THE SPACE RELATIONS TO ATOMS. 503 



from the same source is active, we can only conclude that 

 the former acid consists, not of inactive molecules, but of 

 right- and left-handed molecules in equal quantity ; x and if 

 the temperature of racemisation 2 lies below the ordinary 

 temperature, the difficulty of separating the active isomers 

 is accounted for. This explanation may apply also to those 

 asymmetric molecules previously spoken of as possibly 

 chaotic. 



Apart from these "chaotic " molecules, we cannot admit 

 a single valid exception to the law of the sufficiency of the 

 asymmetric carbon-atom to cause optical activity. 



So far we have mainly followed Le Bel, who bases his 

 conclusions on purely geometrical considerations, resting 

 none of them on any doctrine as to valence or its cause. 

 And the scope of the results arrived at, embracing as they 

 do the theories of optical activity, of geometrical isomerism, 

 and, to a certain extent, of ring-structure, shows that stereo- 

 chemistry may advance in complete independence of any 

 particular theory of valence. 



In fact the question as to what are the places of. the 

 atoms may be treated quite apart from the question as to 

 what holds them in their places ; and to ask chemists to 

 answer the second question before they have informed them- 

 selves as to the first, would be equivalent to asking astro- 

 nomers what caused the arrangement of the planets before 

 they knew what that arrangement was. The discovery of 

 the chemical law of gravitation may be the aim, but it is 

 not the starting-point of stereochemistry. At the same 



1 The proof of this has been given by Walden in a paper (Ber., xxviii., 

 1287) published while the above was passing through the press. By working 

 at moderate temperatures, Walden has succeeded in preparing not only 

 active bromo-succinic acid, and derivatives, but many other active halogen- 

 acids, including a-phenyl-chloracetic acid, and a-chloro- and a-bromo- 

 propionic acids. He concludes that an asymmetric carbon-atom always 

 causes optical activity whatever the nature of the attached radicals. But 

 so long as no active substance is known containing less than three carbon- 

 atoms, this statement is subject to the limitation imposed in the text, con- 

 cerning the molecules provisionally called chaotic. 



2 This term is used to indicate the formation of the inactive aggregate 

 from two isomers of opposite activity. 



