LECTURE X.] HISTORY OF CHEMISTRY. l8l 



(hydrogen) separates out from the one, and unites with an 

 element (oxygen) from the other to produce a stable compound 

 (water), whilst the residues join together. Mitscherlich's nitro- 

 benzene 17 is to be regarded, according to Gerhardt, as pro- 

 duced in this way out of a residue of benzene and a residue of 

 nitric acid. The hydrocarbon gives up hydrogen, and the 

 nitric acid gives up oxygen. Sulphobenzide 18 is also regarded 

 in the same way ; it contains the residues C 2 4H 10 from the 

 benzene, and SO 2 from the sulphuric acid. 19 The SO 2 in this 

 case is not identical with sulphurous acid, as the latter occurs, 

 for instance, in sulphite of lead, but it is contained in the 

 compound in quite a special form, namely as a substituted 

 group. 



Although this latter view is a very peculiar one, still it was 

 eminently suited to supplant the belief in the pre-existence of 

 radicals. The residues were imaginary substances, which were 

 all the more completely deprived of any reality by the fact that 

 they were assumed to be different from the similarly composed 

 atomic groups which occurred in the free state. 



About two years later (in 1841) Mitscherlich 20 propounds 

 similar ideas, which, however, he extends to a much larger class 

 of substances. According to him, also, compounds do not 

 contain any ready-formed radicals which play the part of 

 elements in decompositions ; in the appearance of water, he 

 sees the reason for the observed direction taken by decom- 

 positions ; and therefore he does not seek for this reason in 

 the constitution of the substances employed. The products 

 which are obtained by the action of acids upon bases or 

 alcohols (salts and esters), are likewise considered from this 

 point of view, and it is shown that they decompose again into 

 their constituents by taking up water. 



The idea of the residues was well adapted to explain the 

 phenomena of substitution ; the latter, according to Gerhardt, 

 obeyed the following rule. 21 The element eliminated is replaced 



17 Pgg- Ann. 31, 625. 18 Compare Mitscherlich, Pogg. Ann. 31, 

 628. 19 Dumas' atomic weights : C = 6, O= 16, 8 = 32 etc. 20 Pogg. 

 Ann. 53, 95. 21 Ann. Chim. [2] 72, 196. 



