211 



<>r other, circumstances appear not favorable for spontan< 

 In such cases a second, and from a practical standpoint, the most 

 important method of separation, also found by Pasteur, - 

 lade use of. It is by this method that most substances which may 



:ur in two non-superposable mirror-images, have up till now, 



m resolved into their components. 



The principle on which this method is founded, is, that when 

 two stereometrical arrangements which are non-superposable mirror- 



lages A and A' of each other, are combined in a corresponding 



ly with another stereometrical complex /, also being different 



Dm its mirror-image /', the two figures Af and A'f thus produced 

 will no longer be mirror-images of each other. 



The truth of this can be easily demonstrated; for if Af be reflected 

 in a mirror, it is changed into its mirror-image A'f. This figure 

 A '/' however is certainly different from A'f, because / and /' are 

 non-superposable mirror-images of each other. Therefore Af and 

 A'f can never be mirror-images of each other, unless / and /' be con- 

 gruent, which however is not the case in this method of operation. 



If instead of /, we had used its mirror-image /', we should have 

 obtained the complexes Af and A'f', of course these will not be 

 each other's mirror-images either. But A'f and Af, and in the same 

 way A'f and Af, are truly two pairs of such mirror-images. As we 

 shall see, this last fact can be made use of for obtaining both antipodes 

 of a racemoid by the same method of fission. 



All right and left-handed isomerides have identical scalar pro- 

 perties, and also the same chemical constants. Thus they have the 

 same solubility in the same solvent, identical melting-, and boiling- 

 points, the same affinity-constants in their reactions with optically 

 inactive substances, the same densities, etc. Substances which are 

 not related as mirror-images, have however different solubilities 

 under similar circumstances. It will therefore be possible to separate 

 them by fractional crystallisation; thiu e. g. Af and A'f, or Af and 

 A'f would be symbols for compounds which could be separated 

 in this way. Because only the less soluble compound can be obtained 

 perfectly pure in this way, while the other one always has some of 

 the less soluble substance adhering to it, the pure salts Af and A'f 

 can, properly speaking, only be obtained by successive combination *) 

 of the racemoid A A' with / or /'. However in practice the difference 



i) W. Marckwald, Ber. d. d. Chem. Ges. 29. 43. (1896). 



