GLADSTONE ON CIRCULAR POLARIZATION. XXII1.-On Circular Polarization. A Discourse delivered to the Members of the Chemical Society of London. BY DR. J. H. GLADSTONE, F.R.S. WHEN,at the request of the Council I undertook to bring the subject of circular polarization before the Chemical Society I did not realize the quantity of the observations which have been recently accumulated by the industry of foreign savans. In preparing this discourse I have been well-nigh overwhelmed with materials and I have therefore considered it best to omit all description of the more purely physical questions and to confine myself as closely as possible to those branches of the subject which have R direct hearing on chemical science. Circular polarization was first observed in quartz by Arag o in 1811; it was investigated by B iot,* and the subject speedily attracted the attention of some of the leading physicists of this country.Brewster Herschel and Airy made important discoveries in respect to it; but since that early period very little has been done by the scientific men of Great Britain while in France and other parts of the continent investigations have rapidly extended and circular polarization has been taken advantage of for the solution of many chemical problems. It is true that won after the formation of our Society Leeson read an elaborate * Mem. Inst. 1812 and subsequently. GLADSTONE ON CIRCULAR POLARIZATION communication on the subject;* some of Pasteur’s papers have been translated in our Quarterly Journal,? and the matter has of course been referred to in lectures and treaties on Nattiral Philo- sophy;$ yet there can be no doubt that British Chemists have generally made themselves but little acquainted with the sub- ject.The leading phenomena of circular polarization are simply these. If a slice be cut from a crystal endowed with this property in a direction perpendicular to its axis and it be examined by polarized light it does not exhibit the coloured rings and black cross; and if the emergent polarized ray be analysed by a doubly-refracting prism the two images have complementary colours which change as the prism is made to revolve. If such a slice of crystal or a liquid possessing the same power be placed between the polarizer and analyser of a polariscope the maximum and minimum of light are in fact not attained when the plane of reflection is inclined to that of polarization at 0’ and YO’ but at some other angles 90’ apart.The amount of this deflection will differ with the colour-that is with the refrangibility of the ray ;thus a violet ray will be deflected to perhaps twice the distance of a red ray; and hence when white light is employed a series of coloiirs are ob-served following one another in regular succession as the analyser is made t.ii revolve. If in order to make these follow in their natural order-red orange yellow green blue violet-it is necessary to turn the arialyser to the right-that is to say in the direction of the hands of a watch-the substance is said to exhibit right-handed or positive circular polarization which is usually indicated by the sign A~’or + if on the coutrary the analyser must be turned to the‘”1eft to produce the same result the polarization is left- handed or negative and the sign <I or -is employed.On revolving the analyser beyond the violet rays the same order of colours beginning with red reappears and there is a transition tint called by the French teint de passage or teint sensible which from its sensitiveness is very valuable for obser- vations. * Mem arid Proc. Chem. Soc. ii 26. t Chem. SOC.Qu. J. iii 79; v 62; vi 273 277. $ For instance in Golding Bird’s Elements of Natural Philosophy; in Pereira’s Lectures Pharm. J. ii and iii; in Graham’s Elements of Chemistry 2nd Edit.Vol. ii p. 464;and in a lecture by Maskelyne Phil. Mag. (4) i 428. GLADSTONE ON CIRCULAIL. POLABIZATJOR. For the explanation of these phenomena on the undulating theory I can only refer to Fresnel's Memoirs.* Substances which exhibit Circular Polarization.-T f ie following table contains a list of the substances in which the phenomena of circular polarization have been observed. It is doubtless incom- plete; and it must be borne in mind also that the effects are produced by very many compounds of some of these bodies which are not included in the table. Against the name of the substance is placed the sign -f-or - to indicate the direction of the rotation and in another column the name of the observer who first remarked the property in the substance in question or who has worked upon it to the greatest effect.Where a second observer has discovered the existence of a power of rotation in a direction opposite to that first discovered which is by no means an UZL-common circumstance or has otherwise largely extended our knowledge of the subject his name has generally been added. The last column indicates whether the substance has been ob-served to form hemihedral crystals either itself or in its com- pounds. -Hemi-Substance. Direction. Authority. hedral? --.__I_-Quartz crystal . . .. + or -Arago Biot Brewster Herschel Hem. Chlorate of soda (solid). . + or -Marbach Hem. Bromate of soda (solid). . + or -Acetate of uranium and + or ->J soda (solid) .. 1 Cinnabar.. .. . . + or -Descloizeaux Oil of turpentine .. + or -Biot Seebeck Leeson Oil oflemon .. .. + >2 Oil of hergarnotte .. -+ >> Oil of anise .. .. Oil of carraway ,. . . + >7 Oil of spearmint . . -J? Oil of rue ,. .. -1) ?9 Oil of nutmeg .. .. + Leeson Oil of lavender .. .. -Oil of cubebs .. .. _. W iyh e 1my Oil ofvalerian .. .. -J> Oil ofamber .. .. -CaBtor oil .. .. + Yf Croton oil .. .. -+ )I 1, Balsam of copaiba .. Camphor.. .. .. + or -Biot "C h an tard * Ann. Ch. Phys. (2) xxviii 147. This is abstracted in Daguin's Trait6 El&-mentnire de Physique which contains many of the most recent observations on the subject. GLADSTONE ON CI RCU Lhlt POLARTZATTON. Substances.Direction Authority. Hemi-tedral? -______I_-_-Camphoric acid .. Camphoethylic acid Camphomethylic acid Naphtha .... ..j Cane-sugar . . Milk-sugar .. Grape-sugar . Starch-sugar .. Diabetes-sugar ,. Clucosate of sea-salt Tartaric acid . . Malic acid .. Tartramide .. Asparagine .. Tartramic acid .. Aspartic acid .. Sulphamylic acid Amylic alcohol .. Formobenzoie acid Quinicacid .. Quinine .... Quinjdjne .. Quinrcine .. Cinchonine . . Cinchonidine .. Cinchonicine . . Quinoidine . . Ivforphine .. Brucine .... Strychnine .. Narcotine .. Nicotine .... Santonine .. Hcematoxylin .. Jalapin .... Phloridzin . . Salicin .... Populin .... Codeine .... Narceiue ....Picrotoxinc .. Albumin .. -..; + .. 4-..I -..j t -.I ..I 4-../ +or-../ f I. I. .. .. .. .. .. .. ,I .. .. .. .. .. .. I. .. .. .. .I .. .. .. *. *. .. .. I__ Boucharda t Chant ard Hem. Ma 1 ag 11t i Hem. Loir Hem. Biot , Clerget Hem. , Poggiale Hem. , BQchamp , Clerget Listing Pasteur Hem. Biot Pasteur Arndtxen Hem. P aste 11r Hem. Hem. 7 Hem. 17 Hem. )I 9 Not >f 79 Hem. Bou c h ar d a t Paste 11r 1 ?3 Pasteur Bot~c hard at Pasteur Bou c h ar ds t Paste 11 r Hem. Y , Descloizcsux Wit.h eifmy 99 ?I J. Bou&ardat Biot and Paeteur Biot and Pasteur Bouchardat and Boudet 9) ,1 9 7 A.Becqnerel I have not attempted to express in this table the amount of rotatory force possessed by the different substances not from any lack of numerical data but for the following reasons :-1st. It has been frequently determined differently by different observers. This arises to some extent from actual optical dif- ferences in the samples employed :thus oil of turpentine is known to possess different powers of rotation even when the direction remains the same according to the kind of pine from which it T’OL. XTII. 3 GLADSTONE ON CIRCULAR POLARIZATION. came and the processes adopted in preparing and purifying it. Again the amount of solvent employed is a fruitful cause of diversity. 2nd. Unfortunately in determining the rotatory force no uni-form standard has been adopted.Thus Biot reduces his observa- tions ta a unit of thickness-namely one millimeter and he calculates from the arc of rotation of the rays that have passed througli red glass. He employs the following fiirrnula for deter- mining the specific rotatory power (p) in vhich a stands for the arc of rotation d the density and I the thickness or length in millimeters. a p=-dl And if the substance be dissolved in some optically neutral solvent the proportion of the said substance in one part of the solution (p) has to be taken into account and the formula stands- a P = --d Ip Wilhelmy,* on the contrary laying stress on the molecular nature of the force considers it preferable to take the different substances in quantities proportional to their atomic weights in equal weights of the solvent; and vhere he cannot do this he makes a reduction on the hypothesis that the relative quantity of the solvent has no influence.He makes his calculations also for white light and assumes the molecular rotating power of cane sugar as 100. A table given in one of Wilhelmy's paperst is so good an illustration of the elaborate investigations that have been made into this subject that I have reproduced it here I I Molecular Quantity Atomic rotating Snlxhnce. Solrent. White Red weight. power. dissolved Light. Light. 0 =100. (White -___I -----I1ght.) ni illipr . Cane-sugar ...... 4309 Water +3$5 +250 2154.5 f100 Nicotine ......1301 Alcohol -16 -with hydrochloric acid , >> 4-I(?) Santonin ...... 500 -9 +6'7 3005 -332.3 YJ Hematoxylin .... 1849 ,I .. +13-5 Jalapin ...... 2000 *. -7 7t I I *Pogg. Ann. lxxxi 413 499. $ Ibid Ixxxi 52'7 GLADSTONE ON CIRCULAR POLARIZATION. -MoIecular Degree of rotation. Atomic rotating luaittit Solvent. Wiiite Red weight. power. issolve Light. Light. 0 =100. (White ---llgllt.) milligr Camphor .. .. . 6000 Alcohol + 310 t-22O 963.8 + 30.7 .I Phloridzin .. .. 2377 3) -11 -8.7 2082.5 -59.2 Quinine .. .. . 1782 > -24 -20 }2055.5 -174 .I -.. .. 891 I -12'5 -9.65 -with nitric acid 13 >> -17 .. -241.4 t. -with phosphoric acid . . >> -15 .. .. -213 WaGr & Sulphate of qainine .. 1000 sulp. acid -24.5 -16.7 Valerate of quinine . . Alcohol -16 -13 Hydrochlorate of quinine .. S'O'O -10 3J Cinchonine .. . . . . 454 ,> 4-12 .. 1942 +315.8 -with oxalic acid 494 + 12.5 .. .. + 302 9, -with phosphoric acid .. 952 + 23.5 .. .. + 295 ?> -with nitric acid .. 930 33 + 20 .. .. + 257 -.. 465 79 + 10 99 ?9 Quinoidine.. . . .. 374 1) 4-945 +8 -with nitric acid .. 99 + 9.75 -with phosphoric acid. . 19 + 10 -with hydrochloric acid 97 ?f + 10 1 9, -with sulphuric acid . + 11% I7 Brucine .. .. .. &4 71 -8 .. 3448 -172.5 -with phosphoric acid 0 >> Narcotine with sulphuric 936 Water + 5.5 *. 4684 + 169 acid .. .. 1 It Acetate of morphine .. 490 ,? -4 Hydrochlorate of codeine ..578 -5 Ethereal oil of cubebs . 7460 Al&hol -33.5 ___-, valerian . 7480 Ether -15.4 ' Balsam of copaiva .. .I 7560 Ether with alch. -11.5 Iqluence of state of Aggregation.--The only substance which has been examined in the three different states of aggregation-the solid liquid and gaseous is oil of turpentine and it has been found to exhibit the power in each of these states. It rotates the plane of polarization to the same extent when frozen as when in its ordinary liquid condition ;but Biot who made the experiment on the vapour of turpentine was unable to determine whether the power was then altered in amount. It TVRS necessary as will be readily understood to look through a large amount of the vapour in order to observe any effect on polarized light; in fact an iron tube 15 metres that is 50 feet long was employed for the experiment and unfortunately before the quantitative determina- tion was completed a burst of flame occurred and gave rise t sR 260 GIADSTONE ON CI KCULAH POT~ARIZATJOE.a conflagration which the philosopher could not extinguish without the assistance of the public. Yet while oil of turpentine is so retentive of this power its very existence in some other substances appears to depend on the manner in which they are crystal!ized. Thus silica does not show any effect on polarized light as it exists in the opal or indeed in any other form than that of quartz-crystal. fnfluence of temperature.-Just as the refrangibility of a sub-stance changes with alterations of the temperature and that not pari passu with the change of density so also the amount of rotatory power varies under like circumstances There are how-ever great differences between one substance and another in this respect.Quartz shows an increased power of rotating the plane of polarization when it is heated and that to the extent of 108" or 109.5' for an elevation of 70° C. of temperature. This crystal is known to expand by heat differently in the direction transverse tct what it does in that parallel to its axis. Tartaric acid is similarly affected and that to such an extent that Biot found himself able by reducing the temperature actually to reverse the direction of the rotation in solid amorphous tartaric acid the right-handed deviation diminishing becoming nil and the left-handed commencing before the thermometer sank to 3°C.Grape-sugar in solution is much effected by changes of tem- perature; and in saccharimetry by means of polarized light account must be taken of this circumstance. Clerget,* who has studied this subject finds that the power of rotation decreases-not increases-as the temperature rises and that according to the quantity of sugar quite irrespective of the pro- portion of' water; indeed it is the same whether 100 parts of water dissolve 1 or 130 parts of sugar. He found the decrease between 10' C. and 35' C. to be in the proportion of very nearly 100 to 91 Wilhelrny has determined it for a much longer range of temperature and deduced the formula D' = D [I -0.012 (t' -t)] in which D and D' represents two amounts of rotation corre-sponding with the temperatures t and t'.* Ann. Ch. Phys. [3] xxvi 175. GLADSTONE ON CXRCULAR POLARTZATION. Salts of quinine and many other substances are also known to vary in their power of rotating a ray of polarized light according to the temperature. Yet on the other hand it is stated that the essential oil of turpentine has exactly the same power at 55"C. as at 10" C. and even when frozen. In$uence of Magnetism or Ekctricity .-I cannot help alluding to the beautiful discovery of Faraday that a polarized ray passing through optically inactive bodies may be caused to rotate by the magnetic or electric force and that the rotation in active bodies such as oil of turpentine may be altered or even reversed at will.This part of the subject however I must not pursue further as it belongs purely to the domain of physics. In@xence of SaZution.-S ubstances which exhibit circular polari- zation are affected by solution in three different manners. 1st. There are substances as chlorate of soda bromate of soda and the double acetate of uranic oxide and soda the crystals of which exhibit evidences of circular polarization but are perfectly inactive when dissolved iu water.* These bodies like quartz appear to owe this property to the manner in which their molecules are arranged in the crystals. 211d. There are other substances all of them organic which exhibit the same polarizing power whether they are in the solid state or dissolved in some optically neutral solvent such as water or alcohol.This cannot generally be determined as in most crystals of such bodies the phenomena of circular polarization are masked by the double refraction ; but barley-sugar " has been found to have the same or nearly the same power of rotation as the sugar when dissolved in water; and it has been recently shown that whatever the amount of water in which it is dissolved the rotatory power of sugar is rigorously constant.$ Oil of turpentine belongs to this class. In such cases the substances seem to owe their power of rotating the plane of polarization to the very structure of their molecules. 3rd. There are other substances in which the result partakes to * Marbach Ann.Ch. Phys [3] xliii 252 xliv 41. Biot confirms and draw philosopliical deductions from these experiments. 9 Arndtzen Ann. Ch. Phya. 1311 iv 403. GLADSTONE ON CIRCULAR POLdlCI ZATIONs a certain extent of both these characters where indeed it would seem that the individual action of the molecules is accompanied in the solid state by another action arising from their regular arrangement. Thus sulphate of strychnine* evaporated at a temperature of between 10' C. and 20' C. gives 3 salt which contains 13 atoms of water. It crystallizes in the system of the rectangular prism with a square base and is of such a form as to allow the recognition of the circularly polarizing power. It rotates the ray in fact to about half the extent that left-handed quartz does; but the solution of sulphate of strychnine in water though it also rotates the plane does so only to ,hth or -&th part of the extent to which these crystals do.Tartaric acid when obtained in a transparent solid amorphous state by mixing it with boracic acid was found to exhibit circular polarization ; but different proportions of water cause very re-markable and as yet unexplained differences in the optical character of a solution of tartaric acid. The plane of polarization of the green rays is in fact caused to deviate more than that of any other colour and Arndtzen who has worked out the whole of this matter with much care and ability found when operating with ordinary (that is right-handed) tartaric acid dissolved in alcohol instead of water that he could obtain a left-handed rotation for the blue rays.The same observer found in reference to camphor dissolved in alcohol that the rotatory power augments with the refrangibility of the rays more rapidly than in the case of most active bodies and that the rotatory power decreases regularly with the concentration of the so1ution.f- Influence of Chemical Combination or Substitution.-In most cases this property appears to be so intimately connected with the structure of the molecule itself that it is unaffected or little affected by chemical changes of an important character. Thus an optically active acid such as tartaric acid will carry its rotatory power into its salts; and similarly an optically active base such as quinine will exhibit the phenomena of circular polarization when combined with inactive acids.From observa-tions on neutral and bitartrate of potash and ammonia which are isomorphous Past cur$ has drawn the conclusion that when * Descloizeaux Ann. Ch. Phys. [3j li 361 1. Ann. Ch. Phjs. [3] liv 403. Biot had experimented on these matters pre-viously ; Bee Ann. Ch. Phys. [3] xxxvi 257 405. : Inst. 1850 339. GLADSTONE ON CIltCULAR POLARIZATION. quantities of isomorphous substances corresponding to the equi- valents are dissolved in equal quantities of water these solutions rotate the plane of polarization to an equal degree. But it must not be supposed that as a general rule the optically inactive base or acid exerts no modifying influence on the phenomena exhi- bited by the active element with which it is combined.A glance at the above quoted table of Wilhelmy will show that it is otherwise Bouchardat* has given numbers which indicate that the combination of camphoric acid with soda or with ammonia reduces its rotatory power; some malates have a positive others a negative rotation; and as a further example asparagine if dissolved in pure water or in alkalis turns the plane of polarization to the left but if in mineral acids it turns it to the right.? Oxidation even does not necessarily destroy this power for camphor retains it when it; has been oxidized into the camphoric acid just alluded to; nor does the substitution of a compound radicle for hydrogen as is shown by the optical properties of camphovinicf and camphomethylic acids 0 The conversion of an acid into an amide and that again into the corresponding amidogen-acid is not even fatal to the retention of this optical power as is evidenced by the activity not only of tartaric acid but also of tartramide and tartramic acid; and in a parallel manner not only of malic acid but also of malarnide and mdamic acid- that is asparagine and aspartic acid.11 It must however be supposed in reference to every circularly polarizing substance that there is some amount of chemical change which the molecule cannot suffer without losing this property.In the case of the two acids just mentioned tartaric and malic we find this point passed when by the action of heat they are converted into other acids with evolution of part of their con- stituent elements for pyrotartaric fumaric and maleic acids have no effect on the polarized beam.t And here must be men- tioned one of Past eur’s remarkable observations.From fumarate of ammonia which is inactive aspartic acid may be produced ; but this unlike ordinary aspartic acid is itself inactive ;and again from this may be reproduced malic acid which also in its turn unlike * Comp. rend. xxviii 319. 9 Pasteur Ann. Ch. Phys. [3] xxxi 67. $ Malaguti. 4 Loir Ann. Ch. Phys. [S] xxxviii 483. 11 Pasteur Ann. Ch. Phys. 131 xxxviii 437. Ti Ibid xxxi 67. that derived directly from the plant has no influence 011 the polarized beam.* As yet we have no evidence that an optically active substance has ever been artificially prepared from one that is optically inactive though such is constantly occurring in nature.The tartaric acid which Liebig has very recently prepared has been found to rotate the plane of polarization like natural tartaric acid,? but then it must be borne in mind that it was prepared from milk-sugar which is itself endowed with this property. It will be interesting to determine whether the tartaric acid which Perkin and Duppa have just succeeded in preparing from succinic acid exhibits this power for succinic acid 8s yet is not classed among those bodies which display it. Xet from its analogy to tartaric and malic acids it would appear highly probable that succinic acid should rotate the polarized beam.A specimen said to have been prepared from stearic acid was found by me to be perfectly intctive; but it is quite possible that the succinic acid prepared from malic acid or that which Dessaignesi announces he has prepared from tartaric acid may prove capable of rotating a ray of light. The mutual relationship of the substances here referred to wilt be readily understood by a reference to the following tabl: C=S; 0=8 C=12; 0=16 Tartaric acid . . 2H0.C,H40, C4H606 Malic acid . . 2H0.C8H,0 C4H60 Succinic acid . . 2HO.CSH4o6 C4H6O4 Tartramide . * C8H8N208 C4H8N204 Asparagine . ’ C8H8N206 C4H8N203 Tartramie acid . HO.CsHGK09 C4N,XO5 Aspartic acid . . H0.CsH6N0 C4H,N0 Pyrotartaric acid . H0.C6H30 C3H40 Fumaric acid .2HO.C,H,O6 C4H464 Maleic acid . . 2H0.c,H206 C4H4O4 Relation between Crystalhe Form and the power of Circular Polarization.-As far back as 1820 Herschel found that the crystals of right-handed and of left-handed quartz differ in form a& from the inclination of certain facets he was able to forctel *. Ann. Ch. Phgs. [3] xxxiv 30. .i. Uohn Ann. Ch. Pharni,,Jim 1S60. $ Compt. rend. April 16 1860. GLADSTONE ON CIRCULAR POLARIZATION what would be the direction of the rotation in any particular specimen;* but it was reserved for Pasteur to show that this relation between the crystalline form and the rotatory power is one that generally obtains. It is well known that the law of symmetry does not hold good in all crystals.These exceptional forms have been termed hemi- hedral. It will sometimes happen also that a substance will crj-stallize in two forms which are both unsymmetrical but unsym- metrical in opposite directions-that is to say the one form will appear identical with the other only when it is seen reflected in a mirror. They differ in fact in the same way as one side of our face differs from the other or as our two hands differ. These are variously called not superposable incongruous or opposite hemihedra. From an observation of the opposite hemihedral character of crystals of the double racemate of' soda and ammonia Pasteur commenced that series of observations which led him to the discovery that racemic acid is composed of two tartaric acids with opposite but equal powers of polarization conjoined together as one substance ; to the isolation of left-handed tartaric acid ; to the artificial preparation of racemic acid; and of a neutral and unresolvable tartaric acid$-discoveries which however interesting I refrain from dwelling on as the papers in which they are explained have been printed in our Quarterly Journal and are well known to chemists in this country.The same philosopher has shown by a great number of in- stances that when a substance crystallizes in opposite hemihedra it indicates the existence of two opposite powers of rotation and that active and inactive bodies will not crystallize together however isomeric.2 Yet it cannot be laid down as a universal law that this property of matter in a crystalline state is always accompanied with the power of circular polarization for formate of stroiitia gives opposite hemihedra but neither kind of crystal when separated from the other and dissolved has any influence on polarized light.§ Sulphate of magnesia is an analogous instance.Nor on the otlier hand can it be affirmed that this * See his remarks on this subject in his article on Light in the Encgc. Metrop. par. 1042 in which he almost prophecies more recent discoveries. .t. Ann Ch. Phys. (31 xxiv 442 ; xxviii 56 and Compt. rend. xxxvii 162. .inn. Ch. Phys. [3J xxxviii 437. 3 Illid xxxi 67. GLADSTONE ON CIRCULAR POLARIZATION. optical property is necessarily connected with the power of forming such crystals; for two sulphamylic acids have been discovered which are isomeric and absolutely identical in the crystalline form of their salts vithout showing any disposition to form hemihedra and yet the one is active and the other inactive.* The inactive aspartic acid derived from fumarate of ammonia does not crystallize in hemihedra as the ordinary aspartic acid does; and similarly there are some differences in the crystalline forms of the salts of the natural or active and of the artificial or inactive malic acid.Marbach? has observed that though chlorate of soda crystal- lizes in opposite hemihedra and in that condition exercises an influence on the polarized beam yet a solution of either right- handed or left-handed crystals alone is perfectly inactive and the salt crystallizes out from such a solution in both farms instead of that only from which it was made.POLARIZATION INQUIRIES. CIRCULAR AS APPLIED TO CHEMICAL The phenomena of circular polarization have been applied to many practical purposes in chemical research. These may be conveniently grouped under three heads-the quantitative estima- tion of certain organic products; the determination of what is going forward in a solution; and the examination of isomeric substances. 1at. Quantitative estimation of certain organic products. Curie Sugar.-As cane sugar dissolved in water rotates the ray of polarized light to an extent directly proportional to the amount of sugar in a given depth of liquid and irrespective of the amount of water or of the simultaneous presence of optically inactive substances this property is frequently made use of to effect a quantitative determination.Clerge t$ has worked out this process very fully and others$ have since somewhat extended or modified his observations. Instruments have been devised for * Compt. rend. dii 1259. .t. Ann. Ch. Phys. [3] xliv 41. $ Ibid. xxvi 175. 8 Especially Wilhelmy and Arndtzen in papers previoady referred to; Pohl Wien. Scad. Rericht xxi 492; and MichBelis Journ. Pharm. Chem. lxxv 464. GLL41)8TONE ON CIRCULAR POLARIZATION. the purpose by Biot Solei&* Savart Powell,? Leeson,$ Mitscherlich but the simplest and perhaps the most effective is a modification of Biot's apparatus which Mr. Heisch employs and has kindly lent me on this occasion.It consists essentially of three parts-a polarizer an analyser and a tube for holding the solution. The polarizer is a Nicoll's prism and plano-convex lens to rerider the rays parallel; the nnalyser consists of a small aper- ture in a brass plate a lens and an achromatic prism of doubly refracting spar and it is attached to the vernier of a graduated circle. The tube through which the polarized beam passes between the polarizer and the analyser is a narrow tube of black glass ground in the inside with coarse emery and fitted at each end with covers of perfectly parallel glass. Several such tubes of various known lengths so as to hold different quantities of liquid are provided and the position of the polarizer and analyser can be easily adapted to each.If when the index of the scale is at O" and the extraordinary image is at its greatest obscuration a solution of sugar be placed in the tube it is easy to determine how much the analyser must be now turned round in order to bring the extraordinary image again to its greatest obscuration or rather to the "sensitive tint." It is impossible here to enter into the minuti= of the apparatus or of the treatment of the solution; suffice it to say that Heisch prefers operating by lamp light and has found that it requires 6.09 per cent. of cane-sugar to produce a rotation of 1"in a depth of 1 inch hence the per- centage in the solution examined may be reckoned according to the simple formula a representing the arc of rotation and I the length in inches.An ingenious method of confirming the amount of cane-sugar in a solution or of determining it in the presence of other optically active bodies is founded on the property possessed by this sugar of being converted by acids into a sugar which exhibits the opposite or left-handed rotation. The following * The English reader will find a description of this instrnment by Dr. Bence Jones in the Pharm. Journ. and Trans. xi 455. .t. Phil. Mag. April 1843. :Mem. Chem Soc, ii 26. 268 GLADSTONE OK CIBCLTLAH POLARIZ.\TlON. formula serves for this inverted sugar according to Heisch at 15" C. p = 16,026-a I As glucose rotates the plane of polarization to the right and is not capable of inversion by acids this process will even serve to determine the respective amounts of cane-sugar and glucose if mixed together in solution.Or the same may be effected by determining the rotation of the mixed solution boiling it with potash which destroys the glucose and then again determining the rotation which is now due solely to cane-sugar. Other sugars.-Bence Jones* has published analyses of the different wines of commerce in which he determined the grape- sugar by means of Soleil's saccharimeter. Poggialef has employed a similar method for the estimation of the amount of sugar in milk. Listingf has published optical determinations of the sugar in diabetic urine. The formula of Heisch for this sugar is p = 8.346 -a I If albumin be present it must be removed as it possesses itself an influence on polarized light.A16urnin.-This substance may likewise be estimated by means of its rotating power and A. Becquerelt has in fact measured in this way the proportion of albumin contained in the serum of blood. 2nd. Determination of what is yoiny forward in a solution. The variations in the plane of polarization will often indicate changes in a solution which could not be watched by any other method or even the existence of which might never otherwise have been recognized. Thus Wilhelmp 11 investigated the action of sulphuric phos- phoric nitric and hydrochloric acids on cane-sugar and examined mathematically the progress of the change with reference to time. EIe satisfied himself that the final result of the conversion of the sugar is indcpendent of the quantity of acid originally added.* Proc. Roy. Inst. Vol. i. f Conipt rend. xxviii 505 584. 2 Anii. Cli. Pharm. xcvi 93 101. Compt. rend. xxix 625. )I Pogg. Ann. lxxxi 413 409. .. GLADSTONE ON CIRCULAR POLARIZATION. Bio t investigated a very curious state of combination between tartaric acid and boracir! acid when they are both dissolved together in water and he endeavoured to determine the nature of the transformation which tartaric acid undergoes when acted on by heat. He has applied circular polarization also more widely to the study of the question of the condition of a substance in solution. The papers* which contain these inquiries are very lengthy and include much philosophical reflection on the subject in general.Thus also from some observations of Bouchardat on the effect of polarized light on camphoric acid the same saturated with soda and then supersaturated with hydrochloric acid I was able to derive from a novel source an additional illustration of the reciprocal decomposition of binary compounds in so1utiou.t Be'ch amp 5 too from observations with the pdariscope arrived at the conclusion that crystallized starch sugar is a combination which cannot exist indefinitely except in the solid state and that when dissolved in water it loses its combined water slowly in the cold but rapidly urider the influence of heat resembling in this respect hydrated oxide of copper or hydrated oxide of iron in the presence of water.In fact he supposes that the C,2H,20,2.2H0 when dissolved passes during the lapse of a certain number of hours wholly into C H,202. 3rd. Examination of isomeric substances. Reference has already been made to the researches of Pasteur by which he unravelled the relations between racernic and tartaric acids and enriched our knowledge of many organic compounds by exhibiting isomeric bodies identical with them save in their action on polarized light and perhaps their crystalline form. Their solubility appears ah in some instances to be slightly affected. His discoveries however have not been confined to those already alluded to. Binding that there exist two sulph-amplic acids the one possessed of the rotatory power and the other not possessing it he was able by their decomposition to produce two amylic alcohols differing slightly in their physical properties.fj His very original examination of the cinchona J( *4nn.Ch. Phys. xxviii 215 351 ; xxxvi 257 405. + Chem. Soe. Qu. J. ix 148. X Compt. rend. xlii 640. Dubrunfaut differs from this explanation ibid 739; but 36ehamp defends his own view {bid 896. 5 Comp. rend. xlii 1259. 270 PROFESSORS KIRCHHOFF AND BUNSEN ON CHEMICAL alkaloids and the two isomeric groups at the head of which stand quinine and cinchonine,* I shall only just refer to as his paper was reproduced in our Quarterly Journa1.f Different isomeric essential oils of the composition C,H might be distinguished by this method but the data seem wanting for the practical application of circular polarization to the analysis of them.Berthelott has by the same agency investigated a very remarkable change that takes place in oil of turpentine or oil of lemons by the action of acids alkaline or earthy chlorides or some other salts. This isomeric modification is indicated by a reduction of the rotatory power whether positive or negative; it never entirely disappears but there are grounds for believing that the result is a mixture of unaltered oil and oil which has been rendered inactive. It is not necessary that the salt which produces this singular change should dissolve in the oil for even the insoluble fluoride of calcium has a strongly marked effect.