72 THE PROGRESS OF SCIENCE n 



which also has a close parallel in the history of 

 biological science. If the unit of a compound is 

 made up by the aggregation of elementary units, 

 the notion that these must have some sort of 

 definite arrangement inevitably suggests itself ; 

 and such phenomena as double decomposition 

 pointed, not only to the existence of a molecular 

 architecture, but to the possibility of modifying a 

 molecular fabric without destroying it, by taking 

 out some of the component units and 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, without altering the 

 neutrality of the salt ; just as a cube of bricks re- 

 mains 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 con- 

 ception of " types " of molecular structure, just as 

 the recognition of the unity in diversity of the 

 structure of the species of plants ami animals gave 

 rise to the notion of biological " types." The 

 notation of chemistry enabled these ideas to be 

 represented with precision ; and they acquired an 

 immense importance, when the improvement of 

 methods of analysis, which took place about the 

 beginning of our period, enabled the composition 

 of tin- s>-r;tllrd "organic " bodies to be determined 



