ARCHITECTURE OF MOLECULES 215 



optical activity and asymmetry was discovered by the French 

 chemist J. A. Le Bel. 



The theory is based on the following recognised facts : 



1. The four units of valency of carbon are equal in every 

 respect. In the mono-substitution derivatives of marsh-gas CH 4 

 and ethane C 2 H 6 no isomeric modifications have been discovered. 



2. All compounds of carbon which in the liquid state rotate a 

 polarised ray, or when crystallised produce hemihedral forms 

 which are mirror-images of each other, are found to contain at 

 least one atom of carbon which is united directly to four dis- 

 similar atoms or groups of atoms, and which is therefore said to 

 be asymmetric. 



3. Compounds which are known to contain asymmetric car- 

 bon, and which, nevertheless, do not exhibit optical activity, 

 are generally resolvable by one or other of several known pro- 

 cesses into two compounds^ each of which possesses rotatory 

 power equal and opposite in direction to the rotatory power of 

 the other. 



Succinic acid, for example, 



H 

 HC CO-OH 



I 



HC CO-OH 

 H 



is optically inactive ; but when one of the hydrogen atoms is 

 replaced by hydroxyl, so that the C to which it is attached be- 

 comes " asymmetric," the result is the production of malic acid, 



H 

 HO-C CO-OH 



HC CO-OH 



which exists in two isomeric forms, one of which rotates the 

 polarised ray to the right, the other to the left. 



An apparent exception is represented by mesotartaric acid, 

 which has the same composition as (1) ordinary, dextro-, tartaric 

 acid, (2) racemic acid which is found in the grapes of certain dis- 

 tricts, and (3) Isevo-tartaric acid which is obtainable along with the 

 dextro-acid from racemic acid. Mesotartaric acid is not resolv- 

 able into two acids, like racemic acid, and therefore cannot be 



