) ,occlude methane and ethane slowly, and nitrogen and molecules of similarand smaller cross-section rapidly, at room temperature.Class 3 shownegligible occlusion of methane and ethane, and molecules of larger cross-section, but occlude nitrogen, oxygen, and molecules of smaller cross-sectionrapidly.The three classes of crystal sieve have been used to resolve molecularmixtures, frequently with striking success.For example, by using chabaziteit has been shown that simpler n-paraffins may be separated quantitativelyfrom all iso-paraffins and aromatic hydrocarbons, and that C, and C, hydro-carbons may be removed from propane and higher hydrocarbon^.^^^ 49The molecular sieve method was extended to the separation of polarand polarisable molecules from admixture with other m0lecules.~5 One ormore constituents were removed from 52 different typical liquid mixturescontaining as many as five components, by using a class 1 zeolite; 17 suchseparations were effected by means of a class 2 zeolite; and 6 by class 3zeolites. In many cases these separations are single representatives onlyof whole groups of separations, so that the method is of some generalityand power.The separation depends primarily upon differences in molecularshape and size, and not upon differences in boiling point. In this way itmay supplement distillation technique, e.g., in resolution of azeotropicmixtures, many of which (some of industrial importance) can be separated.The rate of separation varies from extremely rapid to very slow, but pro-vided the components of the mixture do not decompose on heating, rise intemperature frequently accelerates the separation rate many times.Monosubstituted methanes in which the substituent groups are small,such as C1, CH,, OH, CN, NH, and the like, are very rapidly occluded bychabazite, and monosubstituted ethanes with similar substituent groupsenter the lattice considerably more slowly (Table V).Both classes of solutemay be quantitatively separated from molecules in col. 4, section (i), ofTable V, and the monosubstituted methanes can also be partly or com-pletely separated from similarly substituted ethanes. In a natural mordenitethe monosubstituted methanes were slowly occluded (CH,*OH, CH,*NH,,CH,*CN and the like), but monosubstituted ethanes were excluded (Table V).Separations were therefore obtained of one- from two-carbon molecules.On the other hand, the class 3 occlusives, calcium and barium mordenite,did not occlude even molecules with one carbon atom, so that the separations4 9 R.M. Barrer, B.P. No. 548,905; U.S.P. No. 2,306,61046 GENERAL AND PHYSICAL CHEMISTRY.obtained were of simple inorganic molecules (NH,, H20, HCl) from organicgases and vapours.R. M. B.4. CRYSTALLOGRAPHY.i. Introduction.SECTIONS iii and iv of this Report aim at giving a reasonably complete accountof the more important inorganic and organic structures which have beendetermined by X-ray or electron-diffraction methods during the year. Ina few cases, however, only a bare reference is given; and it is possible thatcertain other work may have been missed entirely, because many of thepublications are still rather inaccessible.On the other hand, no attempt is made to cover the wider aspects of thesubject in the form of a systematic Annual Report.Instead, the plan is tohave certain special articles at longer intervals which will review variousbranches of the subject. This year Section ii gives a brief account of ambi-guities which may occur in the analysis of diffraction patterns, a subjectwhich is clearly of importance in all structural work. The large and growingamount of X-ray work on natural and synthetic fibre structures demandssome special treatment, and this is given in Section v. Finally, we includea brief general article on the electron microscope in Section vi. The applica-tions of this instrument to problems of chemical interest are steadily increas-ing. Ultimately we may expect some overlap between electron microscopemethods and the more usual diffraction methods, in the analysis of complexmolecular structures.With this possibility in mind, the present sectionhas been included in this Report and is of an introductory nature.A number of important topics are of necessity entirely omitted from thisReport, but it is hoped that these may in turn be covered by further specialarticles in the future. A,mong these subjects are optical and magneticproperties, lattice vibrations and the physics of crystals in general, thestructure of metals, alloys, and minerals, the investigation of surface problemsby electron diffraction,l the structure of proteins and other complex sub-stances not of the fibre type, the diffraction of X-rays by liquids and amor-phous substances, and general improvements in experimental and interpre-tative technique.The recent extensive work on the structure of diamondsof different types may also merit a special review when the results becomeclearer.The new structure determinations now reported (Sections iii and iv)contain a fairly large number of reasonabIy accurate bond-length deter-minations, notably for halogen derivatives. The whole question of thederivation of bond lengths from the standard covalent radii is now under-0. P. Thomson, J . Inst. Metal8, 1943, 69, 191 (lecture).N. S. Gingrich, Rev. Mod. P h y s k , 1943,15, 90 (review).a (Sir) R. Robertson, J. J. Fox, and A. E. Martin, Phil. Trans., 1943, A , 232, 463;Proc. Roy. SOC., 1936, A, 157, 579; (Sir) C. V. Raman et al., Proc. Indian A d . Sci.,1944, 189; G. D. Preston, Nature, 1945, 156, 69; (Mrs.) K.Lonsdale, ibid., p. 144ROBERTSON : CRYSTALLOGRAPHY. 47going considerable revision. Following the new single- bond covalent radiifor oxygen, nitrogen, and fluorine,* contractions from predicted values arenow seen to be more numerous than was previously thought, and their causeis the subject of much discussion. In general, the effect on the distance ofthe polar character of the bond between unlike atoms is probably greaterthan was formerly realised. The new data now recorded should be of greatvalue in any general rediscussion of bond distances. Amongst new structuredeterminations that of ammonium pentachlorozincate, (NH,),ZnCl,, is ofinterest as it reveals only a four-fold type of co-ordination, the structureconsisting of a packing of NH,+, C1-, and ZnC1,- ions, a situation somewhatreminiscent of the structures of phosphorus penta-chloride and - br~rnide.~In the organic section, the structure of the carboxyl group has receivedconsiderable attention and the latest determinations make the two C-0distances almost as different as for pure single and pure double bonds.That is for non-associated acids ; on association, interesting changas aredetected and the distances become more nearly equal, although still mmain-ing quite distinct.Finally, in the ion, complete equality is to be expected.Other new results indicate that the conjugating power of phenyl groupsmay be less than has hitherto been supposed, and in compounds like diphenylthere appears to be very little contraction in the length of the connectinglink. The tendency for coplanarity of the rings is correspondingly small,and in the vapour the molecule of diphenyl is probably not coplanar.Thecrystal structure should be re-examined. The structure of the highlysymmetrical coronene molecule has now been examined by crystal analysisand the preliminary results are in good agreement with molecular orbitalcalculations of the interatomic distances.J. M. R.ii. Ambiguities in the Analysis of Diffraction Patterns.If a crystal structure is expressed as a continuous distribution of scatter-ing matter a knowledge of the absolute values of the structure amplitudes isnot sufficient to define the distribution. An infinite number of distinctdistributions can clearly be obtained by attaching arbitrary values to theunknown phase constants associated with the observed amplitudes, andevaluating the corresponding Fourier series.I n nearly all actual cases thefundamental assumption is therefore made that the structure is composed ofatoms, of a kind and number indicated by elementary chemical analysis.The problem can then be reduced to finding what co-ordinates must beattached to a limited number of scattering points in order fo explain theobserved diffraction pattern.In single-crystal analysis the number of observed and measurable X-rayreflections is always much greater than the number of atoms whose positiomhave to be discovered. In fact, it should generally be possible to make 60V. Schomaker and D.P. Stevenson, J . Amer. Chern. Soc., 1941, 65, 37; see Ann.Reports, 1943, 40, 86.ti Ibid., 1942, 39, 10148 GENERAL AND PHYSICAL CHEMISTRY.or more observations per atom without difficulty. It might then be thoughtthat atomic positions which yielded perfect agreements between the measuredand the calculated X-ray intensities throughout the whole range of observ-ations would constitute a unique solution of the structural problem involved.That this is not necessarily the case was proved originally by L. Panling andM. D. Shappell for a special point of the space-group Ti - Ia3, wherepositive and negative values of one parameter were found to yield the sameX-ray intensities. A similar property is shown by the correspondingspecial point for the space-group Oy - I ~ 3 d .~ In general, any distinctdistributions of points which possess the same vector distances will obviously@ . . *. . .@a ....... - ....(-J(f+-=. ' *. . . s a :@.* :. . .. . . * * .. .. * . ::L .......-......FIG. 1.Three set8 of homometric quadrupletsfor the cyclotomic set n = 16, r = 8(A. L. Patterson, Physical Rev., loc. cit.).give rise to the same X-ray diffraction pattern. Such sets of points arecalled " homometric."A. L. Patterson has now provided a very useful general discussion ofsuch ambiguities in X-ray crystal analysis. His treatmenf is confinedchiefly to one-dimensional cases. Such linear periodic distributions can beconvenienfly represented by plotting the points on the circumference of acircle, and the cases examined are those obtained by using Q of the n vkrticesof an inscribed regular polygon.Sets of points obtained in this way arecalled " cyclotomic " sets. Out of 2664 cyclotomic sets examined (up ton = 16) Patterson has found a total of 390 homometric pairs, 7 sets of homo-metric triplets, and 3 sets of quadruplets. The last distributions are shown2. Krist., 1930, 75, 128. A. L. Patterson, Nature, 1939, 145, 939.3 Physical Rev., 1944, 65, 195ROBERTSON : CRYSTALLOGRAPHY. 49by the black dots in Fig. 1. I n these sets each member obviously representsa distinct distribution of points, yet all four would give rise to exactly thesame X-ray diffraction pattern.There appears to be considerable difficulty in developing any generaltheory for the occurrence of homometric sets, but the treatment can beextended in various ways.For example, it is sometimes possible to intro-duce a variable parameter between certain points and so produce an infinityof homometric pairs. It is also possible to extend the treatment to two andthree dimensions, but these possibilities have not yet been fully explored.The results so far obtained, however, are very significant and they clearlyreveal the limitations of the X-ray method of crystal analysis. The sametype of ambiguity must also apply, in a more drastic manner, to the resultsof gas-diffraction experiments. There the molecules are widely separatedand scatter independently. The experimental data give in effect the super-position of all the interatomic distances in the molecule, and as these distancesarQ no longer vector quantities, the possibility of ambiguity must be greatlyincreased.For example, the cases illustrated (see Fig. 1) represent onlylinear periodic distributions for crystal analysis, but in gas-diffraction analysiscyclic molecules of these types would give rise to similar ambiguities.These findings emphasise the fact that the results of diffraction experi-mefits should in general be confirmed by other independent lines of physicaland chemical evidence. Optical and magnetic properties, for example,afford valuable auxiliary evidence, and the various experiments should allagree before a structure is finally accepted as being true. The essence of adiffraction experiment is rather to show that a given postulated structure isconsistent with the data, than to attempt to establish a truly unique solution.At the same time if alternative solutions should exist in any given case, itseems very unlikely that they would both be chemically reasonable.The above discussion, of course, assumes complete ignorance of the phaseconstants of the X-ray reflections.If these can be determined, a trulyunique solution to the problem immediately becomes possible. In a largeand growing number of cases this can be done, for example, by the com-parison of members of an isomorphous series, as in the phthalocyanines4or by successive approximations involving the use of a heavy atom in them~lecule.~ The ambiguities discussed above do not arise in such cases, andthe results do represent unique solutions.J.M. R.iii. Inorganic Strwtures.Halogen Derivatives of Tin, Arsenic, and iVitrogen.-A very extensiveinvestigation by electron diffraction of 14 of these derivatives has now beenreported by A. H. Skinner and L. E. Sutt0n.l From 6 to 9 plates were takenon the vapour of each substance, and these showed from 4 to 6 maxima for* J. M. Robertson, J., 1935, 615; 1936, 1195.Idem, Nature, 1939, 143, 75; J. M. Robertson and (Miss) I. Woodward, J., 1940,Trans. Faraday SOC., 1944, 40, 164.3650 GENERAL AND PHYSICAL CHEMISTRY.the various compounds. Both the radial distribution and the correlationmethod of analyses were used. The models employed for the tin compoundsassumed the four valencies to be directed to the corners of a tetrahedron(not necessarily regular).For the compounds of nitrogen and tervalentarsenic non-coplanar valencies were assumed. Free rotation of the methylgroup was assumed in all compounds, and tested with satisfactory resultsin the case of trimethyltin monochloride.The final results regarding interatomic distances and valency anglesare given in Table I, together with some previous results for the tetrahalidesof tin and the trihalides of arsenic.2 The table also gives the length of theb6nd to the halogen as calculated from the covalent radii of L. Pauling andM. L. Huggins; for nitrogen, however, the new radius of 0.74 A. is used.4The small but progressive contraction which occursin the bond lengthwhen more halogens are added to the central atom is very clearly broughtout by these results.(The percentage contraction from the calculatedvalue is given in the last column of the table.) This phenomenon is a verygeneral one and has already been investigated and discussed in other seriesof compounds, such as thesilanes . tiCompound.SnMe,Cl .........SnMe,CI, .........SnMeCl, .........SnCl, ............SnMe,Br .........SnMe,Br, ......SnMeBr, .........SnBr, ............SnMe,I .........SnMe,I, .........SnMeI, .........SnI, ...............AsMe,Cl .........AsCl, ............AsMe,Br .........AsBr, ............AsI, ...............NMeCl,. ...........AsMeJ .........NMe,Cl.. ..........AngleX-M-X.108°f40110 f 5108 f 4109.5109 f 3-109.5109.5 f 2109.5-109.5109.5 f 3109.5 f 2109.598 f 3103 &396 f 3100 f 298 A4100 &2-108 f lfluorinated- methanes and the chlorinatedTABLE I.C-M, A.2-1 9 f 0.032.19f0.052-17 f0-05-2.17-2.17-2.17-----1.47 -+0*02 -* Expansion.M-X, A.obs.2.37 f0.032-34 f 0-032.32 f 0.032.30 f0-032.49 f0.032.48 f0-022-45 k0.022.44 f0.022.72 f0.032.69 f0.032-68f 0.022.64 &O-022-1 8 f0.042.16f0.032*34&0*042.36 k0.042-52 f 0-032-58 IfI0.051-77 f0.021-74 f0.02M-X,calc.2-392-392.392.392.542.542.542.542.732-732.732-732-202.202.352-352.542-541-731.73Contrac-hion, yo.0.82-12.93.82.02.43.53.90-41.51.83.30.91.80.00.00.8-1.6 *-2.3 *-0.6 *As Skinner and Sutton point out, any straightforward explanation ofthis effect in terms of either bond multiplicity or ionic character of bonds isdifficult, but the latter effect appears to be the more important for these2 I,.0. Brockway and F. T. Wall, J. Amer. Chem. SOC., 1934,56,2373; M. W. Listerand L. E. Sutton, Trane. Farachy SOC., 1941, 37, 393, 406.2. KrGt., 1934, 87,205.4 V. Schomaker and D. P. Stevenson, J. Amer. Chem. SOC., 1941, 63, 37.6 L. 0. Brockway and H. 0. Jenkins, ibid., 1936,58,2036.6 L. 0. Brockway and I. E. Coop, Trans. Furuday Soc., 1938,34,1429ROBERTSON : CRYSTALLOGRAPHY. 51cases. Each successive halogen addition will tend to increase the positivechange on the central atom, with corresponding decrease of its normalradius.There will be a similar enlargement of the halogen atom withnegative charge, but this effect on any one of the halogens will tend todiminish as more halogens are added. The net attractive force will alsoincrease between the central atom and each halogen, and so from bothcauses we might expect a small progressive decrease in bond distance. Wemay expect a more quantitative development of this theory in the future.Phosphoryl and Thiophosphoryl Halides.-Further interesting deviationsfrom the predicted covalent bond radii for halogen-phosphorus, oxygen-phosphorus, and sulphur-phosphorus distances have been observed byJ. H. Secrist and L. 0. Brockway in an electron-diffraction investigationof the compounds POBr,, PSBr,, PSFBr,, and PSF,Br.For the first twomolecules trigonal symmetry is assumed and the structures are described interms of two parameters, the Br-P-Br angle and the P-0 or P-S : P-Brbond-length ratio. The calculations are therefore relatively straight-forward. The lower symmetry of the other molecules makes the calculationsmore difficult, but a large number of models have been tested and examined.The radial distribution method has also been employed in each case.The final results for the bond lengths (in A.) and valency angles in thesemolecules are given in Table 11. Allowance being made for all uncertaintiesit is clear that the phosphorus-halogen distances (especially the P-F dis-tance) are considerably shorter than the predicted values.The P-0 andP-S distances are also distinctly less than the sum of the respective double-bond radii. In general, the new results confirm and extend the findings ofprevious investigations on similar compounds.*TABLE 11.CovalentPOBr,. PSBr,. PSFBr,. PSF,Br. radius sum.P S ......... - 1*89&0*06 1-87 f0.05 1.87 f0.05 1.94 *P-F ......... - - 1.50f0-10 1.45f0-08 1.74P-Br ......... 2.06f0-03 2.13f0.03 2.18f0.03 2.14&0*04 2-24 - - 1.57 * P-0 ......... 1.41 k0.07 -A - - BrPBr ...... 108Of3" 106:f3" 100°f3"F@Br ...... - - - 106"f3" -* Calculated for double bond.Silicon Halides.-Further electron diffraction studies have been carriedout by R. L. Livingsfone and L. 0. Brockway8a on silicon dimethyldichloride, silicon methyl trichloride, and trifluorosilicon chloride.TheSi-Cl distances are similar to previous measurements and the Si-C distancesshow some contraction, but details must be deferred.' J . Amer. Chrn. SOC., 1944, 66, 94.L. 0. Brockway and J. Y . Beach, ibid., 1938, 60, 1836; D. P. Stevenson and H.Ruase11, $bid., 1939, 61, 3264; J. Y. Beach and D. P. Stevenson, J . Chem. P h y e b ,1938, 6, 75.8a J . Arner. Chern. XOC., 1944, 86, 9452 GENERAL AND PHYSICAL CHEMISTRY.Silver Salts and the Ag-0 Bond.-The crystal structure of potassium silvercarbonate has been determined by J. Donohue and L. Helmh~lz.~ Thecarbonate group is assumed to have the same configuration and dimensionsas in calcite,1° and the positions of the potassium and silver atoms can bedetermined with considerable accuracy from Fourier analyses of the visuallyestimated intensities.It is found that the silver atoms are surrounded byfour oxygen atoms a t 2.42 & 0.05 A. There is one potassium-oxygendistance of 2.65 & 0.08 A., and others varying from 2.9 to 3.0 A.of the structure of silver carbonate, wherethe silver atoms are surrounded by deformed tetrahedra of oxygen atomswhose distance is estimated at about 2.3 A.Silver oxalate has been examined by R. L. Gr%th.ll The monocliniccrystals appear to be rather similar to some other oxalates, with a short btranslation (3.46 A.), and two molecules in the unit cell (space-group C& -P2,/c). Intensities were estimated visually, and Fourier projections serveto define the positions of the silver atoms with some accuracy, but theoxygen and the carbon atoms can hardly be located at all.Conclusionsregarding the structure of the oxalate group are therefore unreliabk, butthese groups appear to be bound together to form chains by means of twoshort Ag-0 bonds of length 2.17 and 2.30 A. Four other Ag-0 distancesrange from 2.6 to 3.0 A.The average Ag-0 bond distance is found to vary considerably in differentcompounds,12 from 2.51 A. in silver chlorate to 2.06 A. in the oxide. Suchvariations in the Ag-0 distance are of interest in connection with the rulesuggested by K. S. Pitzer and J. H. Hildebrand l3 which relates the colourof a compound formed from colourless ions to the amount of covalentcharacter in the bond between these ions.For Ag-0 the ionic radius sumis 2-46 A. and the covalent radius sum 2.19 A., so that for comparable struc-ture types it should in some cases be possible to make estimates of thebond character. This matter is discussed in the paper by Donohue andLevine . l2Ammonium Pentachloroxincate.-A very full determination of the crystalstructure of this salt, (NH,),ZnC15, has now been made by H. P. mug andL. Alexander.14 The work is of interest in View of the rather infrequentoccurrence of the MX, group in inorganic chemistry. I n the exampleswhich have already been studied the actual co-ordination has usually provedto be some combination of four- or six-fold types. In phosphorus penta-chloride there is a combination of tetrahedral Pa+, and octahedral PC1;;groups,15 and in the pentabromide PBri and Br- ions.16 The crystalA brief report is also given9 J , Amer.Chem. SOC., 1944, 66, 295.N. Elliott, i b d . , 1937, 59, 1380. l1 J . Chem. Physics, 1943, 11, 499.l4 Ibid., 1944, 66, 1056.l2 L. Helmholz and R. Levine, J . Amer. Chem. SOC., 1942,64,354.18 Ibid., 1941, 65, 2472.16 H. M. Powell, D. Clark, and A. F. Wells, J . , 1942, 642; Ann. Reports, 1942, a,l6 H. M. Powell and D. Clark, Nature, 1940, 146, 971.101ROBERTSON : CRYSTALLOGRAPHY. 53structures of TlAlF, and K&lF,,H,O 1' show that infinite chains of AlF,octahedra extend through the crystal in such a way as to make the netcomposition AlF5.Ammonium pentachlorozincate .crystals are orthorhombic, space-groupDG - Pnma, and the unit cell contains four molecules of the composition(NH,),ZnCl,. The analysis was carried out bn about 600 reflections (Cu-Karadiation) obtained from small crystals.The intensities were estimatedvisually from oscillation photographs, the multiple film technique l8 beingemployed. Analysis of the structure by trial proved too difficult, but thepositions of the heavy atoms were finally obtained by evaluating the Patter-son functions. Two-dimensional Fourier projections then gave the positionsof all the atoms with considerable accuracy.The unit cell is found to contain four tetrahedral ZnC1, groups, theaverage Zn-C1 distance being 2.25 & 0.03 A,, a value very close to the sumof the tetrahedral covalent radii l9 (2.30 A,).The remaining chlorineatoms are separated from the zinc atoms by over 4.4 A., and each of thesechlorines is surrounded by a distorted octahedron of NH,+ ions, with anaverage Cl-NH, distance of 3.41 A,, which is rather greater than the sum ofthe ionic radii. Each ammonium ion is in turn surrounded by groups ofchlorine atoms.The structure is thus seen to represent a, packing of NH,+, C1-, andZnC1,- ions, which would be better expressed by writing the chemicalformula as (NH,) ,ZnCl,,NH,Cl. The Zn-C1 bonds are essentially covalent,but the other linkages are ionic (except N-H). Once again, therefore, thecrystal is found t o avoid the difficulty of five-fold co-ordination.Sulphur Compounds.-C. S. Lu and J. Donohue20 have examined anumber of sulphur compounds by the method of electron diffraction.Forsulphur itself they find that the S8 molecule has essentially the same con-figuration as in the crystal,el vix., a regular, puckered, eight-memberedring. It is interesting to note that several systematic distortions of thisring, such as the " tub," " chair," " cradle," and " butterfly '' forms, weretested and shown to be incompatible with the radial distribution curve.Hence, the fraction of sulphur molecules which have these configurationsin the vapour phase must be very small. However, the results do show thatthere must be a rather large thermal vibration associated with the puckeredring structure. The S-S distance of 2-07 & 0.02 A., and the angle S S S of105" -& Z0, are fairly close to previous determinations.Orpiment sublimes at high temperatures apparently to give Aspsgmolecules, for a model based on this structure 22 gives a satisfactory explan-ation of the pattern.In this model As-S = 2.25 & 0.02 A,, angleC. Brosset, 2. anorg. Chem., 1937, 236, 139.18 J. M. Robertson, J . Sci. Instr., 1943, 20, 175.19 L. Pauling and M. L. Huggins, 2. Krist., 1934,87,205.2O J . Arner. Chem. SOC., 1944, 66, 818.21 B. E. Warren md J. T. Burwell, J . Chm. P h y e k , 1936,8,6.2z R. M. Bozorth, J . Amer. Chem. BOG., 1923, 45, 162154 GENERAL AND PHYSICAL CHEMISTRY.ASS-As = 100' &- 2". These molecules also display rather large thermalvibrations.Two other structures, those of sulphur nitride, S4N4, and realgar, Asps4,have been included in this electron-diffraction study, but although usefulresults have been obtained, which should assist in the exact analyses of thecrystals, yet the structures cannot be established with certainty from thegas-diffraction data alone.However, several previously proposed structurescan be definitely eliminated, and it is shown that an alternating eight-membered ring, of " cradle " shaped configuration, can lead iN\/"B to a satisfactory explanation of the diffraction pattern inboth cases. The " cradle " model for sulphur nitride is S\N/'\N/ rather closely related to the formula (inset) proposed byM. H. M. Arnold, J. A. C. Hagill, and J. M. H u t s ~ n , ~ ~ which wouldinvolve resonance among several bond structures.Other Structures.-Finally, reference may be made to a number of in-organic structures which have either been briefly reported, or for which thefull papers are not available to the Reporter.These include magnesiumcarbide,24 zinc cyanide,25 cadmium iodide 26 (which has a random structureif crystallised quickly), and the compounds SrMg,, BaMg,, and CaLi,.27The hydrate SrCI2,6H,O appears to have been fully studied 28 and has achain type of structure with linkages through the water molecules. Ortho-rhombic lead mon0xide,2~ antimony trifiuoride 3O (with an Sb-F distanceof 2.0 A.), natrophilite, NaMnP04,31 colemanite, 2Ca0,3B,03,5H,0,32ammonium ~hloroiridate,~3, sodium and ammonium iodateF4 and the high-temperature modification of sodium nifriteF5 have all received attention.The lattice constants of an isomorphous series magnesium, manganese,ferrous, cobalt, nickel, and zinc metantimonites, MSb,O,, have beenmemured, and the structures are reported to be similar to that of pb,04.36In another isomorphous series, comprising chromium vanadate and nickel,cobalt, copper, zinc, and cadmium chromates, both the metal atoms aresurrounded by distorted octahedra or tetrahedra of oxygen atoms withcommon edges.37J.M. R.23 J., 1936, 1645.24 M. A. Bredig, J . Amer. Chem. Soc., 1343, 65, 1482.25 H. S. Shdanov, Compt. rend. A d . Sci. U.R.S.S., 1941, 31, 352.26 G. Hagg and E. Hermansson, Arkiv Kemi, Min., ffeol., 1943, 17, B, No. 10.27 E. Hellner and F. Laves, 2. Krist., 1943,105, 134.** A. T.Jensen, " 5 Nordische Kemikermode," 1939, 201.29 A. Bystrom, Arkiv Kemi, Min., Qeol., 1944, 17, B, No. 8.30 A. Bystrom and A. Westgren, ibid., 1943, 17, B, No. 2.a2 V. A. Nikolsli, Compt. rend. A d . Sci. U.R.S.S., 1940, 28, 59.33 G. B. Boki and P. I. Usikov, ibid., 1940,26, 782.34 C. H. MacGillavry and C. L. Panthaleon van Eck, Rec. Trav. chirn., 1943, 62,a5 B. Strijk and C. H. MacGillavry, ibid., p. 705.a6 S. Stahl, Arkiv Kemi, Min., Cfeol., 1943,17, B, No. 6.A Bystrom, ibid., No. 4.729.K. Brandt, ibid., 1943, 17, A, No. 6ROBERTSON : CRYSTALLOGRAPHY. 55iv. Organic Structures.Ethylene.-An early study of ethylene 1 provided some single-crystaldata, but the structure proposed is an improbable one because it fails to satisfymodern ideas regarding both interatomic and intermolecular distances.From the original data (which consist of only seven observed reflections)C.W. Bunn 2 has now calculated a reasonable structure, and has shown thata model with a C=C distance of 1.33 A. can give an even better account of theobserved X-ray intensities than the structure originally proposed. In thenew structure the closest approach between carbon atoms on neighbouringmolecules is now 3.8 A., a reasonable value.Xtructure of the Carboxyl Group.-The first detailed diffraction studies ofthe carboxyl group, carried out on the vapour of formic acid,3 indicated thatthe carbon-oxygen distances were equal, at about 1-29 A. A later quantit-ative X-ray investigation of oxalic acid dihydrate crystals gave slightlydifferent distances, of 1.30 and 1.24 A.From an examination of the struc-tures involved in the carboxyl group one would not expect equal distancesin the case of the acids themselves. Later spectroscopic studies on themonomers and dimers of formic and acetic acids 5 have confirmed this view.Equality of the C-0 distances might reasonably be expected, however, forthe free carboxylate ion.have now published a comprehensiveelectron-diffraction study of the monomers and dimers of formic, acetic, andtrifluoroacetic acids. The interesting question of what changes, if any,occur in the C-0 distances on association does not appear to have beenexamined in detail before, and it is almost certainly beyond the reach ofX-ray crystal analysis, where the crystal molecules would normally be asso-ciated in pairs, or in some larger groups, or with water molecules.In thegas-diffraction experiments the monomers were studied by photographingthe equilibrium vapour at approximately 150°, which represents over 90 yoJ. Karle and L. 0. BrockwayTABLE 111.Carboxylic acids (distances in A.).Formic acid.Monomer. Dimer. IA 1C-QH ......... 1-42&0-03 1*36&0*04'C%O ............ 1-24&0*03 1.25f0.03 c-c - -C-F - --OH. ... O=.. . - 2-73 f0.05........................Acetic acid. Trifluoroscetic acid.I ' -Monomer. Dimer. Monomer. Dimer.1.24 f0.03 1.25 f0-03) (average)1*54&0.04 1.54f0.04 - 1.48 f0.03 - - 1.36 f0.05 1.36 f0.03- 2.76&0*06 - 2.76 f0.06A1.43 f0.03 1.36 f0.04 - 1.30f0.03HO-&O ......117Of2" 121°f2" 122-138" 130°f3" - 130"f3"1 W. H. Keesom and K. W. Taconis, Physica, 1935, 2, 463.Trans. Faraday SOC., 1944, 40, 23.L. Pauling and L. 0. Brockway, Proc. Nut. Acad. Sci., 1934, 20, 336, 340.J. M. Robertson and (Miss) I. Woodward, J . , 1936, 1817.L. G. Bonner and R. Hofstadter, J . Chem. Physics, 1938, 6 , 531; M. M. Daviesand G. B. Sutherland, ibid., p. 755.a J . Arner. Chem. SOC., 3944, 60, 57456 QENBRAL AND PHYSICAL CHEMISTRY.monomer. For the experiments on the dimers, the vapour densities indi-cated a 9-15% dissociation, which could be allowed for in the calculations.The results of the diffraction experiments concerning distances andangles are set out in Table 111.For the monomers, i.e., for the unassociatedcarboxyl group, a surprisingly large difference of nearly 0-2 A. is foundbetween the two C-0 distances. In fact, the longer distance (1.42 A.) isabout the same as that observed in ether linkings, whereas the shortercorresponds to that found in ketones. The hydrogen must clearly beattached to one of the oxygens only, with the result that the carbon-oxygenbonds are more or less of the ordinary single- and double-bond types,respectively.In the analysis of the dimers the molecules are assumed to have theplanar centrospmetrical type of bridge (inset),and the agreements obtained on this basis of calcu-dimensions of the carboxyl group, it is significantthat attempts to explain the diffraction pattern with models based on themonomer were unsuccessful.The final results (Table 111) show a veryconsiderable equalising of the C-0 distances, brought about almost entirelyby a shortening of the " single"-bond C-OH distance. However, thereremains a considerable difference between the two C-0 distances, about0.1 A., which is in tolerable agreement with X-ray crystal results on otherstructures. Owing to this difference in C-0 distances, it seems clear thatthe hydrogen atom must remain attached more strongly to one of thepartners in the dimer than to the other. If it occupied an (average) centreposition in the O-H . . . 0 bridge we would expect correspondingsymmetry and equality in the C-0 distances. This is in general agreementwith spectroscopic and X-ray evidence for the type of hydrogen bridgeinvolved in these structures, which has a length (oxygen-oxygen distance)of about 2.7 A.In trifluoroacetic acid the relatively high scattering power of the CF,group prevented a full determinatiofi of structure.The C-0 distancescould not be determined individually, but the average is estimated at 1.30 A.The C-F distance of 1.36 A. and the reduced G-C distance of 1.48 A. are inagreement with other observations on such bonds. It is rather surprisingthat the tendency towards dimerisation and also the length of the hydrogenbridge are apparently but little affected by the increased strength of thisacid.Karle and Brockway also examined deuterium acetate dimer but nodifference could be observed between the qualitative appearance of thephotographs obtained and those of acetic acid.Although an isotope effect(increase in length of the hydrogen bridge on substitution of deuterium forhydrogen) as great as that observed in oxalic acid dihydrate 7 might bedetected by electron diffraction, yet it is unlikely that the acetic acid bridgewould display such a large effect. The effect, if any, would more likely beJ. M. Robertson and A. R. Ubbelohde, Proc. Roy. Soc., 1939, A , 170, 222.-C/O \OH * ' . ' , Ho . o>- lation justify the assumption. With regard to thROBERTSON : CRYSTALLOURAPHY. 57comparable to those observed in succinic and benzoic acids,' which are small.A hydrogen bridge of length 2.76 A . is not expected to give rise to a largeisotope effect.Organic SuZphonates.-A series of interesting single-crystal measure-ments have been made by L.M. Jensen and E. C. Lingafelter on thequarter-hydrates of sodium 1-octane-, l-decane-, l-dodecane-, l-tetra-decane-, 1-hexadecane-, and 1-octadecane-sulphonate. In each case thespace group is C: (Aa) or more probably C$ (A2/a), and the unit cell con-tains 32 molecules of the hydrate or 8 molecules of 4R*S03Na,H,0. Thecross-sectional area of the molecules is somewhat greater than that of sodiumstearate or stearic acid, and decreases uniformly from 20.99 A .z in the octanederivative to 20.08 A . ~ for the octadecane derivative; at the same time thep angle increases by about 6'. The c-axis increases in steps of 10.2 A. from55.39 to 106.46 A,, which corresponds to an increase of eight carbon atomsalong the c-axis for every additional two carbon atoms in the chain.Typicaldimensions are : a = 16.86, b = 10.17, c = 55.39 A., p = 101'39' forC8Hl,*S0,Na,&H20, and a = 16-73, b = 10.05, c = 106.46 A., p = 107" 13'for C,,H,,~SO,~a,~H,O. Such a good series of single crystals of long-chaincompounds has seldom been available for X-ray analysis.Four-membered Rings.-Methylenecydobutane (I) and l-methylcyclo-butene (11) have been studied by W. Shand, V. Schomaker, and J. R. Fi~cher,~by the gas-diffraction method. These two structures are so similar that aunique determination by means of electron diffraction alone would bedifficult or impossible. The diffraction evidence, however, is sufficient toestablish quite definitely that neither of the compounds can have the Gpiro-pentane structure which has recently been advocated by F.Rogowski.1°This is in agreement with the chemical evidence. The final results of thopresent investigation indicate coplanar ring structures. For (I) the distancesare C-C = 1-55 i= 0.02 A., C=C = 1.34 0.03 A., and angle C,ClC2 = 92.5"&- 2O, in very good agreement with values obtained earlier by S. H. Bauerand J. Y. Beach.ll For (11) G C = 1.54 -+ 0.03 A., C=C = 1.34 5 0.03 A.,angle C,C,C, = 125" 5 4 O , angle C,C,C, = 93" 40' & 3". Symmetricalring structures In (11)the bond distortion appears to be distributed equally between the externalC-G-C and C=C-C angles.3" distortion are thus indicated in both cases.4 4(1.) (11.)Polyphenyls and Pheny1enes.-The type of binding between aromaticgroups is a matter of considerable interest, but the evidence available has10 Ber., 1939, 72, 2021.J. Arner. Chem. SOC., 1944, 66, 1946. Ibid., p. 636.11 J . Amer. Chem. Soc., 1942, 64, 1142; see Ann. Reporh, 1942, 39, 10458 GENERAL AND PHYSICAL CHEMISTRY.hitherto been rather unsatisfactory. The crystal structures of diphenyl,l2p-diphenylbenzene,13 o-diphenylbenzene,14 1 : 3 : 5-triphenylbenzene,l5 and4 : 4'-diphenyldiphenyl l6 have all been examined, but not with the accuracyavailable in modern technique, and individual bond lengths remain uncertain.In these early studies the ring size is generally given as 1.41 or 1.42 A., andthe connecting bond lengths as about 1.48 A. In diphenylene more recentelectron-diffraction results l7 give a ring size of 1-41 + 0.02 A., and connect-ing links of 1.46 & 0.05 A.(Miss) I.L. Karle and L. 0. Brockway18 have now made a carefulstudy of diphenyl, o-diphenylbenzene, and tetraphenylene by the electron-diffraction method. Such structures are, of course, much too complex to yieldanything like unique solutions by the gas-diffraction method. The processrather consists of showing that a certain model, or sometimes several differentmodels, are compatible with the data, while other models are not. But themethod is very sensitive, and changes of bond length amounting to 0.02 or0.03 A. are frequently sufficient to destroy the agreement for a given model.In diphenyl three different models satisfy qualitatively the appearance ofthe photographs.The first is planar, with ring size 1.39 A. and inter-ringdistance 1.54 A. The second has a slightly distorted benzene ring and aninter-ring distance of 1-48 A., while the third is non-planar with free rotationof one ring with respect to the other. It is concluded that a non-planarstructure for diphenyl is the most probable, especially as it avoids sterichindrance between the o-hydrogen atoms. The most probable average ringdistances are 1-39 & 0.02 A., and inter-ring distance 1.52 & 0.04 A.In o-diphenylbenzene the molecule cannot possibly be planar, and theaverage position of the two attached rings is thought to be orthogonal to thecentral ring, with possible oscillation of 1 5 O from the normal position.Thesame bond distances apply.In tetraphenylene (111) all planar models provedunsatisfactory. A good solution was obtained from amodel with four regular (1.39 A.) benzene rings and a :u'v) non-planar cyclooctatetraene ring. In the latter the\/\ /\ bonds alternate in length around the ring, between1.39 A. and 1.52 A., the former of course being the sidesof the benzene rings. The benzene rings are directedalternately above and below the average plane of themolecule in such a way that 120" angles are maintained between every pairof bonds.The general conclusion from these studies, based both on inter-ring12 J. Dhar, Indian J . Physics, 1932, 7, 43.Is (Miss) L. W. Pickett, Proc. Roy.Xoc., 1933, A , 142, 333.1 4 C. J. B. Clews and (Mrs.) K. Lonsdale, ibid., 1937, A , 161, 493.l6 B. P. Orelkin and (Mrs.) K. Lonsdale, ibid., 1934, A , 144,630; (Mrs.) K. Lonsdale,1 6 (Miss) L. W. Pickett, J . Amer. Chem. SOC., 1936, 58, 2299.18 J . Amer. Chem. Soc., 1944, 66, 1974.'->/ \(111.)2. Krist., 1937, 97, 91.See Ann. Reports, 1943, 40, 92ROBERTSON : CRYSTALLOURAPHY. 69distances and on the non-coplanarity of the molecules, is that the conjugationeffects between adjacent aromatic rings are less than had hitherto beenthought, and considerably less than between double or treble bonds andaromatic rings.lg More definite conclusions must await really detailed X-rayinvestigations of the crystal structures.A preliminary X-ray examination of crystals of triphenylmethyl chlorideand bromide has been made.20 They belong to the hexagonal system, andthe space group is either C3+H5 or C31-H3, with six molecules in the hex-agonal unit cell.The structures are obviously fairly complex, and althoughthe halogen a!oms have been located approximately by evaluating thePatterson functions, yet nothing definite can so far be said about the con-figuration of the triphenylmethyl group in these crystals.Coronene.-A preliminary X-ray analysis of the crystal structure of thearomatic hydrocarbon coronene (IV) has succeeded indetermining the position oE the carbon atoms with con-siderable certainty by means of a two-dimensional Fourierprojection of the structure along the monoclinic b crystalaxis.21I I distances cannot be measured very accurately, but, a strictly b\/ planar structure being assumed, there is already evidence(IV. 1 that the average C-C distance is slightly greater than theaccepted value for benzene (1.39 A.), and may be rather nearer to the graphitevalue of 1.42 A.The arrangement of the molecules in the crystal is such thatit should ultimately be possible to make very accurate measurements of allthe C-C distances.These findings are of considerable interest in view of detailed calculationsrecently carried out by C. A. Coulson by the method of molecular orbitalson the coronene bond orders and lengths.22 He has computed the energiesof the mobile electrons in terms of the fundamental resonance integral p,23and from these results it is possible to obtain the bond orders and lengthsrelative to the standard values for C-C, C=C, and CGC in ethane, ethylene,and acetylene.The calculated bond lengths are as follows :\/\ At the present stage of the analysis the interatomicCoronene. Graphite. Benzene.Mean length of C-C bonds (A.) ............... 1.406 1.417 1.389Length of central bonds (A,) .................. 1.418 1.417 1.389For structures of high symmetry and links between atoms of the samekind such calculations are obviously capable of considerable refinement.At present the experimental measurements are not nearly good enough toprovide an adequate test of the theory, but we may perhaps hope for someimprovement in this direction.19 J. M. Robertson and (Miss) I.Woodward, Proc. Roy. SOC., 1937, A , 162, 568;20 S. N. Wang and C. S. Lu, J . Amer. Chem. SOC., 1944, 66, 1113.21 J. M. Robertson and J. G. White, Nature, 1944, 154, 605.22 Ibid., p. 797.1938, A , 164, 436.23 Proc. Roy. SOC., 1939, A , 169, 41360 GENERAL AND PHYSICAL CHEMISTRY.Other Structures.-Other investigations in the field of organic structureswhich may be briefly mentioned are unit cell and space-group determinationsfor phloroglucinol d i h ~ d r a t e , ~ ~ the low-temperature form of abieticcodeine and P-methylmorphimethine.26 Ferritin and apoferritin 27 are bothface-centred cubic and contain the same protein. Although the cell size isthe same, the X-ray intensities are different. The iron atoms in ferritinapparently occupy interstices between the protein molecules.The structure of copper NN-di-n-propyldithiocarbamate has been studied,and the copper shown to have planar co-ordination.28 Phytomonic acid,C20H4002,29 has been shown to have a relatively long chain structure and asingle side chain, probably a methyl group. J.M. R.v. Natural and Synthetic Fibre Structures.Introduction.-Fibre structures have not recently been reviewed as suchin these Reports. The present note covers especially the last two years,mainly from the X-ray standpoint. Metals, complex biological fibres, andapplied aspects are omitted. The unity of macromolecular studies has beenreflected in the co-ordination of all relevant physico-chemical techniques intheir problems, which are as basic to colloid science, medicine, and biologyas to the industries of textiles, soaps, plastics, and rubbers.Dominantfeatures are the increasing tendency to treat long-chain molecules fromfundamental first principle&; the extension to fibres of ideas and results(energetics and structure) from the simpler metal and n-aliphatic chainsys,tems; the growing emphasis on secondary structure, aided by newtechniques such as electron microscopy and small-angle X-ray scattering ;and the success in interpreting physical and especially mechanical fibreproperties on a molecular basis. Surveys, varying conditions and environ-ment, still yield more fundamental results with fibres than separate precisionstructure analyses.New 8ourw.-High-polymer studies have brought about the formationof sub-groups of physico-chemical societies,l increasingly participate inothers, and have reoriented a vast applied literature.New governmentaland research associations 2 and two complete abstracting digests dealexclusively with fibres, whose structural problems have been focused in four24 C. R. Bose and R. Sen, Indian J. Physics, 1943,17, 163.26 H. S. Shdanov, M. J. Lazarev, and N. G. Sevastianov, Compt. ~ n d . Amd. SCi.26 L. Castelliz and F. Halla, 2. K ~ i s t . , 1943, 105, 156.27 I. Fankuchen, J . Biol. Chem., 1943, 150, 57.D8 G. Peyronel, Qazzetta, 1943, 73, 89.29 S. F. Velic, J . Bwl. Chem., 1944, 165, 101.U.R.S.S., 1941, 31, 767.E.g., Division of High Polymer Physics, Amer. Physical Soc., June, 1944 ; BritishE.g., Southern Agric.Res. Stn., U.S.A.“Natural and Synthetic Fibres,” M. Harris and H. Mark, Intersci. Publ. Inc.,1944; “ Resins, Rubbers, and Plastics,” H. Mark and E. S. Proskauer, Intersci. Publ.Inc., 1942.Rheologists’ Club, 1940MACARTRDR : CRYSTALLOGRAPHY. 61recent symposia.* Annual reviews have been found necessary in the proteinfield alone.5 Among new journals should be noted J . makromol. Chem.; ofbooks and surveys, Vols. 4, 5 of the “ High Polymer’’ series are fibre-structural landmarks among many.6The Fibrous State.--Ii’ibres are imperfectly ordered solids, occurringusually as discrete filaments, though also in compact form. Genesis and growthare discussed by W. Ostwald.7 They are formed of long-chain molecules,usually high polymers, but comprise also sheet-like plates linked in columnarform,8 by stress-interlea~ing,~ or by mere van der Waals forces.1° Fibresof animal and plant origin have a histological and cytological structure : inwool the fibre structure and properties are those of the cortical cell.ll Chemi-cal composition may vary longitudinally in a single fibre, and markedly SOas between medulla, cortex, and scale.12 Various degrees of order areobtainable with chains whose packing is a function of shape, mobility, andlocal intermolecular forces.The fringed-micelle theory l3 is now preferredto the crystallite 14 and continuous structure 15 pictures. In this, a singlechain may thread its way through alternate crystalline and amorphousregions which really differ only in degree, and alter (though with more thancybotactic permanence) with temperature, stress, pressure, swelling, etc.The crystalline nuclei give a brittle strength, the amorphous tangle tenacityand resilience of orientation, with some plastic flow. Fibre structureanalysis has therefore to deal not only with the phase-ordered crystallites,but also with their shape, size, and relation to the matrix. Special orient-“ Mol.Wt. Distrn. in High Polymers,” Trans. Paraday SOC., 1944,40,217; “ FibreStructures,”X-Ray Anal. Gp., Inst. of Phys., Mtg. Oxford (1944) ; “Physics of Rubberand other H.P.’s,” Amer. Physical SOC., cf. J . Appl. Physics, 1944, 15; “ PhysicalChemistry of Proteins,” Chem. Rev., 1942.Cf. Annual “ Advances in Protein Chemistry,” M.L. Anson and J. T. Edsall,1944, I, Acad. Press Inc., N.Y. ; “ Advances in Colloid Science,” 1942, I; “ Advancesin Enzymology,” I, 11, Intersci. Publ. Inc., N.Y. ; “ Review of Biochemistry,” StanfordUniv. P.O., California.“ Natural and Synthetic H.P.’s,” 1942, 708 pp., K. H. Meyer, Intermi. Publ. ;“ Cellulose and Cellulose Derivatives,” 1943, 1196 pp., E. Ott, Intersci. Publ.; cf.also “ Physical Chemistry of H.P. Systems,” 1940, 353 pp., H. Mark, Intersci. Publ. ;“ Elastic and Creep Properties of Filamentous Materials and other H.P.’s,” 1943,H. Leaderman, Text. Fdn. Washington;) Rep. Prog. Physics, L. R. G. Treloar,2942-43, 9, 113 ; G. Gee, Ann. Reports, 1942, 39, 7 ; J. Needham et al., J . Qen. Physiol.,1944, 27, 201.Kolloid-Z., 1943, 102, 35.E.g., Sodium thymonucleate, W.T. Astbury, Sci. Progr., 1939,133, 1. ’ E.g., Chrysotile asbestos, E. Aruja, J. Sci. Inslr., 1944, 21, 115.lo E.g., Fibrous carbon, J. Gibson, H. L. Riley, and J. Taylor, Nature, 1944, 154,544.H. J. Woods, Proc. Roy. SOC., 1938, A , 166, 76.J. B. Speakman, J . Text. Inst., 1941, 32, 3.0. G8m@osa, I(. Herrmann, and W. Abitz, Biochem. Z., 1930,228,409; K. Herr-and 0. Gemgross, Kautschuk, 1932,8, 181.K. H. Meyer and H. Mark, “Der Aufbau der H.P. in org. Naturst.,” 1930;H. Mark, J. Physical Chem., 1940, 44, 764.l5 H. Staudinger, “ Die hochmol. org. Verbind. Kautschuk u. Cellulose,” 193262 GENERAL AND PHYSICAL CHEMISTRY.ations may occur in cell-wall structures ; l6 normally, the crystallites arelong rods or ribbons with random radial orientation, but approximatelyparallel to the fibre axis; biaxial orientation may be securable in rolledfilm.” X-Radiograms taken perpendicular to the fibre axis are thus inter-mediate to rotating-crystal and powder X-radiograms, the arcing of reflec-tions depending on the degree of tolerance between crystallite and fibre axes.For this reason, meridian reflections, barred in crystal rotation photographs,often appear.Technique.-Fibre cameras are best fitted with stretching frames andconditioning gadgets ; l8 screening foil and vacuum chamber minimisegeneral scattering from air and amorphous fibre regions ; monochromatisedradiation, as in the arrangements of A.Guinier l9 or I. Fankuchen,m is vitalfor small-angle scattering and ordinarily preferable.Moving cameras such asthose of 0. Kratky 21 simplify interpretations, the geometry of which has beensummarised by Y. Go.22 Of recent photometers, that of N. H. Chamberlain 23has many fibre applications. For speedy intensity recording of uniaxial fibrespectra, the G-M counter 24 has advantages. High resolution being requiredfor the fine detail of macromolecular structure and texture, fine slits per-mitting registration up to >600 A. are now frequent practice,25 especially inconjunction with high-power X-ray generators.2s In this connection, pre-cision beam foc~sing,~’ permitting the origin of X-rays to function as theinitial slit, enhances useful intensity. X-Ray power is expedient in allowingquick comparative study of homologues,28 or of fibres maintained at succes-sive small intervals of humidity, pH, swelling, temperature, pressure,extension, etc.; it extends the scope of X-ray methods in following suchtopochemical reactions as mercerisation 2~ or nitration of cellu1ose.m Theuse of glass-capillary slits (0.03 mm.in diameter) providing mirror reflectionhas been extended to the micro-analysis of single fibres and the drawingprocess.31 Of special fibre interest is the application of diffuse low-anglescattering to problems of grain size and texture ( q . ~ . ) , and of the analyticaltechnique for statistical arrangements such as mixed crystallisation, inter-calation, or turbostratic layering in swelling studies. The local chemicalRelative intensities need special settings or calculation.l6 R.D. Preston, Biol. Rev., 1939, 14, 281.l7 C. W. Bunn, Trans. Faraday Soc., 1939, 35, 482.lB W. T. Astbury, unpublished.lo Compt. rend., 1937, 204, 1115; Ann. Physique, 1939, 12, 161.Nature, 1937, 139, 193.0. Kratky, F. Schossberger, and A. Sekora, 2. Elektrochem., 1942,48,409.Bull. Chem. SOC. Japan, 1940, 15, 239. 23 J . Text. Inst., 1944, 35, T61.24 A. Eisenstein and N. S. Gingrich, Rev. Sci. Instr., 1941, 12, 582.25 E.g., R. Hosemann, 2. Elektrochem., 1940, 46, 535.26 I. MacArthur, Electron. Eng., 1944, 17, 272; 1945, 17, 317; W. T. Astbury and27 A. Guinier and J. Devaux, Rev. Sci., 1943, 341 ; Compt. rend., 1943, 217, 682.28 I. MacArthur, Mtg. X-Ray Anal. Gp., Inst. of Phys., Oxford (1944).30 M.Mathieu, Compt. rend., 1941, 212, 80.31 I. Fankuchen and H. Mark, J . Appl. Physim, 1944, 15, 364.I. MacArthur, Nature, 1945, 156, 108.E. Green, Ph.D. Thesis, Leeds University (1938)MACARTHUR : CRYSTALLOGRAPHY. 63composition and density of reflecting regions, required in structure analyses,are not necessarily easy determinations. Valuable analytical developmentsare new and improved methods for amino-acids 32 (protein fibres) ; theelectron probe with its transmitted velocity spectrum peaked by the excit-ation of X-ray K, L, or M levels33 will be of future interest. Porosity,preferential swelling, and adsorption of the immersion medium by the non-crystalline matrix are germane in density determinations ; comparativeresults of the usual immersion methods and the helium procedure of P.M.Heertzes 34 are described.35Inorganic.-Common alumina and silicate fibres are of membrane typewith liquid c0ntent.3~ Technical methods described for producing (e.g.,in glass) suitable strength and texture include high temperature centrifugalspinnerets, and surface and admixture treatrnent~.~~ Filamentous carbon,prepared by cracking of methane, has given a fibre X-radiogram,lo thelamellar hexagon layers, - parallel to the fibre axis, showing the usual3.5 A. graphitic spacing. A pioneer in the technique of diffuse scattering,A. Guinier extends the work on amorphous carbons and coa1F8 determiningmicelle dimensions and distribution ; as expected, activated carbons showa very fine state of division which varies with treatment.39 Similar studieson fibrils of chrysotile 40 reveal the hexagonal close-packing of parallel rods,earlier found in n-paraffins 41 and n-alcohols,42 in a discrete pair of longspacings of ratio fi : 1. Fibril diameters vary between 195 and 250 A.Further independent work on chrysotile asbe~tos,~~ 43 reveals novel features.The monoclinic cell has a 14.64, b 9.22, c (fibre axis) 5.33 A., p 93.2".TheSi4OI1 chains originally proposed yield place to a duplex sheet structure ofSi,05Mg3( OH), consisting of a branch-type sheet of O,( OH),Mg6( OH), linkedto O,Si,O,(OH), (hexagonal net of tetrahedral SiO, with OH in the planeof the tops of the tetrahedra) by the-common O,(OH),. Certain diffusereflections reveal irregularly stacked packing : their positions and asymmetricnature accord with two-dimensional net scattering and, together withthose of the congener antigorite, have been used by E.Aruja to interpretfine detail. Cell and space-group of giimbelite45 are determined. An32 A. J. P. Martin and R. L. M. Synge, Biochem. J . , 1941,35,91,1358; A. H. Gordon,A. J. P. Martin, and R. L. M. Synge, ibid., 1944,38,65 ; R. Consden, A. H. Gordon, andA. J. P. Martin, ibid., p. 224; J. R. McMahon and E. E. Snell, J . Biol. Chem., 1944,162, 83; A. C. Chibnall et al., Biochem. J . , 1943, 37, 354, 360, 372.33 J. Hillier and R. F. Baker, J . Appl. Physics, 1944, 15, 663.34 Rec. Trav. chim., 1933, 52, 305.35 Idem, ibid., 1941, 60, 91, 329; 1942, 61, 761; R. P.Rossman and W. R. Smith,38 W. Ostwald, Kolloid. Z., 1943, 102, 181.38 B.C.U.R.A. Conference, 1943, London; Proc., 1944.39 H. Brusset, J. Devaux, and A. Guinier, Compt. rend., 1943, 216, 152.40 I. Fankuchen and M. Schneider, J . Arner. Chem. SOC., 1944, 66, 500.41 A. Muller, Proc. Roy. SOC., 1932, A , 138, 514.42 I. MacArthur, unpublished.44 Idem, Phyeiccrl Rev., 1941, 59, 693.Ind. Eng. Chem., 1943, 35, 972.B.P. 563,678; U.S.P. 2,338,473, 2,331,944/5/6.43 B. E. Warren, Amer. Min., 1942, 27, 235.4 5 E. Aruja, Min. Mag., 1944, 27, 1164 GENERAL AND PHYSICAL CHEMISTRY.interesting discussion of gelatinising silicates in terms of lateral radical andnetwork structure is made by K. J. Murata.46n-AZipliatic Unbranched Chains.-(a) Soaps. Soaps fibre with the paraffinchains perpendicular, not parallel, to the fibre axis.47 Further X-rayevidence 48 confirms their existence in gel or solution as head-to-head bimole-cules of standard chain cross-section, giving “ long spacings ” (L) whichincrease on dilution, or on addition of benzene or dye solubilisation, byintercalation at heads and tails respectively.Micelle aggregates arelamell~e?~ with the dominant cohesion along the smallest axis. As withmontmorillonite, thixotropy is evidenced, e.g., by magnesium stearate inbenzene,50 the temperature of the sol-gel transformation increasing linearlywith soap concentration. The equilibrium has been studied spectro-phot~metrically.~~ The most direct evidence of gel structure and fibre sizeand texture is still the excellent electron micrographs (EM) for sodiumlaurate,62 statistical analysis of whose interlocked mesh of ribbon fibrilsshowed a multi-pile lamellar structure of parallel, slightly hydrated, bi-molecular chains.Fibering of metal soaps in mineral oil 53 is favoured byfree acid. Calcium and aluminium cations yield very small fibres, but, forsodium soaps, large fibres are favoured by low-viscosity polar oils, un-saturated soap chains, and shearing stress in combination with polaradditants such as glycerol. Growth mechanisms and energetics have been * Transparent soap is not amorphous, but is composed ofultra-fine random fibres (y-form, q.v.) with free glycerol.55Characterisation of the numerous phase structures and their stabilityranges and transitions, complicated as they are by water content, continuesby X-ray and thermal methods.For the sodium palmitate-water systemF. G. Chesley s6 finds six transitions. In addition to the liquid-crystallinestate, four crystalline modifications have been given X-ray criteria for thesodium stearate-water system. 5’ and many well-known features of longaliphatic chains with polar end-groups have been confkmed and extended byfibre X-radi~grams.~~ Transitions variously indicate change of water content,46 Amer. Min., 1943, 28, 545.47 S . Ross, J . Physical Chem., 1942, 46, 414 ; 0. E. A. Bolduan, J. W. McBain, and48 H. Kiessig, Kolloid-Z., 1941, 96, 252; J. W. McBain and K. E. Johnson, J .49 W. Philippoff, Kolloid. Z., 1941, 96, 255.6o B.S. Kandelaki, Izv. Gruz. Ind. Inst. Kirova, 1940, 13, 109.J. Stauff, 2. Elektrochem., 1941,47, 820.62 L. Marton, J. W. McBain, and R. D. Vold, J. Amer. Chem. SOC., 1941, 63, 1990.53 W. Gallay and I. E. Puddington, Canadian J. Res., 1944, B, 22, 66, 90, 103.64 J. Stauff, Kolloid-Z., 1941, 96, 244.6 5 J. W. McBain and S . Ross, Oil and Soap, 1944, 21, 97.56 J . Chem. Physics, 1940, 8, 642.6 7 R. H. Ferguson, F. B. Rosevear, and R. C. Stillman, Ind. Eng. Chem., 1943, 35,6 8 J. W. McBain, 0. E. A. Bolduan, and S. Ross, J. Amer. Chem. SOC., 1943, 65,S . Ross, J . Physical Chem., 1943, 47, 528.Amer. Chem. SOC., 1944, 66, 9.1005.1873; J. W. McBain, A. de Bretteville, and S. Ross, J. Chem. Physics, 1943, 11, 179MACARTHUR : CRYSTALLOGRAPHY.65change of inclination of paraffin chain axes to polar layer planes, genotypictransformations (1 -dimensional melting), confirmed by the sudden variationin thermal expansion and onset of effects on the surface tension and viscosityof surrounding polar and the inception at the waxy-superwaxy trans-ition of the hexagonal symmetry of " molecular rotation." 6* Temperature-spectra showing the latter effect together with some L variation have beenmade for sodium stearate.61 R. H. Ferguson believes the water is present onlyin solid solution ; M. J. Buerger,62 using de Jong and precision Weissenbergtechnique on sodium stearate crystals prepared by A. de Bretteville, has deter-mined the a, p, y forms as -$H,O, -hH,O, and anhydrous respectively.Sodium stearate, +H,O is monoclinic, a 9.16, b 8-00, c 1 0 3 .9 6 ~ . , p 93" 43',space-group A2/a. The variety of crystal forms and packing detail adoptedby n-aliphatic polar molecules is much wider than is commonly supposed.While broad essentials are obtainable from fibre and powder X-radiograms,particularly by use of the anisotropic thermal expansion in temperature-spectra,28 complete crystallographic analyses, such as those of A. Miiller 63and C. W. Bunn,l7 could well be extended to other forms in a field where somuch understanding of molecular structure and forces can be gained by theprecise use of X-ray, thermal, polarisation, and force-field methods.While higher paraffins fibre with some tenacity, lowermembers give moderately tough and rubbery fibres only on admixture, e.g.,with lithium stearate.64 Work continues on the lines of A.Miiller andA. R. Ubbelohde, crystalline forms, transitions, stability ranges, and pre-melting being discussed by C. G. Gray, A. van Hook, and H. FrOhli~h.~~Dielectric measurements extend previous experimental results ; molecularff exure, invoked to account for the entropy changes involved, is found such asto leave - half the chains untwisted at the second-order transition. Therecent infra-red conclusion that in polythene some branching occurs-inparticular -2% of methyl groups 66-may bear on certain systematicanomalies met l7 in its crystal structure determination.Branched-chain Hydrocarbons and Rubbers.-(a) General and Macro-structure. Rubber-like materials consist of long flexible chains weaklycross-linked.They are distinguished from ordinary liquids and solidsrespectively by these few links (chemical, vulcanised, sterically knotted) andby their weak control of form through entropy rather than energy. Localcrystallisation, impeded by plasticisers and temporary knots, is promoted by(b) n-Parafins.5D W. Gallsy and I. E. Puddington, Canadian J . Res., 1943, B, 21, 211, 225.6O L. Pauling, Physical Rev., 1930,36,430 ; T . E. Stern, Proc. Roy. SOC., 1931, A , 130,s1 A. de Bretteville and J. W. McBain, J . Chem. Physics, 1943, 11, 426.62 M. J. Buerger, A. de Bretteville, F. V. Ryer, and L. B. Smith, Proc. Nat. Awd.63 Proc. Roy. SOC., 1928, A , 120, 437.64 Foote Min. Co., Rev. Sci. Instr., 1944, 15, 157.6 5 H.Frohlich, Trans. Faraduy Soc., 1944, 40, 498; A. van Hook and L. Silver,6 6 H. W. Thompson, J . , 1944, 183.551.Sci., 1942, 28, 526; M. J. Buerger, ibid., p. 529.J . Chem. Physics, 1942, 10, 686; C. G. Gray, J . Inst. Petr., 1943, 29, 226.REP. VOL. XLI. 66 GENERAL AND PHYSICAL CHEMISTRY.addition of active fillers such as carbon or dry silica.67 Normally, crystallitesare absent or randomly oriented ; stretching favours fibering.Secondary structure in such systems, in particular its relation to elasticity,has been widely studied, mainly by statistical and thermodynamic methods.68I n such treatments, the properties of bulk polymer are usually referred tothose of individual Gaussian or free-link chains involving the chain length (asdetermined by osmotic pressure, viscosity, or light scattering 69 methods)or its mean end-to-end distance (A).A is unaffected by a symmetrichindrance potential; 70 for unsymmetric steric effects, the form of the Adistribution function remains unchanged, but its magnitude increases. 70C. Sadron’s treatment 71 replaces the free-link chain by its “ equivalentparticle,” a time-average of its mean geometric articulation ; this surface ofrevolution, whose shape reflects chain rigidity, is then treated by Brownianmovement methods. Fundamental constants of diffusion, orientation,translation, viscosity, and temperature effects are expressible. Actual casesrequire individual correction because of the special importance of steric andpolar factors.72 Model experiments on threaded beads to simulate the effectof varying chain length, concentration, solvent nature, e t ~ ., ~ ~ give generalconfirmation.An interesting treatment of the melting of high polymers 74 explains interms of degree of polymerisation (P), latent heat per link, crystalline-amorphous ratio, and structural co-ordination numbers-(a) convergence ofmelting points with increase in P, ( b ) melting ranges, (c) stable coexistence ofamorphous and crystalline regions in a composite phase; data by N. Bekke-dahl 75 corroborate for rubber, Recent determinations of crystallite size(rubber) have been made by means of line-broadening (A. Taylor’s analysis 76)and small-angle diffuse 77 The values (80, 52-81 A. perpendi-cular to, -300 A.parallel to, stretch) confirm older figures.78 EM deter-minations show & gel fibril structure of minimum width 100-400 A,, but, in67 P. Rebinder, G. A. Ab, and S. J. Veiler, Comp. rend. Acad. Sci., U.R.S.S., 1941,31, 444; C. E. Hall et al., I n d . Eng. Chem., 1944, 36, 634.6 8 H. M. James and E. Guth, J . Chem. Physics, 1943, 11, 455, 531: F. H . Miiller,Angew. Chem., 1940, 53, 425; Kolloid. Z., 1941, 96, 326; W. and H. Kuhn, Helu.Chim. Acta, 1943, 26, 1394; F. T. Wall, J . Chem. Physics, 1943, 11, 527; P. J. Floryand J. Rehner, J . Chem. Physics, 1943,11, 512; R. Simha, Ann. N.Y. Acad. Sci., 1943,44, 297; J. J. Hermans, Kolloid. Z., 1943, 103, 210; J. D. Ferry, Ann. N.Y. Acad. Sci.,1943, 44, 313; J. J. Press and H. Mark, Rayon Text.Monthly, June-Aug., 1943;R. Houwink, J . Physical Chem., 1943, 47, 436; L. R. G. Treloar, Trans. Faraday SOC.,1944, 40, 109; and especially ref. 6.*@ P. Debye, J . Appl. Physics, 1944, 15, 338; W. W. Lepeschkin, Kolloid. Z., 1943,105, 141.70 P. J. Flory and J. Rehner, Ann. N . Y . A m d . Sci., 1943, 44,419.71 C. Sadron, J . Phys. Radium, 1943, 14, 92.72 C. W. Bunn, Proc. Roy. SOC., 1942, A , 180, 67, 82.73 H. A. Stuart, Natumoiss., 1943, 31, 123.7 4 E. M. Frith and R. F. Tuckett, Trans. Paraday SOC., 1944, 40, 251.7 5 J . Res. N u t . Bur. Stand., 1936, 15, 503.77 s. D. Gehman and J. E. Field, J . Appl. Physics, 1944, 15, 371.78 J. Hengstenberg and H. Mark, 2. Krist., 1928/9, 69,271.7 6 Phil. Mag., 1941, 31, 330MACARTHUR : CRYSTALLOGRAPHY.87addition, amorphous nodules also appear, often of considerable size.79 Fora standard 600% stretch in rubber, crystalline fraction and tensile strengthincrease as crystallite size decreases. The increase in crystallisation ofrubber with stretch is determinable by the parallel increases of magneticanisotropy 8o and fibre X-radiogram intensity. Lattice size is very constantduring stretch and density increases > 1 %.81 Cell expansion with temper-ature is anisotropi~.~*Important results on the phenomena of second-order transitions withtheir time effects and structural implications are those of R. F. Boyer andR. S. Spencer 82 (rubbers, nylon, polythene, etc.) and of H. Mark et(polystyrene). The former authors, for polymers, copolymers, and mixtures,determine by dilatometry the rates of expansion (PI? p,) below and abovethe transition point (Tt); the relations found are applicable to the deter-mination of crystalline-amorphous ratios, nature of phases, and somethingof the variation in P.Mark finds that above the polystyrene Tt, the specificvolume V-T curve is fixed; below Tt, aV/aT is fixed (= pl) but V itselfdepends on the speed of cooling, Tt being higher the quicker the speed.On quick heating from the lower temperatures, any particular V-T curvais retraceable, but on maintaining T fixed in a wide range (40-80") V fallsto a minimum curve a t a rate increasing with T. The phenomena, whichrecall comparable cyclical results of R. Buckingham 84 and, in another field,of W.A. emphasise the effect of time of relaxation phenomena inlong-chain molecules. This is vital in analysing load-extension and hysteresisphenomena, especially in textile fibres,86 is instanced in mechanical depoly-nierisations by supersonic frequencie~,~~ and also in Tt variations, whichare closely related to viscosity.88 Tt increases with increasing difliculty ofmovement. Thus Tt is raised by bulky groups or rings as side chains(polystyrene), by primary valency cross-linking (polydivinyl benzene), or bydipole forces (polyacrylic acid); Tt is lowered by mobile non-polar sidechains (polybutadiene) or by screening of dipoles (polyacrylic esters).Tb x pz a=. const. for many series.82Structures for rubber, rubber hydrochloride, poly-chloroprene and p-guttapercha have been noted.89 By electron diffractionof thin (250 A.) films of guttapercha, K.H. Storks finds the macromoleculein concertina-like Structures based on fibre X-radiograms with(b) Fine structure.7D C. E. Hall, E. A. Hauser, D. S. LeBeau, F. 0. Schmitt, and P. Talalag, Ind. Eng.Cliem., 1944, 36, 634.E. Cotton-Feytis, Compt. rend., 1942, 214, 485; 215, 299.81 W. H. Smith and N. P. Hanna, J . Res. Nut. Bur. Stand., 1941, 27, 229.82 J. Appl. Physics, 1944, 15, 398.83 T. Alfrey, G. Goldfinger, and H. Mark, dbid., 1943, 14, 700.84 Trans. Paraday SOC., 1934, 30, 375.85 S. L. Smith and W. A. Wood, Proc. Roy. SOC., 1941, A , 178, 93.86 E.g., H. Leaderman, ref. 6.G. Schmid and E. Beuttenmuller, 2. Elektrochem., 1943, 49, 325.E.Jenckel, Kolloid. Z., 1942, 100, 163.Ann. Reports, 1942, 108; 1943, 96. Bo Bell Lab. Rec., 1943, 21, 39068 GENERAL AND PHYSICAL CHEMISTRY.limited and partly unresolved reflections cannot attain the precision of crystalanalyses : G. A. Jeffrey's p-guttapercha structure 91 differs from that ofC. W. Bunn in that a planar distribution of the C-C bonds about the doublebond is found, with no distortion of the methyl group out of the plane; andorientation of the CH,-CH, bond to the plane of the double bond is ratherlarger (80" v. 63"). The C-atom parameters listed yield the more normalbond lengths and angles of C-C 1-54, C-C 1.33 A., LC-C-C 122",LC-C-C 109".Stretched polyisobutylene yields possibly the finest fibre X-radiogram yetfound for high polymers.C. S. Fuller et aLg2 find a rhombic cell with a 6.94,b 11-95, c 18-03 A. From intensity analysis, two chains penetrate the cell,and the isobutyl residues are held to coil spirally, each CMe, group rotating45" about the fibre axis (c) with respect to its predecessor to remove methylgroup steric interference. It may be notedg3 that certain (001) absencesbasic to this structure appear on permitting meridian reflection, that mutualmethyl group interference is still considerable, and that some adjustment ofthe methyl groups brings the structure in closer agreement, on force-fieldtheory, with the remarkable anomalies found for the heat of formation.94Polpethers, -esters, -amidea.-In these polymers the chain itself is modi-fied by the insertion of oxygen or nitrogen atoms.The mutual adjustmentbetween alkyl chain and polar layer forces depends on average P, and type,mobility, size, shape, density and distribution of the groups inserted. Theforce fields being differently responsive to variations in physical conditions,all grades of elasticity and plasticity occur, crystalline forms and transitionsare numerous and individualistic, and amorphous, fibrous, mesornorphous,and crystalline regions occur.When polar groups are small and their distribution dense and regular,good fibre diagrams are obtainable. Thus polyoxymethylenes (-CH,-O-),are hexagonal, CS2, with a fibre period of 17.25 A. Chain orientation is non-planar tub form with helical progression along the fibre axis.95 Similarly,polyethylene oxide (-CH2-CH,-O-), (I) is monoclinic, a 9.5, b (fibre axis)19-5, c 12.0 A.*~ 101"; 4 chains per cell, 9 units per fibre period, involvingagain a folded chain.95, 96 Less order is shown in X-ray and electron diffrac-tion studies on methacrylate fibres.97 With a low density of dipoles, theparaffin chain packing becomes more dominant ; for the decamethyleneseries 98 (11) C. S.Fuller finds monoclinic types with normally-packed zigzagchains parallel and also inclined to the fibre axis. The cells of the tri-O1 Trans. Faraday SOC., 1944, 40, 517.92 C. S. Fuller, C. J. Frosch, and N. R. Pape, J. Amer. Chem. SOC., 1940, 62, 1905;93 W. T. Astbury and H. J. Woods, unpublished.94 M. Polanyi, unpublished.D5 E. huter, 2. physikal.Chem., 1933, B, 21, 161, 186.O 7 H. A. Robinson, R. Ruggy, and E. Slantz, J . Appl. Physics, 1914,15, 343 ; G. D.cf. also R. Brill and F. Halle, Nuturwiss., 1938, 26, 12.H. and M. Staudinger, ibid., 1937, B, 37, 403.Coumoulos, Proc. Roy. SOC., 1943, A , 182, 166.Chem. Rev., 1940, 26, 143MACARTHUR : CRYSTALLOQRAPHY. 69methylene poly-esters of dibasic acids usually consist of normally-packedparaffin chains lying parallel to the cell long asis (itself at -30' to the fibreaxis) with the planes of dipoles perpendicular to the fibre axis.99 A slightlydifferent end-group packing from those of (I), (11), is indicated by a lower L,but the same ZCH, increment. Odd and even members show finer distinc-tions. On stretching the fibre, cells aline nearer the fibre axis, and equatorialand meridian reflections resolve to non-axial pairs.On strain release, theprocess is reversible. In the 1.16 member, chains show only the parallelalinement of the mesomorphic state characterised by the 4-2 A. lateral spacing.Structural analyses have been made for several nylons,lOO especially thecondensate of NH,*[CH,],*NH, and C02H*[CH2]p*COZH, but no satisfactorystructure is yet released in detail. The strong equatorials at 4.4 and 3.8 A.conform with one type of paraffin chain packing; meridians indicate anaxial periodicity (16.8 A.) rather less than that demanded by a vertical zigzagchain. Hydrogen-bonding at the CO*NH links emphasises the parallel with proteins. C. s.Fuller's extensive studies 2 attempt to relate structure and physical properties.The same features appear as in poly-esters, e.g., the transition from extendedto balanced retracted chains with inclination of chains to fibre axis a functionof tension.Forms range over amorphous, mesomorphous, and fibrous, fromcoplanar dipoles with radially random chains to parallel chains with randomdipoles. Co-polymerisation and weakening of polar forces (e.g., by N -methylation) affect side-packing less than fibre-axial period. IncreasingN-methylation lowers melting point and elastic modulus, increases watersorption, and broadens the 3 . 8 ~ . spacing. The retracted chain form isfavoured by high temperature annealing, high N-methylation, low polargroup density along the chain, relaxed longitudinal strain, swelling, andpolar plasticisers. As with polythene, rolling (which induces the extendedchain form) gives biaxial orientation.The mobility of such chains has beenshown by many methods. For the condensate of adipic acid and (CH2)sN,,R. Brill finds the transition from monoclinic lattice to the hexagonal formcharacterising " rotating chains " to be lowered in temperature and speededby moisture. Similar polarisation studies 4 to those on ketones also confirmchain mobility, while spontaneous adjustments and coiling are seen in theelectron diffraction of films,90a in film deposition 6 and in other phenomena.2-Altering the balance of stresses in such systems may lead to macro-@O C. s. Fuller, J . Amer. Chem. SOC., 1942, 64, 154.loo I.Sakurada and I. Hizawa, J . SOC. Chem. Ind., Japan, 1940, 43, B, 348; L. E. R.R. Brill's unit cell 1 has a 9.66, b 8-32, c 17.2 A., y 65".Taylor, unpublished.Z . physikal. Chem., 1943, B, 53, 61.W. 0. Baker and C. S. Fuller, J . Amer. Chem. SOC., 1942, 64, 2399; 1943, 65J . pr. Chem., 1942, 161, 49.W. 0. Baker and W. A. Yager, J . Arner. Chem. SOC., 1942,64,2164, 2171.A. Muller, Proc. Roy. SOC., 1937, A , 158, 403; 1940, A , 174, 137.F. H. Muller, Kolloid. Z., 1943, 103, 144.W. B. Bridgman and J. W. Williams, J . Amer. Chem. SOC., 1937, 59, 1579.112070 GENDAAL AND PFIYSICAL CEEMISTRY.structures along the fibre axis sufficiently regular to give diffraotion pheno-mena. For nylons and ply-estera respectively, macrospacings by H. Mark 31aand by K.Hess and H. Kiessig,s which increase irreversibly on heating, andothers by E. Ott 8h for polyoxymethylenes have no relation to the well-defined fibre axis periods, but are of the order of crystallite size determinedfrom electron diffra~tion.~ Degraded nylon fibres do not yield the fine-diameter (50-100 A.) proto-fibrils so characteristic of natural celIuloses.10Curbohydrutes.--Cellulose, with its great proportion of active polar groupsand regular chain of cellobiose units, forms fibres with well-markedcrystallites. P vmies greatly, -2000 being usual for untreated cotton,ramie, etc.,ll compared with -250, >low, for starch amylose and amylo-pectin respectively.12 Very long L are not found, though weak links atregular 2600 A. intervals have been ~1airned.l~ EM determinations ofwidth l4 confirm the X-ray values (30-100 A.) for the finest fibrils.InEM'S, the branched polysaccharide dextran shows periodic swellings at800 A. intervals.15 Three crystalline structures, cellulose, hydrate-cellulose(mercerised), and water-cellulose are well differentiated. The first nativeplant cellulose found in the mercerised state is the halicyst membrane.l6Two types of cellulose cell are distinguished,l7 (a) ramie type : a 8-28,b 10.3, c 7-89 A., p 84"; (b) coltsfoot type : a 8.05, b 10.3, c 7.98 A., p 89.0".A further variation suggested 1* has been shown to be due to waxy contamin-ation.19 Fibre diagnosis by X-radiograms is now standard practice forcotton and jute.20Determination of micelle size by low-angle diffuse scattering 31a andelectron microscopy 14 confirm early results by X-ray line-broadening(ramie, -50 x >600 A.; viscose, -40 X 350 ~ . ) . ' * a In this field, intensivestudies by the former method have been made by A. Guinier,21 R. Hose-mann,22 0. Kratky et aZ.,23 and H. M;~rk.~la The theory of scattering asapplied to gases can be applied t o other particles; if these are loose-packedand amorphous, each particle scatters with a " particle form factor " akin tothat of an atom and its electrons; scattering for the whole is coherent andNaturwim., 1943, 31, 171.9 M. v. Ardenne, E. Schiebold, and G. Giinther, 2. Physik, 1943, 119, 362.10 E. Husemann and A. Carnap, J . makromol. Chem., 1943,1, 16.l1 E.g., 0. A. Battista, I d .Eng. Chern. Anal., 1944, 16, 361.le J. F. Foster, Iowa State Coll. J . Sci., 1943, 18, 36.l3 0. V. Schulz and E. Husemann (with H. J. Lohmann), 2. physikal. Chem., 1942,l4 E.g., D. Beischer, Discussion, 2. Elektrochem., 1940, 48, 650; A. Hamann, Kolloid.l 5 B. Ingleman and K. Siegbahn, Arkiv Kerni, Min., Qeol., 1944, B, 18, No. 1.l6 W. A. Sisson, Contr. Boyce-Thompson Inst., 1941, 12, 31.l7 K. H. Meyer and A. van der Wyk, 2. Elektrochem., 1941, 47, 353.N. Gralth, S. Berg, and T. Svedberg, Ber., 1942, 75, 1702.l9 K. Hess, H. Kiessig, and W. Wergin, Ber., 1943, 78, 449.2o Ann. Rep. Ind. Assoo. Cult. Sci., 1943.22 2. Elektrochern., 1940, 48, 535; 2. Physik, 1939, 113, 751.23 0. Kratky, Naturwiss., 1942, 30, 542.B, 52, 23, 50.Z., 1942, 100, 248.Ann.Physique, 1939, 12, 161MACARTKUR : UBYSTALLOQRAPHY. 71additive. For particles of micelle size, this scattering is intense only at verylow angles. By comparing the intensity distribution found with that calculablefrom geometrical systems (e.g., elongated ellipsoids for fibres), size, shape,and orientation of the macrostructures may be determinable. Treatmentsdiffer, Hosemann following Guinier with increased precision, Kratky, doubtfulof the interference of internal particle structure and close-packing, preferringa statistics of interdistances. Conditions can be improved by suitableswelling agents or inter-micellar deposition of heavy scattering material.25, 31aBy such means, n-paraffin lamelke have been-found 400 A. thick, and distri-butions for ramie and cellulose triacetate have maxima at 3000, 2 0 0 ~ .respectively (length) and <400 A.(width).22 In favourable cases (e.g.,colloid sols of spherical chymotrypsin) precision may reach 3 3 A,=The crystalline-amorphous ratio (C/A = +) is important in physicalproperties of cellulose fibres. Alternative methods of defining " crystalline "and '' amorphous '' are not necessarily identical, for fringe intermicellarmaterial, especially on stretching, has more than random order thoughwithout exact phase relationship. Broadly speaking, the crystalline regionshave lower swelling, energy, entropy, little accessibility to dyes and mildchemical reagents, greater compactness and density, and measurementsinvolving these are indicative.Where C + A (rubber), 4 is simply got byX-ray diffraction ; 26 where C *- A (cellulose), calibration may be made fromlike mixtures, e.g., sugar (C)-sugar glass (A) .27 Measurements of density(rubber), latent heat of fusion (polythene), vapour pressure isotherms(nylon, cellulose) have been employed ; swelling anisotropy, magneticanisotropy, birefringence are applicable. Chemical methods for celluloseinclude peroxidation 28 and thallation in akyl ethers.29 Indications are alsoderivable by use of the Tuckett equation,7& or in certain cases from volumechanges above a second-order transition point .82aStructural indications are given well by swelling phenomena in coii-junction with X-ray analysis. The transition cellulose + hydrate-cellulosehas been followed by X-rays.30 Swelling by n-alkyl primary amines takesplace exclusively between the (101) by the creation of 0 H N links, DlO1increasing regularly with length of alkyl ~hain.3~ By the use of nitrogenpentoxide, important results for the nitrocellulose systems have been ob-tained by something approaching X-ray cinematography ; 3a the nature of24 0.Kratky, B. Baule, A. Sekors, and R. Treer, Kolloid. Z., 1942, 98, 170 ; 0.25 0. Kratky and F. Schossberger, 2. physikul. Chem., 1938, B, 39,1451 ; 0. Kratky,2 6 J. E. Field, J. Appl. Physics, 1941, 12, 23.2 7 I. Fankuchen and H. Mark, Rec. Chem. Prop., 1943, July-Oct., 54.2s G. Gol&nger, H. Mark, and S. Siggia; I n d . Eng. Chem., 1943,85, 1083.29 A. G. Assaf, R. H. Haas, and C.B. Purves, J . Amer. Chem. SOC., 1944, 66, 59.3o T . Kubo, Kolloid. Z., 1940, 93, 338.31 W. E. Davis, A. J. Barry, F. C. Peterson, and A. J. King, J . Amer. Chem. SOC.,3a M. Mathieu, Compt. rend., 1941, 212, 80.Kratky and A. Sekora, Naturwiss., 1943, 31, 46.A. Sekora, and R. Treer, 2. Elektrochern., 1942, 48, 587.1943, 65, 129472 GENERAL AND PHYSICAL CHEMISTRY.chain slip and the growth of the di- and tri-nitro-derivatives is followed, whileon swelling with acetone gelatiniser, definite lattice structures exist at1 mol./glucose, 1 mol./O*NO,, 3 mols./glucose, beyond which the fibrestructure breaks down.33 Similar experiments with series of ketones, esters,and alcoholic nitrates show unidirectional swelling, only u of the monocliniccells varying appreciably.Water is of special structural importance. Forcellulose, adsorption isotherms have been determined, and heats of sorptionnoted.35 The heat of adsorptionfor hydrate-cellulose a t 3-5 kg.-cals./mole '=. twice that for native cellulose(1.60), giving, per mole of water, the heat of adsorption value for hydrationof p-sugars and dcohol-water mixtures. The energy difference betweennormal and superfinely ground sugars equals the heat of solvation, superfinegrinding also destroying the lattice.36 Thus heats of sorption are an indexof + in a fibre. For cellulose, water sorption falls into three categories 37-interpreted as a unimolecular layer on theThe adsorption curves are 3-stage sigmoid.(a) 1 H20 per primary OH(b) 1 H20 per secondary OH 1 accessible surface.( c ) Collection in capillaries : non-structural.At stage (a), compression of water takes place, presumably by attachmentthrough two hydrogen bonds.The hydrate-cellulose lattice is not distorted.A treatment using the Langmuir and Brunauer-Teller mono- and multi-layer adsorption isotherms is that of J. D, Babbitt.38 Bound water has alsobeen surveyed by K. C. Blan~hard.~~An interesting application of specific rotatory power locates the copperin the cuprammonium cellulose complex.40 By comparison with glucosides,sufficient conditions for similarly enhanced rotations are shown to be freehydroxyls at positions 2 and 3 and substitution at 4.For this seaweed fibre consisting ofa succession of P-d-mannuronic acid residues,41 the rhombic cell has a 8-60,b (fibre axis) 8-72, c 10.74 A,, space-group V3(P2,22,), almost V4(P21212,).42The cell contains, in addition to 4 residues, -4H,O which extend a only;for the dry cell, u = 7-75 A.Using standard bonds and angles and thechair form of glucose ring, two pairs of positions for glucosidic O-linkingpermit a linear chain extension parallel to the fibre axis. One pair gives aperiod of 4.35 A,, hence probably holds for alginic acid (cf. 2 x 4.36) andprobably for pectin also (because of the stereochemical relation of side groups33 M. Raison and M. Mathieu, Gompt. rend., 1941,212,157 ; G. V. Schulz, 2. phylsikal.B, 52, 253.34 T. Petitpas, These, 1943, Paris.35 K. Lauer, R. Doderlein, C. Jiickel, and 0.Wilde, J . makromol. Chem., 1943,36 J. Gundermann, KoZZoicE. Z., 1942, 99, 143.37 A. 0. Assaf, R. H. Haas, and C. B. Purves, J . Awr. Chem. SOC., 1944, 66,66.38 Canadian J . Res., 1942, A, 20, 143.39 Cold Spring Harbor Symp. Q. Bwl., 1940, 8, 1.O0 R. E. Reeves, Science, 1944, 99, 148.41 E. L. Hirst, J. K. N. Jones, and W. 0. Jones, J . , 1939, 1880.42 W. T. Aatbury (in press).Fine Structure.-(a) Alginic acid.1, 76MACARTHUR : CRYSTALLOQRAPHY. 73in a-galacturonic and @-mannuronic residues). A full spatial analysis willbe of interest.The second pair of positions gives an extension of 5.18 A.,suggesting a normally built structure for cellulose (and chitin). While G. L.Clark’s analysis 43 confirmed the Meyer-Misch stru~ture,4~ models show somediscrepancies.The Astbury-Davies skeletons for alginic acid and cellulosereveal the possibility of interchange with only slight bond strain,45 withobvious repercussions on carbohydrate stereochemistry. Other recentmodifications have been suggested. P. H. Hermans’s m0de1,~6 using standardbonds and angles, provides a similar bent cellobiose radical to fit the fibreperiod, and does not permit parallelism in successive glucose rings. Chainshave alternating polarities in successive layers along c ; along b chain layersare staggered by 3 4 A. F. T. Peirce4’ proposes three modifications ofthe Meyer-Misch model :(i) A pyranose ring with perpendicular ,0, valencies and more nearlycoplanar C’s, though precision analyses on simpler sugars strongly favour theunstrained chair model.(ii) With the principal plane of the rings in the ab plane, the primaryalcohol groups can form OH bonds with the pair of secondary alcohol groupsof the adjacent parallel chain; an alternative bond with 2 0 of the samechain is held to account for the slight departure from rhombic symmetry.(iii) The pyranose rings are turned out of the ab plane of similarlyoriented chains towards that containing fhose of alternate sense, with OHbonding between the two sets of chains instead of within one.This modelis claimed to explain better identity period,-symmetry, and the facts ofregenerated cellulose.Structures for lignin 48 and pectin 49 have been advanced; the fibreperiod of the latter is 8-7 A. as with alginic acid.Lignin is a polycapillary(30-20,000 A. diameter) lamellar disperse organophilic mixture.The two components of starch, amylose and amylopectin,are respectively linear and branched chains. P of amylose and of the inter-branch length of amylopectin are determinable by measurements of themolecular extinction coefficient and h of peak absorption.50 Starch chainsare more flexible than the cellulose type 51 and configurations vary withtreatment.52 In the fibrous “ B ” modification, chains are fully extendedin a rhombic cell with a 16.0, b (fibre axis) 10.6, c 9 . 2 ~ . , probable space-group V(1).52 Butanol-precipitated amylose and the starch-iodine complex,(b) Cellulose.(c) Starch.43 S. T. Gross and G. L. Clark, 2. Krist., 1938, 99, 357; Text.Res., 1938, 9, 7.4 4 K. H. Meyer and L. Misch, Helv. Chim. Acta, 1937, 20, 232.4 6 W. T. Astbury and M. M. Davies, Nature, 1944, 154, 84.4 6 P. I€. Hermans, J. de BOOYS, and C. J. Maan, Kolloid. Z . , 1943, 102, 169.4 7 Nature, 1944, 153, 586.4 8 R. Jodl, Brennstoff-Chem., 1942, 23, 163, 178; cf. Ann. Reports, 1942, 39, 142.49 F. A. Henglein, J . makromol. Chem., 1943, 1, 121.50 R. R. Baldwin, R. S. Bear, and R. E. Rundle, J . Amer. Chem. SOC., 1944, 66, 8461 R. E. Rundle and L. W. Daasch, ibid., 1943, 65, 2261.52 R. E. Rundle, L. Daasch, and D. French, ibid., 1944, 66, 130.c 74 GENERAL AND PHYSTUAL mMISTRY.however, reveal a helical starch chain of pitch and diameter 8 and 13.7 A.respectively, with six glucose residues per turn. The helices are approx-imately close-packed, alternate helices oppositely alined, both space-groupsprobably v4-P212121.53 Like the iodine, the butanol is enclosed lengthwaysinside the helix, absorption optics confirming this.54 With glycerolas plasticiser, another fibrous form of amylose (period 7.5 A.) is obtained.51Polarisation optics differentiate this from the ‘‘ V ” form, and a linear foldrather than a helical chain is indi~ated.~l Starch cannot absorb iodinevapour when in the “ A ’’ or ‘‘ B ” configuration, but on conversion intothe “ V ” configuration, by alcohol precipitation, iodine is taken up up to 1 I,per six glucose rings.S5Proteins.-( a) General. Despite difficulties in recording, resolving,indexing, and analysing fibre X-radiograms of large and protean chains liableto polyphase and cytological structure, mixed crystallisation, polymorphism,disorder of strain, layering, and swelling, giving possibly only local crystall-isation of many-parameter elements of uncertain chemical content, structuralprogress continues by the combined use of all relevant physico-chemicaltechniques. Structural unity in crystalline and fibrous proteins is nowrecognised as based on the packing of polypeptide and side-chain elements.D. Wrinch’s structural basis 56 is the polypeptide chain mesh itself, with itscyclisations and folding to cage fabrics ; its success in interpreting structuralprotein features and X-ray data, e.g., for insulin, has been variously estim-ated.57 W, T.Astbury’s basis 5* is the packing of side chains, the polypeptideskeleton adopting the configuration permitting close-packing and thepresentation of the specific patterns so characteristic of enzyme action.Forkeratin, a lamellar structure completely reoriented with reference to thefibre axis has been suggested.59 Recent general analyses are due toM. L. Huggins 6O and D. G . DervichianaG1 Using principles such as pseudo-hexagonal close-packing, standard bonds and angles, N = . H * * 0 bridging,Z-configuration of residues, and a fibre screw axis (to prevent any asymmetricbending tendenoy), the former derives conceivable skeleton structures for silkfibroin, p- and a-keratin, and collagen. The favoured a-structure resemblesAstbury’s and adheies to a 100% intramolecular extension in the tc + atransition.The collagen model consists of linked spirally-coiled polypeptidechains with a pseudo-rhombic cell having a 4.5, b 5-8, c 22 A. Dervichianpostulates for proteins a two-dimensional double layer of amino-acids, withpolar and non-polar groups on opposite sides of a layer. Polar groups nor-mally form the external surface. Internal structure is imposed by symmetrical53 R. E. Rundle and F. C. Edwards, J. Amer. Chem. Soc., 1943, 65, 2200.54 R. R. Baldwin, Iowa State Coll. J. Sci., 1943, 18, 10.5 5 R. E. Rundle and D. French, J. Amer. Chem. SOC., 1943, 65, 1707.5 6 Phil. Mag., 1940, 30, 64; Proc. Roy. SOC., 1937, A , 161, 505; A , 160, 59.5 7 I. Langmuir, Proc. Physical SOC., 1939, 91, 592; I. Langmuir and D. Wrinch,ihid., p.613; J. D. Bernel, ibid., p. 618; L. Pauling and C . Niemann, J . Amer. Chem.SOC., 1939, 61, 1860; D. Wrinch, ibid., 1941, 63, 330.5 8 J . , 1942, 337. 59 L. W. Janssen, Protoplasma, 1939, 33, 410.6o Chern. Rev., 1943, 32, 195. 61 J . Chem. Phy8k8, 1943, 11, 236MACARTHUR : URY STALLOGRAPHY. 76hexagonal packing of parallel rod-like side chains. One plate is thus theminimum unit, its component amino-acids necessarily in 2m3u proportions.The passage from globular to fibrous state is simply the smectic-nematictransition of liquid crystals.62 In denaturation, a plate is ruptured and thepolypeptide chain, presenting hydrophobic groups also, becomes the dominantelement.Examination of the 2m3n rule for the occurrence and distribution ofamino-acid protein components continues.The rule, established by ultrn-centrifuge measurements,63 follows structurally from the geometry ofWrinch’s cages and Dervichian’s symmetric packings ; support for itsspatial regularity is given by analysis of keratin fibre X-radi~grams.~~Prom a wealth of chemical analyses by methods of improved 65it is now dear 66 that the rule is followed mostly with high precision, butwith equally precise exceptions, which, however, do not necessarily invalidatethe rule for component lamella. A mathematical analysisG7 of the con-ditions that permit the periodic chain congruences upheld by M. Bergmann,6sshows that the numbers 2 and 3 have no special privilege in the general caseand, further, that for certain proteins, if constituted as a &ngk chain, such adistribution cannot occur.Specific evidence against the full rigidity of theBergmann-Niemann hypothesis is seen in the observed multiple functionof certain residues (serine in fibroin,6e cystine in keratin 70) and in the com-ponents of partial hydr~lysis.~~ Dipeptides so formed often corroboratethe polar-nonpolar sequence assumed in various 61 The possi-bility that protein amino-acids may not be exclusively of one optical con-figuration 72 does not lessen structure-analytical difficulties.Denaturation X-ray studies 73 continue for their structural interest andas an aid in the production of strong protein fibres with wool-like qualities.Protein complexes are formed with detergents such as alkyl benzenesulphon-ate 75 or sodium dodecyl ~ulphitte.~~ The detergent ( a ) unfolds globular62 Trans.Faraday SOC., 1933, 29, 881.64 I. MacArthur, Nature, 1943, 152, 38; Wool Reu., 1940, 9.6 5 D. Bolling and R. J. Block, Arch. Biochem., 1943, 2, 93; 3, 217; H. B Vickery,G6 A. C. Chibnall, Proc. Roy. SOC., 1942, B , 131, 136.6 7 A. G. Ogston, Trans. Faraday Xoc., 1943, 39, 151.6 8 M. Bergmann and C. Niemann, J. Biol. Chem., 1937, 118, 301; 1938, 122, 577;Chm. Rev., 1938, 22, 423.A. H. Gordon, A. J. P. Martin, and R. L. M. Synge, Biochern. J., 1943, 37, 538;S. Blackburn, W. R. Middlebrook, and H. Phillips, Nature, 1942, 150, 57.70 W. R. Middlebrook and H. Phillips, Biochem. J., 1942, 36, 294; W. C. Hoss andM. X. Sullivan, Arch. Biochem., 1943, 3, 53.71 A.H. Gordon, A. J. P. Martin, and R. L. M. Synge, Biochem. J., 1941,%, 1369;1943, 37, 92 ; R. L. M. Synge, Chem. Rev., 1943, 32, 135.7 2 R. L. M. Syngo, Biochem. J., 1944, 38, 285.M. Spiegel-Adolf and G. C. Hsnny, J . Physical Chem., 1941,45,931; 1942,4$, 581.74 F. R. Senti, C. R. Eddy, and T. G. Nutting, J . Amer. Chem. SOC., 1943, 85, 2473.7 5 H. P- Lundgren, D. W. Elam, and R. A. O ’ C o ~ e l l , J. BioE. Chem., 1943,149,183,7~3 F. W. Putnam and H. Neureth, ibid., 1943,160,263.T. Svedberg, Proc. Roy. SOC., 1939, 3, 127, 1.Ann. N.Y. Acad. Sci., 1941, 41, 8776 GENERAL AND PHYSICAL CHEMISTRY.proteins, ( b ) allows precipitation with a minimum of inorganic ion, and(c) prevents crystallisation in drawing the fibres. Conditions for effectivefibering are a function of protein, nature and concentration of detergent,temperature, and pH.77 Egg albumin, edestin, zein, casein, pepsin, lacto-globulin, feather keratin, soybean, and peanut proteins have been used; onremoval of salt and detergent the steam-extended fibres develop considerablestrength.74 K. J. Palmer’s X-ray results for egg albumin are important.78The complex shows L (30 A.) of detergent and 4.7 and 10 A. rings of protein;detergent removal leaves the disoriented P-form as in heat-denaturation ;500% extension in steam gives the p-form oriented with the definition anddegree of silk. Orientation being here a chain not a miceZle phenomenonexplains the difference in strength and orientation from Astbury’s films.79The facts of electrophoresis and the curious compositions and phase stabilityare quantitatively explained by structures in which denatured egg albuminforms a polar-apolar monolayer with a covering unimolecular layer of close-packed detergent chains held by their hydrophobic ends, and the Astbury-Pauling 4-plate structure of native egg albumin contains detergent as aparallel bimolecular tail-to-tail layer, held close-packed between the appro-priate hydrophobic albumin groups so that detergent chains are unidirec-tionally parallel to the plates and perpendicular to the stack.Detergent-gelatin complexes show a single detergent chain forest-layer polar- bonded tothe basic nitrogen of the protein, followed by a covering reverse layer beforepeptisation.81The prevalence of intercalation, mixed systems, and mutual stoicheio-metric accommodation in organic fibres is due to a similarity in C, N, and 0bonds and angles ; e.g., the aliphatic chain interdistance (4.6 A.near m. p.) .=the backbone of a polypeptide grid (4.65 A.), while the relations of the fibrerepeat of the common units of structure (Table) are equally striking :Component unit. Increment (I). n. nI. ....................................... Aliphatic (CH,), 2.548 A. 4 1 0 . 1 9 A .p-Keratin ................................................... 3-38 3 10-14Nucleotides ............................................. 3.34 3 10.02a-Keratin ................................................... 5.14 2 10.28Muscle (normal) ..........................................5-13 2 10.26Cellulose, chitin .......................................... 5-16 2 10-30Alginic acid, pectin .................................... 4.36 2 8-72Collagen ................................................... 2-88 3 8.64Muscle (L-P) ............................................. 2.83 3 8.48Viscose-casein, protein-polyamide , polyamide-ester, polyamide-cellulose,L. Pauling 82 has7 7 H. P. Lundgren, J. Amer. Chern. Xoc., 1941, 63, 2854.78 K. J. Palmer and J. A. Galvin, ibid., 1943, 65, 2187; K. J. Palmer, J. Physical?# W. T . Astbury, S. Dickinson, and K. Bailey, Biochem. J., 1935, 29, 2361.80 W. T. Astbury, Nature, 1936, 137, 803; L. Pauling, J. Amer. Chem. SOC., 1940,81 I(. G. A. Pankhurst and R. C. M. Smith, Trans. Pura&ay Xoc., 1944, 40, 565.8, L.Pauling and D. H. Campbell, Bcknce, 1942, 95, 440.and cellulose-protein systems have industrial importance.Chern., 1944, 48, 12.82, 2643MACARTHUR : CRYSTALLOGRAPHY. 77used the mutual effect of globulin and denaturant to synthesise in vitroantibodies specific to the phenylarsonic group. Chromosomes have not yetyielded to X-ray structural analy~is.~3 Structural results on lipids, nerve,and plasmosin fibres have been recently reviewed.84Water content and its nature are structurally important in protein fibres.J. R. Katz early showed 85 that water entered between the side chains of thegelatin lattice, the spacing increase being linear up to 35% content.A. Weidinger,s6 using Co(CoC1,) t 2[Co( H20)6]C12, as indicator for free andbound water, identifies these as intra- and inter-micellar by X-rays.In furtherstudies, S. E. Sheppard 87 and 0. L Sponsler et aLS8 find, in addition to theside-spacing increase, a new fixed spacing at 7.5 A., no change in the fibre-axial 2-8 A., but a fall in the backbone from 4.4 to 3-3 A. The latter authors,from consideration of the amino-acid content of gelatin and the normal water-co-ordination of the hydrophilic groups involved, locate the adsorbed waterfirst on the hydrophilic side chains and later on the peptide links, corrobor-ating this by infra-red spectroscopy. J. B. Speakman’s analysis 89 of the woolkeratin adsorption isotherm shows similar fractions : (i) a-H,O (up to 6%)combining exothermically with hydrophilic side-chains without effect on suchmechanical features as rigidity, (ii) P1-H20, which associates with the peptidegroups up to a 1 : 1 limit and affects mechanical properties, and (iii) p,-inter-stitial H20.A similar study of protein hydration in general, with a spatialmolecular interpretation of the energetics of adsorption layers, has importantstructural implications. I n a favourable instance, admirable use has beenmade of hydration for phase deterininations in Fourier synthesis .gl* Aswith soaps 92 and viruses,93 intermicellar water may reach great dimensionsin lipid-protein complexes without greatly disturbing regularity of array.94The interpretation, on a molecular structural basis, of elasticity andplasticity in protein fibres is difficult, mainly because of their compositecrystalline-amorphous nature and the time effect in mechanical tests.Despite its successes and improved or-form, Astbury’s or + p transition inwool, with its reversible 100% intramolecular extension, is not yet universallyaccepted.The histological objection has been negatived by H. J. Woods.95It has been variously suggested that the transformation is an ordinary gel-sol83 J. B. Buck and A. M. Melland, J . Hered., 1942, 33, 173.81 F. 0. Schmitt, “ Advances in Protein Chemistry,” 1944,1,25.8 s J. R. Katz and J. C. Derksen, Rec. Traw. chim., 1932, 51, 513.8 6 A. Weidinger and H. Pelser, ibid., 1940, 59, 64.8’ J . Physical Chenb., 1940, 44, 185.8fi Trans. Faraday SOC., 1944, 40, 6.fio H. B. Bull. J .Amer. Chem. SOC., 1944, 66, 1499.9 1 J. Boyes-Watson and M. F. Perutz, Nature, 1943,151, 714.O2 K. Hess, W. Philippoff, and H. Kiessig, Kolloid. Z., 1939, 88, 40.03 J. D. Bernal and I. Fankuchen, J . Gen. Physiol., 1941, 25, 111.94 K. J. Palmer, F. 0. Schmitt, and E. Chargaff, J . Cell. Comp. Physiol., 1941, 18,43 ; R. S. Bear, K. J. Palmer, and F. 0. Schmitt, ibid., 1941,17,355 ; K. J. Palmer andI?. 0. Schmitt, ibid., p. 385.0. L. Sponsler, J. D. Bath, and J. W. Ellis, ibid., p. 906.s5 Proc. Roy. SOC., 1938, A , 166, 7678 GENERAL AND PHYSICAL CZIEMISTRY.is interpretable statistically like that of vulcanised rubber,Q6 cannot give100% fibre extension unless cross-links are broken,96, 97 is really irreversible,g6and can be obtained without fibre extension a t all; 97 that proto-fibril andmacroscopic fibre (e.g., collagen) need not be elastically parallel; 84 that theintramolecular interpretation fails to account for the importance of water inthe transition,98 the correlation of poor elasticity and high c r y ~ t a l l i n i t y , ~ ~ ~ ~ 96and the absence of long spacings in the p-form; and that its biologicalgenerality and 3-residue a-fold are disproved by the Picken structure foraged muscle.99 Astbury’s point of view stresses the absence of loq-mngereversibb extension in straight chains such as fibroin, the specificity ofproteins as compared with rubber, the evidence of optical birefringence anddenaturation,79 the generality of the relation in the keratin-myosin groupindependent of chemical composition as such, 58 and its thread of biologicalsequence, as all referring it to a fundamental configurational chain feature,the 100% molecular extension best fitting all the evidence.Apart fromfurther recent X-ray w0rk,4~a the developing chemistry of keratin cross-links,100, 1 consideration of their relation to elasticity and setting in thoseregions available to acid dyes,2 and the remarkable properties of the cupr-ammonium-keratin compIex,3 promise valuable information on thesestructure-elasticity relations.(b) Specijc. Fibroin. M. Bergmann’s analytical figures supporting the2m3n distribution rule have undergone review,’lb and E. Abderhalden main-tains a diketopiperaeine structure. rejects tyrosine from thecryetalline region and favours there a regular alternation of alanine andglycine (with 1 serine replacing 1 alanine in 12) to fit the Kratky-Kuriyamawhich may, however, be a simple fractional relation of the real one.R.Brill holds that the cell structure is like that of the polyamides, with anextension of b to accommodate side chains, and lateral connection of thezigzag chains in the ac plane by N H 0. Preliminary results of a recentstudy suggest chains (I, 1300 A. ; N, 33,000) mainly of regular periodicity,but with two symmetrically-set regions containing the four proline residuesin a tyrosine-rich section. Specific structures based on chain folds a t6-co-ordinated copper atoms (2H20, 2 prol., 1 tyr., 1 other) are proposedfor the chains dissolved in cupriethylenediamine (1 Cu to 2 peptide N).I(.H. Meyer9 G H. B. Bull and M. Gutmsn, J . Amer. Chem. SOC., 1944, 66, 1253.97 W. Harrison, Chenz. and Ind., 1941, 558.s 8 H. J. Woods, €‘roc. Leeds Phil. Soc., 1940, 3, 577.OD W. Lotmar and L. E. R. Picken, Helv. Chim. Acta, 1942, 25, 538.loo T. Barr and J. B. Speakman, J . SOC. Dyers Col., 1944, 60, 335; J. L. Stoves,Trans. Faraday Soc., 1942, 38, 254, 501 ; 1043, 39, 294, 301 ; M. Harris et al., J. Res.Kat. Bur. Stand., 1941, 27, 89.M. Harris, L. R. Mizell, and L. Foort, ibid., 1942, 29, 73.G. H. Elliott and J. B. Speakman, J . SOC. Dyers and Col., 1943, 59, 124C. S. Whswell and H. J. Woods, Nature, 1944,154, 546.2. physiol. Chem., 1940, 265, 23.K. H. Meyer, M. Fuld, and 0. Klemm, Halv. Chim. Ada, 1940, 28, 1441.0. Kratky and S.Kuriyama, 2. physikal. Chem., 1931, B, 11, 363. ’ D. Coleman and I?. 0. Howitt, Nature, 1945, 155, 78MACARTHUR : CRYSTALLOGRAPHY. 79Pibin. This insoluble fibrous form of fibrinogen is now shown to be amember of the keratin-myosin group.8 Normally a-form, the p-form isproduced by stretching or lateral pressure. The mechanical properties havebeen considered on the basis of a mesh s t r ~ c t u r e . ~EM’S lo confirm the difference in disorder between scale andcortex, proto-fibrils of the former showing a minimum diameter of -1000 A.without evidence of fine structure. Mercury and water at 80” hydrolysepreferentially the extra-micellar regions of wool (to 48%), no changesshowing in the cc- or P-pattern, stretched or unstretched.11 Acidic reagents andhydrogen peroxide produce a degraded structure, held by E. Elod12 to be ad-keratin distinct from the p-form.I. MacArthur’s precision recording 626of the super-structure of porcupine quill yields a very complete spectrumwith a fibre period of either 658 or 198 A. (ratio 10/3). The first alternativehe shows t o contain, with its strong sub-orders, 2m3n numbers of amino-acidresidue lengths, and suggests a multi-ladder structure with spatial regularitiesaccording with much of the Bergmann-Niemann theory ; this interpretationis adopted by Astb~ry.~~bs l3 The smaller figure is preferred by R. S. Bear l4on the basis of similar X-ray measurements. Positions of constituentamino-acid residues have not yet been located in complete accordance withthe intensity distribution.R. S. Bear’s feather keratin period l4 (95 A.)scarcely includes the full spectrum. A less compact a-keratin structurecontaining 2 not 3 residues per 5.14 A. period has been suggested.60bP 96b, 99bThe nature of the muscle proteins, of which myosin is the chief,has been recently re~iewed.1~ By ultracentrifuge analysis, two long mono-disperse rod-shaped myosin proteins are found l6 and to these the diffractionsof muscle are attributed. EM’S show protofibrils clown to 50-100 A. thick.17These have a regular banded structure,18 especially on treatment with osmicacid,lQ and of mean period 360 A . ~ ~ Long X-ray spacings by Bear reveal aregular periodicity of 726 A. up to a t least the 40th order, with characteristicintensification a t 5 m intervals.20 Such complementary use of EM andX-ray methods is a promising one in protein fibre analysis.On the groundsof the relation of the 5.1 A. period of muscle to this 726 A. spacing, Mac-Arthur’s extension of possible 2”3“ periodicities to muscle 64b is rejected.Keratin.Myosin.K. Bailey, W. T. Astbury, and K. M. Rudall, Nature, 1943, 151, 716.U. Ebbecke, KoZZoid. Z., 1940, 91, 134.lo €1. Zahn, TextiZber., 1942, 23, 157; C. W. Hock and H. F. McMurdie, Amer.Dyes Bepr., 1943, 32, 433, 451.l1 E. Elad, H. Nowotny, and H. Zahn, TextiZber., 1940, 21, 385.l2 Idem, ibid., p. 617.l3 “ Advances in Enzymology,” 1943,.3, 63.l4 J. Arner. Chem. SOC., 1943, 65, 1784..l5 H. B. Bailey, “Advances in Protein Chemistry,” 1944, 1, 289; E.Wohlischl6 0. Schramm and H. H. Weber, ibid., 1942, 100, 242.l7 M. v. Ardenne and H. H. Weber, ibid., 1941, 97, 322.l 8 F. Sjostrand, Nature, 1943, 151, 725.l9 M. A. Jakus, C. E. Hall, and F. 0. Schmitt, J. Anter. Chem. SOC., 1944, 66.313.2o Ibid., p. 2043.Kolloid. Z., 1941, 96, 26180 GENERAL AND PHYSICAL CHEMISTRY.But, its exact origin apart, this 7 2 6 ~ . spacing is just the 128th multiple(2730 as for a-keratin) of the other muscle fibre period, R. 0. Herzog’s earlyresult 21 having now been confirmed with the precision of a ramie photographfor an aged preparati~n.~~b Recording conditions are critical, new preparationsgiving only the usual a-keratin structure. From 18 reflections, W. Lotmarand L.E. R. Picken derive the monoclinic cell a 11.70, b (fibre axis) 5 . 6 5 ,c 9.85 A., p 73” 30’, with four amino-acid residues per cell if density = 1.27.The probable space-group C22 affords the dyad screw axis along b desiredby M. L. Huggins.606 A statistical distribution of particular amino-acids ispostulated. A structure is found in which cis-type peptide residues aline witha fibre-axial extension of 2.83 A., and are laterally bonded by N H 0 links.That the maximum intramolecular extension of 25% is insufficient to explainthe elastic range observed is met by allocating residual extension to the amor-phous regions. The cis-formation and axial residue length recall Astbury’scollagen form; 22 the normal myosin structure (period 5.1 A.) is held torepresent 2 not 3 residues.The crystallite size in the L-P structure mustbe >> the diameter of the fibre bundles (estimated a t -100 A. from the peak oflow-angle scattering by 0. Kratky), and weak equatorial spacings observedat 33,42, and 66 A. are attributed 23 to regular side-spacings, though similarvalues found in nerve fibres have been referred to persistent tissuelipids.-Collagen. Gelatin. The diketopiperazines found in gelatin hydrolysatesmay be attributable to artefacts from dipeptide~.~* The macrostructure haabeen examined by X-rays by 0. Kratky and A. Sekora 25 and by R. S. Bear.26Both find in collagen many orders of a fibre-axial period of 642 A., absent ingelatin. The period varies from 680 to 615 A. according as swelling or tanningprocedures are applied, without any corresponding variation in the dominant2-86 A.period. Thus the lattice and superlattice may have no close commonorigin. The large period is not removed by swelling in acid or alkali andredrying, but disappears on thermal or chemical shortening of the fibre. Thealternation of intensities in odd and even orders when wet, disappears ondrying. That the macrostructure seems more a function of protofibril thanlattice is confirmed by EM. The 6 4 4 ~ . period so found2’ with possible160 A. sub-peaks, can be greatly extended by tension, which also alters theratio of the lengths of dark and light bands. Further elucidation of thenature of this banded superstructure will be of the greatest interest.Viruses. EM’S 28 have confirmed X-ray findings.No further analysishas been presented of the wide-angle pattern of tobacco mosaic virus, whichseems based on rhombohedra1 units, though further X-ray data indicate a lowerZ1 R. 0. Herzog and W. Jancke, Naturyiss., 1926,14, 1223.22 J . Int. SOC. Leather Trades Chemists, 1940, 24, 69.23 0. Kratky, A. Sekora, and H. H. Weber, Naturwiss., 1943, 31, 91.24 A. H. Gordon, A. J. P. Martin, and R. L. M. Synge, Biochem. J., 1943, 37, 92.25 J . makromol. Chem., 1943, 1, 113.26 J . Amer. Chem. SOC., 1944, 66, 1297.27 F. 0. Schmitt, C. E. Hall, and M. A. Jakus, J . CeZZ. Comp. Physiol., 1942, 20, 3 1 .W. M. Stanley, J . Biol. Chem., 1942, 146, 25PRESTON : CRYSTALLOGRAPHY. 81molecular weight than R. B. C o r e y ’ ~ .~ ~ Optical analysis of solutions underpressure 30 suggests that the ribonucleic acid and protein frameworks combineso that the planes of the purine and pyrimidine rings are mainly parallel toeach other and to the plane of the indole ring of tryptophan, and probablyperpendicular to the long molecular axis. I. MAcA.vi. Electron Microscopy.The electron microscope is one of the new tools available for the studyof the fine structure of matter. Although the fundamental identity ofgeometrical optics and the dynamics of a particle was known to Hamiltona century ago, the idea of using electrons instead of light in a microscope isa very recent development. Looking back, it now seems curious thatthe discovery of the electron in 1896 was not very quickly followed by theinvention of electron-optics.The knowledge of Maxwell’s electromagneticequations and of Hamilton’s optical and dynamical theories, together withthe existence of the charged particle, provided all the essential features forthe inception of the idea of electron-optics. Nearly 30 years passed beforede Broglie, Thomson, Davisson, and Germer showed that electrons, like X-rays, are diffracted by crystals just as light is diffracted by a ruled grating.Even the discovery that the motion of an electron is governed by a wavedid not lead immediately to the idea of using electrons in a microscope. Thefirst step towards the practical realisation of the electron microscope mayprobably be recognised in a paper by A. Busch in 1926, in which he showedthat electrons could be focused by electrostatic and magnetic fields, just aslight is focused by a lens.It is an interesting fact that, although the scienceof light developed from the ray treatment to the wave theory with all itsconsequences-interference, diffraction, polarisation-yet with electronsthe historical sequence is reversed. Following the period in which theywere treated as massive particles, which is analogous to the idea of thephoton, the diffraction of electrons, i.e., that aspect of their behaviourwhich is attributable to a wave, received attention whereas their treatmentas rays has only been fully studied in the last ten years or so.The arrival of the electron microscope probably owes something todevelopments in many branches of scientific investigation which created ademand for such an instrument.The study of the fine structure of matterreceived an enormous impetus from the application of the technique of X-raycrystal analysis, but the step from magnitudes that were optically visibleto the knowledge of atomic spacings and arrangements was so large that agap was left. The limit of resolution of the optical microscope may be set,in round numbers, at about em. as a lower limit. Particles smallerthan this may be seen as diffraction discs when suitably illuminated but theirC. G. Vinson, D. K. McReynolds, and N. S. Gingrich, Missouri Agr. Exp. Stn.Res. Bull., 1939, No. 297, 11 pp.30 A. Butenandt, H. Friedrich-Freksa, S. Hartwig, and G. Scheibe, 2. physw2.Chem.,1942, 274, 276.1 Ann. Physik, 1926, 81, 97482 GENERAL AND PHYSIUAL UHEMISTRY.size and shape will not be determinable. X-Ray diffraction carries us a tone step down to orders of magnitude of about cm., but the method isnot very suitable for telling us much about structural detail in the region of10-6 cm. It is here that we may hope the electron microscope will step inand show us a world not accessible at all to light waves and difficult of accessby X-ray methods.It is perhaps worth while briefly considering the fields open to in spectionby the three methods-light, electrons, and X-rays. The following tableem.Limit of unaided vision 10-8Bacteria ..................................................................... 10-4 = 1 IL.red ............................................ 8 x 10-6Wave-length of light violet ......................................... 4 x 10-6ultra-violet ................................2 x10-6Inter-atomic distances in solids ...................................... 3 x 10-8Wave-length of X-rays ..................................................Wave-length of 60-EV. electrons ..........................................................................................Viruses, protein molecu { es, smoke part cles, oxide films .........lo-’ = 1 A.5 x 10-10shows a few of the magnitudes of small objects together with the wave-lengths of light, X-rays, and electrons. There are, of course, X-rays ofwave-lengths other than cm., but for crystal analysis the wave-lengthsused are commonly about 1.5-24 x cm.The K characteristicradiations of copper, iron, chromium, and cobalt fall in this range.The range of magnitudes open to inspection by light is limited a t thelower end by the wave-length of light itself. The figure quoted above,10-6 cm., is one-mh of the wave-length and is probably optimistic. Itwill be seen that there is a wealth of material, organic and inorganic, belowthis limit the structure of which cannot be ascertained by optical means.Other things being equal, the limit to the resolving power of X-ray or electronmicroscopes would be the same, Le., one-fifth of the wave-length. Lookingat the table, we see that the X-rays used in crystal analysis have a sufficientlyshort wave-length t o enable us to see atoms-always, of course, subjectto the above specified condition The difficultywith X-rays is that we have no lenses to bend and focus them in the waythat light is focused by the objective and eye lenses of a microscope.Allwe can do is to record a diffraction pattern and then send out for a mathe-matician who, given sufficient data, can see with his mind’s eye and willproduce an electron map of the crystal for us. The resolving power is, infact, just about the same fraction of a wave-length as in the case of light.The synthesis carried out by the mathematician can be effected by anoptical device due to Sir W. Lawrence Bragg,2 which he has called an X-raymicroscope.The wave-length isextremely short ; it is less than that of light by a factor of but wecannot yet make use of this enormous potential increase of resolving power.Unlike X-rays, but like light, electrons can be deflected from their straightother things being equal.”With electrom the case is somewhat different.Nature, 1939, 143, 678; 1942, 149,5470PRESTON : CRYSTALLOGRAPHY.83paths-not by material lenses, which would stop an electron beam com-pletely, but by magnetic or electrostatic fields. These electron lenses sufferin an exaggerated degree from the defect known in optics as sphericalaberration. There are other defects of the image but this seems to be themost important one. Up to the present, electron lenses free from this defecthave not been produced and to minimise its consequences recourse hasbeen had to the device of narrowing the aperture of the lens and using anextremely fine pencil of electrons to illuminate the specimen. The netresult is that the resolving power obtained in practice is about 50 A., thoughin favourable circumstances it seems that a figure of 20 A.may be attained.This means that the resolving power is about 100 times greater than that ofan optical microscope.The growth of electron microscopes from small experimental laboratorymodels t o the type of commercial product which is now available (or would bebut for the war) has been fairly rapid and has gone on simultaneously andindependently in this country, America, and Germany. Here, L. C. Martin,3in collaboration with Metropolitan Vickers, produced an experimentalmodel.B ~ r t o n , ~ and his collaborators,6 were a t work in Toronto anddemonstrated clearly the practical possibilities of high-resolution electronmicroscopy. In Germany, both Siemens and the A.E.G. had investigatorsa t work and just before the war the former firm had a model for sale, but sofar as the Reporter is aware none wits imported into this country. TheA.E.G. instrument was of the electrostatic type, i.e., the electron lenseswere electrostatic fields. This type is stated to be rather simpler in designand construction than the magnetic type, in which the lenses are magneticfields produced by oircular coils carrying a current. Published photographsby H. Mahl and others at magnifications up to 10,000 are certainly of goodquality. The magnetia instruments appear to be capable of rather highermagnifying power but the stabilisation of the electric supplia presents adifficult problem.The Toronto instrument has been developed commercially by the RadioCorporation of America, and several of these instruments are now in servicein this country.The lenses are magnetic and their functions are closelyanalogous to the lenses of the optical microscope. An image of the electron~ource, a hot tungsten filament, is focused on the specimen by a condenserlens; an objective lens produces a magnified image of the specimen and thisimage is subjected to a further stage of magnification by a projector lens.The final image at from 2000 to 30,000 diameters is viewed on a fluorescentscreen and can be recorded photographically.Usually for high magnifi-cation work an electron magnification of about 10,000 is used, followed by8 L. C. Martin, R. V. Whelpton, and D. H. Parnum, J . Sci. Instr., 1937, 14, 14.4 E. F. Burton and W. H. Kohl, “The Electron Microscope,” Reinhold Carp.,6 E. F. Burton, J. Hillier, and A. Prebus, Physical Rev., 1939, 56, 1171; A. Prebus6 f;. tech. Physik, 1939, 20, 316.New York, 1942.and J. Hillier, Canadian J . Res., 1939, A , 17, 4984 GENERAL AND PHYSICAL CHEMISTRY.optical enlargement up to 10 times. These R.C.A. instruments are arrangedso that specimen and photographic plate can be introduced into the vacuumthrough air locks; these operations can be carried out quickly withoutdestroying the vacuum in the body of the microscope column.Recently the R.C.A.' have described a simpler and smaller model, alsoof the magnetic type. The G.E.C. of America 8 have produced a simplifiedelectrostatic model. Both firms have realised that there will probably bea demand for a cheaper and smaller instrument in industry, hospitals, anduniversities, with not quite the same high resolving power, but still capableof giving results much in advance of anything obtainable optically. TheG.E.C. model is particularly simple. The electron magnification is about500, and the image on the fluorescent screen is photographed outside thevacuum, a further stage of optical magnification up to a total of about 5000being possible. These smaller instruments would certainly appear to havesome points in their favour for routine work where the highest resolvingpower is not required.The instruments mentioned above are all of the '' optical " type in thesense that there is a very close analogy between the function of the electronlenses and the lenses of the optical microscope. There are several otherdevices, which may be called microscopes since they are intended to revealfine structure, two of which deserve mention, M. Benjamin and R. 0.Jenkins have described a method of investigating the auto-electronicemission from fine metallic points which produces a very high magnification.In principle, the apparatus consisted of a very small spherical tip of metal atthe centre of a large evacuated spherical vessel coated internally with a thinlayer of fluorescent material. A difference of potential was maintainedbetween the metal point and the sphere sufficient to drag electrons from themetal. The electrons starting with negligible velocity follow the straightlines of electrostatic force, and an image of the point is projected on to thesurface of the containing vessel. The magnification is the ratio of the radiiof the containing vessel and the metal tip. V. K. Zworykin,lo J. Hillier, andR. L. Snyder lo have described an electron microscope of a novel kind suitablefor the examination of opaque, massive specimens, such as metals. Theidea is to use the electron lenses to produce a very fine beam or pencil ofelectrons. This is moved over the area to be examined, and the secondaryelectrons, released from the bombarded surface, are accelerated on to afluorescent screen where they produce light, the intensity of which dependson the number of secondaries and therefore on the nature of the very smallarea subject to bombardment. The light fluctuations from the fluorescentscreen are converted into electrical impulses by a photo-electric cell, and theseimpulses after amplification are recorded on a drum where a magnifiedpicture of the surface is built up. The instrument will produce magnifiedimages up to some 5000 diameters.7 V. K. Zworykin and J. Hillier, J . Appl. Physics, 1943, 14, 658.8 C. H. Bachman and S. Ramo, ibicl., p. 155. @ Proc. Roy. Soc., 1940, A , 176, 262.10 Amer. SOC. Testing Mat., Aug., 1944; C. J. Overbeck, J . Sci. Imtr., 1944, 21, 1PRESTON : CRYSTALLOURAPHY. 85Applications.-(a) Biological. One of the most interesting fields towhich electron microscopy is being applied is in microbiology. There areobviously serious drawbacks to be overcome ; the necessity of placing thespecimen in a vacuum and bombarding it with high-speed electrons is to betaken into account, and due care must be exercised in ensuring that theresults are not affected by changes induced by drying of the material. Not-withstanding the perishable nature of the material and the manifest ill-treatment to which it is subjected, there seem to be grounds for believingthat these drawbacks are not in fact so serious as might at first sight appear.Bacteria, viruses, and tissue seem to withstand a reasonable amount ofbombardment in a vacuum without undergoing any progressive change inappearance. I. M. Abraham and J. W. McBain l1 have described a methodof enclosing the specimen in a cell which is thin enough to allow the electronsto pass without much scattering and yet strong enough to enclose the speci-men in a film of water. Applications of the electron microscope to biologicalproblems have been recently reviewed by G. E. Donovan,12 who gives a listof recent papers on this aspect of the matter. There seems to be a widefield for investigation, and it is being quickly explored.Under this heading there appear to be a t present twomain uses for the electron microscope. The first is the determination ofgrain size and shape in smokes, powders, clays, etc. The great opacity ofsmall grains to electrons makes their use particularly apt; particles ofcolloidal dimensions, 100 A. in diameter, can be detected, and the thicknessof material which crystallises in flakes, can be estimated. As examples ofthis type of investigation attention may be directed to a paper by T. Marxand G. Wehner l3 on the shape and size of particles of Mg( OH) ,, which appearto be flakes about 100 A. thick. Carbon soot from different sources has beenexamined by various workers,14~ l5 and at a recent conference called by theBritish Coal Utilization Research Association a short account of the applica-tion of the electron microscope to the study of coal was given.16 The changesin shape and size of particles in the transformation of y-FeO*OH into y-Fe ,O,and or-Fe203 have been studied by R. Fricke, T. Schoon, and W. Schroder.17Mention must be made of the work of C. E. Hall and A. L. Schoen 18 at theEastman Research Laboratories on the structure of silver bromide grains inphotographic emulsions. The change in the bromide crystals during exposureto the electron beam is recorded and the production of filaments of silverin the development of the exposed bromide is shown.The second type of problem in the inorganic world is metallurgical.l1 J . Appl. Physics, 1944,15, 607.l2 Nature, 1944, 154, 356.l3 Kolloid-Z., 1943, 105, 226.l4 U. Hofman, A. Ragoss, and F. Sinkel, ibid., 1941, 96, 231.l5 M. von Ardenne and U. Hofman, Z. physikal. Chem., 1941, By 50, 1.l7 G. D. Preston and F. W. Cuckow, 1943 Conference on the Ultra-fine Structure ofla J . Opt. SOC. Amer., 1941, 31, 281.(b) Inorganic.Ibid., p. 13.Coal and Cokes, B.C.U.R.A86 GENERAL AND PHYSICAL CHEMISTRY.The examination of an etched metal surface requires a special technique l9which appears likely to yield valuable results. To examine the surface, it isreproduced in the form of a thin cast or replica in a film of some suitablematerial such as nitrocellulose or '' formvar '' (polyvinyl formal). A dilutesolution of one of these materials is allowed to dry on the surface of the metaland is then detached and used in the electron tnicroscope as a specimen.Surface irregularities developed on the metal by etching are reproducedin the film as variations of thickness which muse differences in the intensityof the transmitted electron beam. A highly magnified image of the surfacecan thus be obtained. In certain cases a thin oxide film may be detachedfrom the metal and used as the specimen in the same way; this techniquehas been used successfully with aluminium alloys. The variation of surfaceelevation and the thickness of small particles have also been investigated byR. D. Heidenreich and L. A. Matheson,20 using a stereoscopic method.Changes in elevation and thicknesses of 150 A. can be measured.The intensive development of synthetic polymers in recentyears has focused attention on the large molecular aggregates in these sub-stances. Some attempts have been made to examine these structures bymeans of the electron microscope. Photomicrographs of polyoxymebhylene 21crystals degraded by acids and alkalis have been published. The same papercontains some observations on chemical and bacterial degradation of cellulosefibrils.The above very brief summary of the uses to which the electron micro-scope has been put shows that up to the present the work carried out hasbeen of an exploratory nature. Both the type of problem suitable forinvestigation and the technique of obtaining specimens in a form suitablefor examination must inevitably first be found. At the same time theobservations have t o be correlated with existing knowledge obtained byoptical methods on the one hand and by X-ray and electron diffraction onthe other. Progress is undoubtedly being made quickly and it may be hopedthat with the advent of simpler instruments electron microscopes will be ascommonly found in hospital, industrial, and university laboratories as aretheir optical prototypes. A very valuable bibliography of this rapidlygrowing subject has been compiled by C. Marton and S. Samza(c) Organic.G . D. P.A. E. ALEXANDER.R. M. BARRER.C. N. HINSHELWOOD.I. MACARTHUR.G. D. PRESTON.J. M. ROBERTSON.19 V. J. Schaefer, Physical Rev., 1942, 68, 496; 3. A w l . Physics, 1942, 13, 427;2O l b i d . , 1944, 15, 423.21 M. Staudinger, Chem. Ztg., 1943, 67, 316.R. D. Heidenreich and V. G. Peck, ibid., 1943, 14, 23.J . Appl. Physics, 1943, 14, 522; 1944, 16, 575