184 HISTORY OF CHEMISTRY. [LECTURE X. 



holds only for the coupling of a single equivalent, 26 and that the 

 equation must be applied twice in order to ascertain the correct 

 basicity of the product obtained by the coupling of two equiva- 

 lents of one substance with one of another. Thus sulphuric 

 acid, for example, which Gerhardt now regards as dibasic, can 

 form with neutral substances both acids and neutral products. 

 The ether of sulphuric acid belongs to the latter class of sub- 

 stances ; it is produced from two equivalents of alcohol and 

 one equivalent of acid. Its basicity, B, is obtained from the 

 following equations, in which B^ represents the basicity of ethyl- 

 sulphuric acid : 



- i =o. 



Strecker, in 1848, thought he had brought the rules into a 

 more general form when he made a statement to the following 

 effect : 27 The basicity of the coupled compound is equal to 

 the sum of the basicities of the compounds, less one-half of 

 the number of hydrogen equivalents removed, 28 or the basicity 

 is diminished by one unit for each pair of hydrogen atoms 

 removed. But in this way of stating the matter, the same 

 result as was required- by Gerhardt's rule was, necessarily, 

 always obtained. It can only be regarded as a simplification 

 (not as a wider generalisation) in which a single application was 

 sufficient in all cases. 



Although it was afterwards shown that even this form of 

 the law of basicity does not always lead to accurate conclu- 

 sions, 29 and although the exceptional position which was given 

 to certain classes of substances in consequence of the idea of 

 coupled compounds, was more recently recognised as incor- 

 rect, 30 still, it cannot be denied that the assumption of the 

 copula played a definite part in the historical development of 



26 Gerhardt at that time employed the word equivalent in Gmelin's 

 sense, so that for us it often means atom and often molecule. 2 " Annalen. 

 68, 51. 28 Strecker assumes the equivalent of water = 9, compared with 

 that of hydrogen chosen =i. 29 Compare Becketoff, Bulletin phys.-math. 

 de 1 s Academic de St Petersbourg, 12 (1854), 369. 30 Compare Kekule, 

 Annalen. 104, 130. 



