Mr. M. M. Patttson Muir on Chemical Classification. 91 



actions by assigning to each substance such a hypothetical 

 structure as should include in itself the widest possible gene-» 

 ralization. 



And, proceeding thus, the upholder of the unitary system 

 soon began to see that compounds might be distributed into 

 certain classes, at the head of each of which might be placed 

 one compound as typical of all the members of the class. 

 Many substances might be derived from water by direct or by 

 indirect substitution ; what more natural than to regard thes^ 

 substances as constituted on the water type ? So arose the 

 types of hydrogen, water, ammonia, and marsh-gas. 



But if the genetic relations between compounds were ex- 

 pressed by the use of types, it was still true that vast differ- 

 ences in actual properties existed between many of those sub- 

 stances which were classed under the same type. Hence the 

 chemist who employed types was obliged to admit that the 

 nature of the substituting element or radicle powerfully influ- 

 enced the nature of the compound produced, although the 

 change might not be carried so far as actually to destroy the 

 equilibrium of the system. 



Thus we see how the new chemistry borrowed some of its 

 ideas from the old — how, although at first it appeared as the 

 destroyer of the system of Berzelius, it was eventually seen to 

 be founded upon the truths which had underlain the dualistic 

 theory, and was really the development of so much as was 

 lasting in the generalizations which had preceded it. 



10. But the theory of types was incomplete : it was ab- 

 sorbed in that of valency or atomicity. 



If one half of the hydrogen in water could be replaced by a 

 certain radicle, while another radicle was capable of replacing 

 the whole of the hydrogen in that compound, it was natural 

 to regard the latter radicle as in a certain sense equivalent to, 

 or capable of doing as much work as, two of the former. So, 

 if one third, two thirds, or the whole of the hydrogen in am- 

 monia was replaceable by one, two, or three atoms of certain 

 radicles, while other radicles could replace the hydrogen only 

 as a whole, it was no far-fetched idea to regard the latter 

 radicles as equivalent to three of the former. 



Having got hold of the idea of varying equivalencies, che- 

 mists recalled the old experiments of Wenzel and Richter upon 

 the saturation of bases by acids, in which it had been shown 

 that different weights of such bases as lime, potash, or soda 

 were required in order to saturate equal quantities of the sam^ 

 acid. These experiments led also to the idea of varying equi- 

 valents. And if it were justifiable to view compound radicles 

 as possessed of different capacities of saturation, surely the saniQ 



