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ether surrounding the molecules, considerable enough in reference to the wave-length of the transmitted ray, and conditioning, of course, a particular molecular structure of the substance. The connection between the direction of rotation and the appearance of right or left-handed hemihedric planes in active crystals has led to the supposition that their ultimate parts are superposed so as to form right-handed or left-handed helices. This view, suggested by Pasteur, Rammelsberg, and others, appears highly probable from experiments first instituted by Reusch, and more recently further extended by Sohncke. If a number (12 to 36) of thin laminæ of optically biaxial nica be superposed in the form of a spiral, so that the principal section of each may form a certain angle (45°, 60°, 90°, or 120°) with that of the preceding one, an optical combination is produced, which causes rotation in a ray of polarized light precisely like an active crystal, the direction of the rotation being to the right or left hand according as the plates are arranged in a right or left-handed spiral. The optical properties of such mica-combinations were minutely investigated by Sohncke, who has arrived at the conclusion that provided we use sufficiently thin laminæ, we shall obtain combinations exbibiting rotation-phenomena more nearly obeying the laws found to hold good for quartz and other active crystals. Hence, Sohncke considers as, to say the least, probable, that rotatory crystals possess a structure analogous to that of these mica-combinations.

§ 13. As to the constitution of active liquids, we are driven to seek for the peculiarity of structure on which their power of rotation depends in the arrangement of atoms in the molecule. Now, Pasteur supposes that molecules-like all other material objects—may be divided, in respect of shape and the repetition of their symmetrical parts, into two great classes, viz. :-1. Those whose images are superposable by the bodies themselves (as straight flights of steps, dice, &c.). 2. Those whose images are not superposable (as winding stairs, screws, irregular tetrahedrons, &c.), and which may possess either of two structurally opposed (or enantiomorphous) shapes. Molecules of the former class possess symmetry of structure; those of the latter class have their atoms disposed asymmetrically, and accordingly exhibit optical activity. In 1848, Pasteur made the important discovery that inactive para-tartaric acid is separable into right-rotating and left-rotating tartaric acids; and the sodiumammonium salts of these two acids are distinguishable from each other by the presence of dextro-hemihedric and lævo-hemihedric planes respectively. Moreover, these salts retain their opposite characters in solution, by exhibiting opposite rotatory powers.

| Pasteur : Recherches sur la dissymétrie moléculaire des produits organiques naturels. Leçons de Chimie professées en 1860. Paris, 1861.

2 Rammelsberg : Ber. d. deutsch. chem. Gesell. 2, 31.
3 Reusch : Pogg. Ann. 138, 628.
4 Sohncke: Pogg. Ann. Erg. Bd. 8, 16.
5 Pasteur: Recherches, &c. p. 27.

Hence we may suppose that the property of asymmetrical structure of opposite kinds, such as we have seen in crystals, may occur in molecules also, and the precise nature of the arrangement of the atoms, or rather atom-groups, may reasonably be assumed to be here also of a helical kind. Whether the phenomenon of circular doublerefraction, as exhibited by crystals, occurs also in active liquids is still an undecided point, several experiments made by Dove on sugar solutions and on oil of turpentine having led to no conclusive result.

Hence, according to Pasteur's views, the different optical modifications of tartaric acid may be explained on the supposition that in dextro-tartaric acid the atoms which go to form the molecule are grouped in right-handed helices, whilst in lævo-tartaric acid they are grouped in helices, equal in size, but left-handed in direction : and hence, too, the inactivity of racemic (para-tartaric) acid on the ground of its being formed by the union of equal molecules of the two former modifications. But besides these, other forms are well known, optically inactive, but not separable into the two optically active acids. To explain the existence of these, some other assumption is necessary, either that the helical structure is in their case abolished (untwisted), as Pasteur suggests, or that their inactivity arises from compensation within the molecule which is composed of two atom-groups possessing opposite rotatory powers. As to chemical structure, however--that is, the distribution of affinities between the atoms-they do not differ from the other isomeric acids.

Analogous optical modifications have been observed in a few other substances, which have been brought together in the table on

page 22.

1 Pasteur: Ann. Chim. Phys. [3] 24, 442; 28, 56; 38, 437.
2 Dove: Pogg. Ann. 110, 290. 3 Pasteur : Recherches, &c. p. 38.

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Similar conditions are found to exist in other substances, with the difference that the two oppositely active isomers exhibit unequal rotatory powers.

Thus, the following occur in dextro-rotatory (+), lævo-rotatory (-), and inactive (0) forms :-glucose (as dextrose +, lævulose -, and glucose obtained by heating cane-sugar with water to a temperature of 160° Cent., 0); terpenes (australene, the English oil of turpentine +, terebenthene, the French oil –, terebene 0); amyl-alcohol (that formed from the lævo-alcohol by conversion into the chloride and reconversion into alcohol +, fermentation-amyl-alcohol –, and the

modification obtainable from either by distillation with caustic potash 0). See Le Bel4; Balbiano.5

In many substances one of the two active modifications is unknown. For example : ethylidene-lactic acid (from muscle juice +, by fermentation or synthesis 0); cymol (from oil of Roman cumin +, synthetic cymols 0); mandelic acid (from amygdalin from benzoic aldehyde 0); aspartic acid (from active asparagin in acid solutions +, from fumaric or maleic acids 0).

1 Bremer: Ber. d. deutsch. chem. Gesell. 1875, 1594.

2 Chautard : Comptes Rend., 38, 166 ; 56, 698. Erdmann : Journ. für prakt. Chem. 90, 251.

3 Chautard : Jahresb. für ́Chem. 1863, 394 ; Jungfleisch : Jahresb. für Chem. 1873, 631. Wreden: Liebig's Ann. 167, 302.

4 Le Bel : Bull. soc. chim. [2] 25, 545. 5 Balbiano : Jahresh. für Chem. 1876, 348.

Lastly, in a few substances the inactive form is still unrecognized, as, for example, in borneol (as dryobalanops camphor +, as blumea (Ngai) camphor, and camphor from fermentation of maddersugar -); carvol (from cumin-oil and dill-oil +, from mint-oil —). See Flückiger. A whole series of ethereal oils exist in both dextrorotatory and lævo-rotatory modifications.

Optically different modifications of particular substances are found, in some cases, to exhibit differences in their other properties. Thus, the salts of active para-lactic acid are distinguished from those of inactive fermentation-lactic acid by different amounts of water of crystallization and somewhat different degrees of solubility (Wislicenus). Para-tartaric acid is more difficult of solution than the active tartaric acids. In their behaviour with inactive substances, Pasteur finds no difference between dextro- and lævo-tartaric acid ; thus their potassium, sodium, and ammonium salts, tartar emetics and tartramides exhibit no difference beyond opposite rotatory powers and the occurrence of incongruous hemihedry in the crystals. But it is otherwise when the two acids are allowed to react with active substances, as asparagin, quinine, strychnine, sugar, &c. Where combination takes place the compounds formed differ from each other in crystalline form, specific gravity, water of crystallization, and in the readiness to decompose under the action of heat. Dextro-tartaric acid forms with asparagin a highly crystallizable substance, lævo-tartaric acid does not: lævo-acid malate of ammonia combines with dextro-acid tartrate of ammonia to form a crystallizable double salt, but not with the lævo-tartrate : lævotartrate of cinchonine is more difficultly soluble in water than the dextro-tartrate : dextro-tartrate of ammonia is decomposed by ferment-action, whilst lævo-tartrate undergoes no fermentation, and lævo-tartaric acid can, in consequence, be obtained in this way from para-tartaric acid, and so on. To illustrate these peculiarities, Pasteur suggests the case of two screws-one right-handed, the other left-handed—driven into separate pieces of wood. When the fibres of the wood are rectilinear (inactive substance), two systems of the same kind will be produced ; but this will no longer be the

| Flückiger: Ber. d. deutsch. chem. Gesell. 1876, 468. 2 Pasteur : Comptes Rend. 46, 615.

case when the fibres are themselves arranged helically, and especially when the helices take opposite directions in the two pieces.

Incongruous hemihedric faces are found in most crystallizable active substances. Pasteur has observed them not only in the tartaric acids, tartrates, tartramides, and amic acids, but also in the crystals of acid malates of lime and ammonia, valerianate, and chloride of morphine, &c. In other cases, however, they are absent, as in active amyl-sulphate of barium.o Incongruous hemihedry, moreover, is found to occur in some crystals exhibiting no rotatory power, as formiate of strontium and magnesium sulphate. The two characteristics are, therefore, not inseparable.

C. Dependence of Optical Activity upon Chemical

Constitution.

$ 14. On this question Hoppe-Seyler and also Mulder, proceeding on the ground that the rotatory properties of natural organic substances appear to be to some extent inherited by their derivatives, have expressed an opinion that optical activity is not dependent on the whole atomic structure of the molecule, but only on a particular part of it. The original compound they assume to include one or more active radicles, which in the derivatives may either appear unchanged or transformed into new but still active groups, or are eliminated altogether.

A theory has lately been proposed by Le Bel, and nearly at the same time by van't Hoff,? which is much more plausible, and inasmuch as it brings into direct connection the rotatory powers and the constitutional formulæ of substances, is of special significance to chemistry. Le Bel first suggested, that when a carbon-atom occurs in combination with four different radicles, a molecule of asymmetrical shape is constituted, which, as such, should exhibit rotatory properties. Van't Hoff, proceeding on a hypothesis of his own respecting

1 Pasteur: Ann. Chim. Phys. [3] 38, 437; 42, 418. Comptes Rend. 35, 176. 2 Pasteur : Comptes Rend. 42, 1259. 3 Pasteur : Ann. Chim. Phys. [3] 31, 67. 4 Hoppe-Seyler : Journ. für prakt. Chem. 89, 274. 5 Mulder : Zeitsch. für Chem. 1868, 58. 6 Le Bel : Bull. Soc. Chim. [2] 22, 337 (1874).

7 J. van't Hoff: Bull. Soc. Chim. [2] 23, 295 (1875). La chimie dans l'espace. Rotterdam, 1875. German ed. by F. Hermann, Die Lagerung der Atome im Raum. Braunschweig, 1877.

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