50 THE ADVANCE OF SCIENCE 



replacing them by others. The class of 

 neutral salts, for example, includes a 

 great number of bodies in many ways 

 similar, in which the basic molecules, or 

 the acid molecules, may be replaced by 

 other basic and other acid molecules with- 

 out altering the neutrality of the salt ; 

 just as a cube of bricks remains a cube, 

 so long as any brick that is taken out is 

 replaced by another of the same shape 

 and dimensions, whatever its weight or 

 other properties may be. Facts of this 

 kind gave rise to the conception of 

 'types' of molecular structure, just as 

 the recognition of the unity in diversity 

 of the structure of the species of plants 

 and animals gave rise to the notion of 

 biological ' types.' The notation of chem- 

 istry enabled these ideas to be repre- 

 sented with precision ; and they acquired 

 an immense importance, when the im- 

 provement of methods of analysis, which 

 took place about the beginning of our 



