LECTURE X.] HISTORY OF CHEMISTRY. 183 



special signification upon it. It appeared to me of interest 

 to follow the historical course of an expression which has been 

 employed in senses so numerous and so varied. 



Gerhardt, in 1843, regards all compounds as coupled which 

 are prepared by the action of acids upon alcohols, hydrocarbons, 

 etc., and in whose formation the substances unite with the 

 elimination of water. 24 Coupled compounds were, accordingly, 

 no longer addition products, obtained by the union of two com- 

 pounds, but they were formed by the joining together of two 

 residues. They were, therefore, substitution products a view 

 which Gerhardt, however, does not adopt. With him, they 

 still constituted a special class and were not compared with 

 the original substances, principally, no doubt, because they 

 possessed a different saturating capacity. With respect to the 

 latter also, Gerhardt has now become of a different opinion, 

 and states that the basicity of the coupled compound is equal 

 to the sum of the basicities of the substances coupled, less one. 

 From this statement, which is advanced as an axiom, the 

 dibasic character of sulphuric acid follows. Coupled with 

 neutral substances, such as alcohols, or hydrocarbons, sul- 

 phuric acid gives rise to monobasic acids, whereas acetic acid, 

 nitric acid, hydrochloric acid, etc., do not possess this pro- 

 perty, and are hence regarded by Gerhardt also as monobasic. 



In 1845, Gerhardt endeavours to show the general appli- 

 cability of the law of basicity mentioned above. 25 He now 

 designates as coupled, all compounds which are formed by the 

 union of two substances with the elimination of water and 

 decompose into their constituents again by taking up water ; 

 and, therefore, he reckons in this class, the neutral ethers, the 

 acid ethers, etc., and formulates the law 



3 = (b + V)--L t 



where B represents the basicity of the coupled compound, and 

 b and b' the basicities of the substances which take part in its 

 formation. Gerhardt expressly remarks here that this equation 



24 Comptes Rendus. 17, 312. - 5 Comptes rendus des travaux chimiques 

 par Laurent et Gerhardt, 1845, 161. 



