10 PRINCIPLES AND DEFINITIONS 



dissymmetric,^ and are defined as objects possessing non- 

 superposable mirror images. 



Dissymmetric objects can exist in two forms, right and 

 left. 



When the two forms of dissymmetric molecules are rep- 

 resented in equal concentrations (racemic mixture),' the 

 aggregate of molecules is symmetric. When they are rep- 

 resented in unequal concentrations, the aggregate is non- 

 symmetric. There may be a predominance of the right 

 forms (dextrality) or of the left forms of molecules (sinis- 

 trality). Pasteur did not propose any special term for such 

 a deviation of the molecular aggregate from the racemic 

 state. His views on this subject were somewhat uncer- 

 tain.' Following Emil Fischer (1894) and Japp* (1898), 

 we shall designate this condition by the term asymmetry. 



1 In the German (1891) and in the English (1897) translations of Pasteur's 

 work (lectures of 1860), the word dissymmetry was arbitrarily replaced by 

 the word asymmetry. 



2 Eecently Findlay (1937) pointed out that the term acide racemique, as 

 applied to tartaric acid, was due to Gay-Lussac (1828), but its use in the 

 sense accepted at present originated with Pasteur (1861). Pasteur wrote in 

 1860 : ' ' We still need a word in chemical terminology to express the fact 

 of a double molecular dissymmetry concealed by the neutralisation of two 

 opposite dissymmetries, the physical and geometrical effects of which com- 

 pensate each other exactly. ' ' 



3 Pasteur did not distinguish sharply the dissymmetry of individual mole- 

 cules from the asymmetry of their aggregates in the sense given above. For 

 him the molecules acquire dissymmetry by receiving a ' ' twist ' ' in living 

 organisms or in contact with products of living organisms and they lose their 

 dissymmetry by being untwisted. In 1860 he wrote that "the twisted organic 

 group can be untwisted and so assume the ordinary character of artificial and 

 mineral substances. ' ' The ' ' twisting ' ' was considered as characteristically 

 ' ' vital ' ' and destructible by energetic chemical reactions. According to mod- 

 ern views, these reactions, instead of ' ' untwisting ' ' the molecules, produce a 

 racemisation or an equalization in the concentrations of the right and left 

 forms of a substance. While Pasteur is the discoverer of the fact that 

 ' ' Racemic tartaric acid of chemists, inactive as to the optical rotatory power, 

 consists of two acids, the rotations of which mutually neutralize each other, 

 as one of them rotates to the right and the other to the left, and both in the 

 same degree," (Pasteur, 1848, p. 458) he thought that the molecules of the 

 racemates were symmetric by their very nature, and that they became dis- 

 symmetric in their separation from the racemate by crystallization of the anti- 

 podes under the action of some dissymmetry forces which might be furnished 

 by ' ' organic dissymmetric particles on the surface of the crystallization dish ' ' 



