STEREO-CHEMISTRY AND PHYSIOLOGY 479 



The two groups c are identical. Now, if the right-hand c be 

 replaced by d we shall have one compound, while if the left- 

 hand c be chosen for replacement we shall have the optical 

 antipode. Since the optical antipodes are identical in chemical 

 properties and differ physically only in respect to their crystal- 

 line forms, there is no reason why the dextro form should be 

 formed rather than the laevo form ; and there is no reason why 

 one c should be replaced rather than the other, since they are 

 symmetrically situated with regard to one another. It is, 

 therefore, only natural that in one half of the total number 

 of molecules the right-hand c is replaced by d, while in the 

 others the left-hand c is chosen, the result being a mixture of 

 the two antipodes in equimolecular proportions. 



We may take it as a general rule, then, that from sj^mmetrical 

 substances only symmetrical products are formed in the course 

 of our laboratory reactions. On the other hand, when we 

 examine the constituents of living tissues we find that a very 

 large proportion of them are optically active substances, so that 

 at the first glance there appears to be a profound difference 

 between chemical actions in the laboratory and those which are 

 carried out within living organisms. At present those chemists 

 who take an interest in the matter are divided into two schools. 

 The one school holds that there is no fundamental difference 

 between the chemical processes of vital action and the chemical 

 processes of the laboratory ; while the other school maintains 

 that the question of molecular asymmetry sharply divides the 

 two sets of phenomena. Up to a few years ago, neither view 

 could call any experimental evidence to its assistance. The case 

 of sodium ammonium racemate was cited by the " uniform " 

 school as a proof of their case. This substance, if allowed to 

 crystallise from its solution under certain conditions of tem- 

 perature, separates spontaneously into two sets of crystals 

 which, owing to the difference in the disposition of their 

 hemihedral faces, can be separated by hand from each other. 

 The upholders of the vitalistic theory of molecular asj-mmetry, 

 however, pointed out that in order to separate the two vital 

 action was necessary, since a human being was a vital agent. 

 It is unnecessary here to enter into the details of the argu- 

 ments employed on both sides ; they are to be found in the 

 correspondence in Nahtre,'^ and were chiefly theoretical. 



' Nature, vols. 58 and 59 (1898-9). 



