90 Mr. M. M. Pattison Muir on Chemical Classification. 



Let us for a moment trace the deyelopment of these ideas. 



Reactions were observed which could be best explained by 

 supposing that a group of elements had moved from one com- 

 pound into another without the members of the group becom- 

 ing separated from one another, and without disturbing the 

 equilibrium of the molecule of the compound into which the 

 group entered. These groups might, or might not, be isolated ; 

 the theory did not demand any proof of their actual existence 

 in the free state ; they were regarded merely as hypothetical 

 existences which served to bring out analogies between groups 

 of reactions which could not find an explanation in terms of 

 any other theory. 



The application of this theory of substituting radicles was 

 soon attended with the discovery of relationships between 

 bodies which appeared to have but slight analogies with one 

 another when viewed in the light of the older theory of Dualism. 

 The researches of Liebig and Wohler upon the benzoyl com- 

 pounds found their interpretation in the tliepry of substituting 

 radicles. Berzelius had also spoken of radicles: he had indeed 

 represented all compounds as built up of radicles, simple or 

 compound ; but the radicles of dualism had almost nothing in 

 common with those of the new school. Although the substi- 

 tutionist had once denied the existence of compound radicles, 

 he was now obliged to use these radicles as corner stones in 

 building up the edifice of the uew chemistry. But, in place 

 of regarding the compound as consisting of two separate ex- 

 istences held together by a mysterious force, aided, it might 

 be, by a third separate existence interposed between the other 

 two, the substitutionist looked upon the molecule of a com- 

 pound as essentially a whole ; only he was prepared to ac- 

 knowledge a structure of some kind, he would not deny the 

 existence of parts in this whole, and he was even ready to 

 admit that the various parts were probably grouped around a 

 central mass to which he gaxe the name of compound radicle. 

 The followers of Dumas, Laurent, and Gerhardt did not appeal, 

 as the upholder of dualism was ready to do, to the electrolysis 

 of salts, and claim that the separation of the salt into two or 

 more parts (themselves compounded of simple elements) was 

 proof of the existence of these parts in the original salt. 

 He was aware that this method of proof could be turned 

 against the man who employed it, and that a dozen different 

 constitutional formulae could be assigned to the same substance 

 according to the reaction which was for the moment contem- 

 plated. He was content to group together the general ana- 

 logies of classes of compounds, and, without insisting upon the 

 necessary truth of his views, to endeavour to explain these re- 



