4 i8 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



It will be seen at once that this conception is much 

 broader than Dumas's Law of Substitutions, which was 

 restricted to those special cases of copulation in which the 

 reagent was an element, the original compound a hydride, 

 and the product a chloride, etc., derived from it by substi- 

 tution, e.g. 



(C 7 H 5 C))H + C1 2 = HC1 + (C 7 H 5 0)C1 



Oil of bitter almonds. Benzoyl 



Benzoyl hydride. chloride. 



Gerhardt (loc. tit.} expressed the contrast as follows : 

 "If the reagent is an element in the free state, the 

 material removed is replaced, as Dumas has said, equivalent 

 by equivalent ; if, on the other hand, it is a compound, the 

 element removed is replaced by the residual elements of the 

 reagent" e.g. 



SO 2 from SO S minus O ; NH 2 from NH 3 minus H ; 

 NO 2 from HNO 3 minus OH ; C 7 H 5 O from C 7 H 5 OC1 minus Cl. 



Gerhardt's conception of copulated compounds led him 

 very far in the direction in which a solution of the complex 

 problem of structural chemistry was ultimately found. It 

 showed him that the products of copulation (or CONDENSA- 

 TION, as it is now called) must be regarded as formed 

 from the mutilated RESIDUES of two molecules residues 

 which might well be incapable of separate existence. 

 His residues (or radicals) thus differed essentially from the 

 radicals of Dumas and Boullay and of Wohler and Liebig, 

 which were always regarded as real substances, capable 

 of separate existence, as in the case of ammonia and 

 ammonium amalgam. Again, the older radicals were con- 

 sidered to have a real existence in their compounds ; but 

 Gerhardt was content to regard his residues or radicals as 

 expressing merely the possible methods of formation and 

 decomposition of a compound ; he was therefore prepared 

 to write barium sulphate indifferently as BaSO 4 or BaO + SO 3 , 

 or BaO 2 + SO 2 or BaS + O 4 , according as he wished to 

 interpret one of its chemical changes or another (Treatise on 

 Chemistry, 1856, IV. 561). 



