454 



NATURE 



[September 8, 1898 



as occur in only half their possible number ; and in the case of 

 non-superposable hemihedry, to which class that of the tartrates 

 belongs, there are always two opposite hemihedral forms pos- 

 sible : a right-handed or dextro-form, and a left-handed or Itevo- 

 form. Which is right, and which is left, is a matter of con- 

 vention ; but they are opposite forms, and differ from one 

 another exactly as the right hand of the human body differs 

 from the left': that is, they resemble one another in every respect, 

 except that they are non-superposable — the one cannot be made 

 to coincide in space with the other, just as a right hand will 

 not fit into a left-hand glove. The one form is identical with 

 the mirror image of the other : thus the mirror image of a right 

 hand is a left hand. Such opposite hemihedral crystalline forms are 

 termed enantiomorphs ; they have the same faces and the same 

 angles, but differ in the fact that all positions in the one are 

 reversed iii the other for one dimension of space, and left un- 

 changed for the other two dimensions ; this being the geometrical 

 transformation which an object appears to undergo when 

 reflected in a plane mirror. Enantiomorphism is possible only 

 in the case of asymmetric solid figures ; these alone give non- 

 superposable mirror images. Any object which gives a mirror 

 image identical with the object itself— a superposable mirror 

 image — must have at least one plane of symmetry. 



The hemihedry of the tartrates discovered by Pasteur is in every 

 case in the same sense — that termed right-handed — provided 

 that the crystals are oriented according to two of the axes which 

 have nearly the same ratio in all the tartrates. 



Pasteur was inclined to connect the molecular dextro-rotatory 

 power of the tartrates with this right-handed hemihedry ; since 

 in the racemates both the hemihedry and the rotatory power 

 were absent. A similar connection, which, however, held good 

 only for the crystalline condition, had, as he points out, been 

 already observed in the case of quartz, the crystals of which 

 occasionally exhibit small asymmetric (tetrahedral) faces, 

 situated in some specimens to the right and in others to the left ; 

 the former specimens being dextro-, the latter, laevo-rotatory. 

 The possibility of this connection was first suggested by Sir 

 John Herschel. 



Pasteur's views were confirmed by an unexpected discovery 

 which he made shortly after. Mitscherlich had stated, in 1844, 

 in a communication to Biot, which the latter laid before the 

 French Academy of Sciences, that sodium ammonium tartrate 

 and sodium ammonium racemate were identical, not merely in 

 chemical composition, but in crystalline form, in specific gravity, 

 and in every other property, chemical and physical, except that 

 the solution of the former salt was dextro-rotatory, that of the 

 latter inactive. And to make his statement still more definite, 

 he added : " The nature and the number of the atoms, their 

 arrangement, and their distances from one another, are the same 

 in both compounds." 



At the time this passage appeared, Pasteur was a student 

 in the Ecole Normale. He tells us how it puzzled him, as being 

 in contradiction to the views universally held by physicists and 

 chemists that the properties, chemical and physical, of sub- 

 stances depended on the nature, number, and arrangement of 

 their constituent atoms. He now returned to the subject, 

 imagining that the explanation would be found in the fact that 

 Mitscherlich had overlooked the hemihedral faces in the tar- 

 trate, and that the racemate would not be hemihedral. He 

 therefore prepared and examined the two double salts. He 

 found that the tartrate was, like all the other tartrates which he 

 had investigated, hemihedral ;• but, to his surprise, the solution 

 of the racemate also deposited hemihedral crystals. A closer 

 examination, however, disclosed the fact that, whereas in the 

 tartrate all the hemihedral faces were situated to the right, in 

 the crystals, from the solution of the racemate they were 

 situated sometimes to the right, and sometimes to the left. 

 Mindful of his view regarding the connection between the sense 

 of the hemihedry and that of the optical activity, he carefully 

 picked out and separated the dextro-and laevo-hemihedral crys- 

 tals, made a solution of each kind separately, and observed it in 

 the polarimeter. To his surprise and delight, the solution of 

 the right-handed crystals was dextro-rotatory ; that of the left- 

 handed, laevo-rotatory. The right handed crystals were iden- 

 tical with those of the ordinary (dextro-) tartrate ; the others, 

 which were their mirror image, or enantiomorph, were derived 

 from the hitherto unknown Itevo-tartaric acids. From the 

 dextro- and Isevo-salts, thus separated, he prepared the free 

 dextro- and Ijevo-tartaric acids. And having thus obtained 

 from racemic acid its two component acids — dextro- and 



NO. 1506, VOL. 58] 



lasvo-tartaric acids — it was an easy matter to recompose 

 racemic acid. He found that, on mixing equal weights of the 

 two opposite acids, each previously dissolved in a little water, 

 the solution almost solidified, depositing a mass of crystals of 

 racemic acid. 



These two tartaric acids have the same properties, chemical 

 and physical, except where their opposite asymmetry comes 

 into play. They crystallise in the same forms, with the same 

 faces and angles ; but the hemihedral facets, which in the one 

 are situated to the right, are, in the other, situated to the left. 

 Their specific gravities and solubilities are the same ; but the 

 solution of the one is dextro-rotatory ; of the other, laevo- 

 rotatory. The salts which they form with inorganic bases 

 also agree in every respect, except as regards their opposite 

 asymmetry and opposite rotatory power. They are enanti- 

 omorphous. 



Pasteur, discussing the question of the molecular constitutior* 

 of these acids, anticipates in a remarkable manner the views' at 

 present held by chemists. " We know, on the one hand," he 

 says, "that the molecular structures of the two tartaric acids- 

 are asymmetric, and on the other, that they are rigorously the 

 same, with the sole difference of showing asymmetry in opposite 

 senses. Are the atoms of the right acid grouped on the spirals- 

 of a right-handed helix, or placed at the solid angles of arv 

 irregular tetrahedron, or disposed according to some particular 

 asymmetric grouping or other ? We cannot answer these ques- 

 tions. But it cannot be a subject of doubt that there exists an. 

 arrangement of the atoms in an asymmetric order having a non- 

 superposable image. It is not less certain that the atoms of the 

 left acid realise precisely the asymmetric grouping which is the 

 inverse of this." 



The idea of the irregular tetrahedron is, it may be explained,, 

 derived from the hemihedral facets. Imagine these to develop 

 in the case of dextro-tartaric acid until the other faces of the 

 crystal disappear, and there results an irregular tetrahedron. 

 Repeat the process with a crystal of Isevo-tartaric acid, and the 

 enantiomorphous tetrahedron — the mirror-image of the former 

 — is obtained. We shall see later that the idea, on the one 

 hand, of two asymmetric tetrahedra, and, on the other, that of 

 two opposite helices, given as alternatives by Pasteur to explain 

 the grouping of the atoms within the molecules of dextro- and 

 laevo-tartaric acids, are in reality identical. 



The precision of Pasteur's views as to the asymmetry of these 

 acids enabled him to discover two further methods of separating, 

 them. Thus he points out that although these acids will possess 

 equal affinity for any given symmetric base, such as potash, or 

 ammonia, or aniline, yet their affinities will not be equal if the 

 base, like quinine or strychnine, is itself asymmetric ; because 

 here the special one-sided asymmetry of the base will modify its- 

 mode of combination with the two enantiomorphous acids. The 

 solubility is different in the case of the dextro- and Isevo- 

 tartrates of the same asymmetric base ; the crystalline form, the 

 specific gravity, the number of molecules of water of crystallisa- 

 tion, may be all different. Potassium dextro- and Icevo-tartrates- 

 are mirror-images of one another ; quinine dextro- and Isevo- 

 tartrates are not. Pasteur employed in his experiments the 

 asymmetric base cinchonicine, which he converted into its acid 

 racemate, and allowed the solution to crystallise. The first 

 crystallisations consisted of pure leevo-tartrate of cinchonicine, 

 whilst the more soluble dextro-tartrate remained in the mother 

 liquor, from which it finally crystallised in forms totally distinct 

 from those of the lasvo-tartrate. 



Pasteur's third method is of physiological interest, and is, 

 moreover, the stepping-stone to his later work on ferments. As 

 we shall see presently, he regarded the formation of asymmetric 

 organic compounds as the special prerogative of the livii^ 

 organism. Most of the substances of which the animal and 

 vegetable tissues are built up — the proteids, cellulose— are 

 asymmetric organic compounds, displaying optical activity. 

 Pasteur had shown that two compounds of inverse asymmetry 

 behaved differently towards a third asymmetric compound. 

 How would they behave towards the asymmetric living 

 organism ? 



It had frequently been noticed that impure calcium tartrate, 

 when mixed with organic matters, as is the case when it is 

 obtained in the process of preparing tartaric acid from argol, 

 readily underwent fermentation. Pasteur examined the actiori 

 of the ferment (apparently a Pefncillium) on ammonium 

 tartrate — a substance which had the advantage over calcium 

 tartrate of being soluble — and finding that the fermentation here 



