182 HISTORY OF CHEMISTRY. [LECTURE X. 



either by an equivalent of another element, or by the residue of 

 the reacting substance. The application of this rule was limited, 

 however, for Gerhardt, besides substitutions, is also acquainted 

 with additions, and these of two kinds. Firstly there are those 

 in which the saturating capacity is altered, and the formation 

 of salts is reckoned amongst these ; and then there are addi- 

 tions in which this is not the case. Gerhardt directs his 

 attention chiefly to the products of the latter kind of addition, 

 and calls them coupled compounds (corps copules). To this 

 class there belong, in particular, the substances produced by 

 the action of sulphuric acid upon organic compounds, such, 

 for example, as benzoyl sulphuric acid and its salts, discovered 

 by Mitscherlich. 22 The acid is produced by the action of sul- 

 phuric acid upon sulphobenzide. According to Gerhardt, the 

 two substances become coupled, whereby the saturating capacity 

 of sulphuric acid, which was still regarded at that time as 

 monobasic, remains unchanged. Thus : 



C 94 H 10 (SO a ) + SO,H a O- C M H 10 (S0 2 ) . SO 3 . H 2 O. 



Sulphobenzide. Sulphuric acid. Hyposulphobenzidic acid. 



Sulphovinic acid (ethyl sulphuric acid) is regarded as a 

 coupled compound of sulphate of ethyl and sulphuric acid, 

 and is written C 8 H 10 (SO. 2 )O. 2 . SO 8 H 2 O ; whereas sulphobenzoic 

 acid, the basicity of which is supposed to be equal to the sum 

 of the basicities of its constituents, and in the formation of 

 which the saturating capacity has remained unchanged, is 

 reckoned amongst the conjugated acids, a class of substances 

 which were first distinguished by Dumas. 23 It may, however, 

 also be regarded as produced by the coupling of a substituted 

 benzoic acid, CasH^SC^C^, with sulphuric acid. 



The view announced above with respect to coupled sub- 

 stances, is very soon abandoned by Gerhardt. He retains the 

 name, but gives it a different signification. But I have inten- 

 tionally stated the older notion, because the word was also 

 employed by Berzelius and by Kolbe, who again bestowed a 



<22 l >0 gg- Ann - 3I> 283 and 634. M Dumas and Piria, Annalen. 44, 

 66 ; Ann. Chim. [3] 5, 353. 



