Januaet 19, 1917] 



SCIENCE 



51 



discovered what had escaped the eyes of 

 even so keen an investigator as de La Pro- 

 vostaye, namely, that the crystals of the 

 tartrates possessed truncating planes anal- 

 ogous to those observed in the quartz crys- 

 tals. He found, however, that in the case 

 of the crystals of the salts of the optically 

 active tartaric acid (dextro-tartaric acid) 

 these planes all occupied the same relative 

 position so that the crystals were all iden- 

 tical. On the other hand, the crystals of the 

 salts of the optically inactive tartaric acid 

 (racemic acid), like the quartz crystals, 

 could be divided into two groups according 

 to the position of the planes, the members 

 of the one group being related to those of 

 the other as object is to image. It was but 

 a step to show that, of the two groups of the 

 crystals formed by the salts of the inactive 

 acid, the one group rotated the plane of 

 polarization to the right while the other 

 rotated it an equal amount to the left ; that 

 the acid prepared from the group of crys- 

 tals which rotated the plane of polarization 

 to the right was in every way identical with 

 the well-known dextro-tartaric acid and 

 that the acid prepared from the remaining 

 group of crystals was identical with the 

 dextro acid in every respect except that it 

 was levorotatory. 



Here then, lay the secret of the optical 

 inactivity of racemic acid, as well as the 

 secret of the relation of the racemic acid to 

 the ordinary active tartaric acid. 



Pasteur realized the importance of his 

 results. Vallery-Radot in his fascinating_ 

 life of Pasteur gives a vivid account of the 

 discovery. After referring to the final ex- 

 periment in which Pasteur had tested the 

 activity of the two sets of crystals in the 

 polarizing apparatus, the biographer con- 

 tinues : 



His excitement was such that he could not look 

 at the apparatus again; he rushed out of the lab- 

 oratory, not unlike Archimedes. . . . Never was 

 there greater or more exuberant joy on a young 



man's lips. He foresaw all the consequences of 

 his discovery. The hitherto incomprehensible con- 

 stitution of paratartaric or racemic acid was ex- 

 plained; he differentiated it into right-hand tar- 

 taric acid, similar in every way to the natural tar- 

 taric acid of grapes, and left-hand tartaric acid. 

 These two distinct acids possess equal and oppo- 

 site rotatory powers, which neutralize each other 

 when these two substances, reduced to an aqueous 

 solution, combine spontaneously in equal quantities. 



The discovery was received with the 

 greatest interest by the French scientists. 

 The story is well known of how Biot, then 

 an old man, wishing to learn more about 

 the work before presenting it to the French 

 Academy, invited Pasteur to repeat the ex- 

 periments in his laboratory, using solutions 

 which Biot himself had prepared, and how 

 upon the completion of the experiments he 

 grasped Pasteur's hand, saying: 



My dear child, I have all my life so loved this 

 science that I can hear my heart beat with joy. 



Pasteur had connected the different 

 effects of the dextro- and levo-tartaric acids 

 with the asymmetrical character of their 

 molecules. I quote from him : 



We know, in fact, on the one hand, that the 

 molecular arrangement of both tartaric acids is 

 asymmetric; on the other, that they are entirely 

 the same, with the exception that the asymmetry 

 is shown in opposite senses. Are the atoms of the 

 dextro acid arranged in the form of a right-handed 

 spiral, or are they situated at the corners of an ir- 

 regular tetrahedron, or do they have some other 

 asymmetric grouping? This we do not know. But 

 without doubt the atoms possess an asymmetric 

 arrangement like that of an object and its re- 

 flected image. 



Following the work of Pasteur, "Wisli- 

 cenus in 1873 made an extended study of 

 the different isomeric lactic acids and con- 

 cluded that the existence of these could be 

 explained only upon the assumption that 

 their differences are dne, not to different 

 groups of atoms, but to a different arrange- 

 ment of the same groups in space. 



It remained for van 't Hoff and Le Bel in 

 1874 to propose a theory which gives a satis- 

 factory explanation of the facts. These 



