81(3 KEPORT — 1898. 



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 (tetartohedral) faces, situated in some specimens to the right 

 and in others to the left ; the former specimens bein» dextro-, the latter, Itevo- 

 rotatory. The possibility of tliis 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 

 ammoniiim 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 arrange- 

 ment, 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 

 substances depended on the nature, number, and arrangement of their constituent 

 ntoms. He now returned to the subject, imagining that the explanation would be 

 found in the fact tliat ^Mitscherlich had overlooked the hemihedral faces in the 

 tartrate, 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, how- 

 ever, 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 activitv, he carefully piclced out and separated the dextro- and laevo-hemi- 

 hedrai crystals, made a solution of each kind separatelv, 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, liievo-rotatory. The right-handed 

 crystals were identical with those of the ordinary (dextro-) tartrate; the others, 

 which -were their mirror image, or enantiomorph, were derived from the hitherto 

 unknown Isevo-tartaric acid. From the dextro- and l^vo-salts thus separated he 

 prepared the free dextro- and Isevo-tartaric acids. And having thus obtained from 

 racemic acid its two component acids — dextro- and la-vo-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 di-^solved 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 plav. They crvstallise 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, Inevo-rotatory. The salts which they form with 

 inorganic bases also agree in every respect, except as regards their opposite 

 asvmmetry and opposite rotatory power. They are enantiomorphous. 



Pasteur, discussing the question of the molecular constitution of these acids, 

 nnticipates 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 tar- 

 taric 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 rio-ht acid grouped on the spirals of a right-handed helix, or placed at the 

 solid angles of an irregular tetrahedron, or disposed according to some particular 

 asymmetric grouping or other ? We cannot answer these questions. But it can- 

 not be a .subject of doubt that there exists an arrangement of the atoms in an 

 asymmetric order having a non-siiperposable image. It is not less certain that 



