285 



duce optically inactive substances, if we start from the ordinary, 

 optically inactive reagents. The explanation of this is, that the 

 mechanical stability of both enantiomorphous atomic configurations 

 with respect to forces which have themselves a symmetry of the 

 second order, is evidently the same, and, therefore, the chance 

 that both enantiomorphous molecules be produced by such forces, 

 is the same also. Thus in our ordinary chemical reactions between 

 optically inactive molecules, equal quantities of both molecules 

 will be produced in a certain lapse of time; or at least, the acci- 

 dental deviations from equal quantities will be so slight that the 

 final product will appear optically inactive, even if examined by 

 our most exact experimental methods. 



On the other hand, because, starting from optically inactive 

 materials we can in our chemical laboratory-processes only pro- 

 duce optically inactive substances l ) which, if resolvable at all, - 

 can be separated into their antipodes by only a few and, moreover, 

 delicate and tiresome manipulations, --we can from this assume, 

 that in all probability the agencies which play a role in such 

 chemical processes, have really such symmetry-properties of the 

 second order, and that in consequence they determine chemical 

 and physical conditions during the reaction, which do not differ 

 from the mirror-images of these special circumstances. 



Of course, conditions are different when optically active mole- 

 cules take part in the reaction. Here we can distinguish three cases: 



a. An optically active substance acts upon an optically inactive, 

 non-resolvable compound, in such a way, that there is merely a 

 substitution of one of the atoms or radicals placed round the 

 asymmetric carbon-atom of the active molecule by another atom 

 or radical. 



b. An optically active compound acts upon an optically inactive 

 substance in such a way, that a new asymmetric carbon-atom is 

 produced in the active molecule besides the one already present. 



c. An optically active compound acts upon another optically 

 active substance, or upon an inactive, racemic or externally com- 

 pensated substance. 



We shall consider these three cases subsequently in the following 

 paragraps. 



2. If an optically active molecule acts upon an optically 



J. A. Le Bel, Bull, de la Soc. Chim. Paris, 22, 346, (1874). 



