1Q4 HISTORY OF CHEMISTRY. [LECTURE X. 



quantity of sulphur, one and a half times as much iron as the 

 neutral sulphate of the peroxide of iron (ferric sulphate). 

 Thus we may express it that 28 parts of iron in the ferrous 

 salt can take the place of one part of hydrogen, while the 

 latter can also have its place taken by i8| parts of iron, in 

 the ferric salt; both quantities are, therefore, equivalent to 

 one part of hydrogen. If we distinguish, as Laurent and 

 Gerhardt did, by the terms ferrosum (Fe = 28) and ferricum 

 (fe = .28) the equivalents of iron in the ferrous and in the 

 ferric salts respectively, then the formulae of ferrous sulphate 

 (Fe 2 ) SO 4 and of ferric sulphate (fe 2 ) SO 4 become comparable 

 with each other. A similar thing holds for other metals such 

 as copper, mercury, tin, etc. ; in the salts corresponding to the 

 lower and to the higher oxides of these metals, different 

 equivalents, of which the one is double the other, 59 must be 

 assumed in each case. 



Complete analogy is attained in the mode of writing salts 

 when equivalent formulae are employed for acids also. We 

 then have : 



Ferrous sulphate Cupric chloride Mercurous chloride 



S0 4 (Fe 2 ) . Cl e (Cu 2 ) Cl 2 (hg 2 ) 



In this mode of writing formulae, the differences between 

 monobasic and polybasic acids disappear ; and it certainly is 

 an advantage of the molecular formulae that they permit these 

 highly important peculiarities to become prominent. Laurent 

 and Gerhardt recognised this very fully, and it was the latter, 

 especially, who endeavoured, and with much success, to bring 

 about the separation of these classes of substances by more 

 definitely advancing new characteristics. 60 



The formation of double salts with non-isomorphous bases 

 did not appear to Gerhardt sufficient for fixing the basicity of 

 an acid ; he points out that dibasic (and polybasic) acids can 



59 Laurent's views with respect to the mode in which the existence of 

 several equivalents for the same element may be explained are given in 

 his Methode de Chimie, 127 ; E. 103. 60 Laurent and Gerhardt, 



Comptes rendus mensuels des trauvaux chimiques 1851, 129 ; J. pr. Chem. 

 S3, 460. 



