LECTURE X.] HISTORY OF CHEMISTRY. 193 



basis of a system. With Laurent, the molecule is the smallest 

 quantity of a substance which is required in order to give 

 rise to a compound, and which, in the form of vapour, 

 always (or at least with few exceptions) occupies double the 

 volume of an atom of hydrogen. The atom is the smallest 

 quantity of an element which occurs in compounds, whilst the 

 equivalents represent quantities of analogous substances which 

 have the same value in reactions. 56 



I shall try to give an idea of the importance to chemistry of 

 these definitions, by stating some of the conclusions that were 

 drawn from them in view of the experimental work of the period. 



The consistent application of the idea of equivalents 

 necessitated the assumption by Laurent and Gerhardt of 

 several equivalents for many of the metals. 57 "The idea of 

 the equivalent includes the notion of an identity of function ; 

 we are aware that one and the same element can play the part 

 of two or of several others, whence it must occur that different 

 weights also correspond to these different functions. On the 

 other hand we find different weights of the same metal, such 

 as iron, copper, mercury, etc., replacing the hydrogen of acids, 

 and, in doing so, forming salts which contain the same metal but 

 possess different properties. These metals have, consequently, 

 different equivalents." 



This idea was not new, 58 but as really equivalent formulae 

 had never been employed, it had not up to this time had any 

 further consequences. Now, when Laurent and Gerhardt in- 

 troduce this mode of writing formulae, it acquires a certain 

 value. Thus, for example, these reformers of chemistry seek 

 to find analogies where they have hitherto remained concealed; 

 the formulae of the sesquioxides can be assumed to be similar 

 to those of the normal bases, and a uniformity can thus be 

 introduced into the way of regarding salts which has not been 

 possible hitherto. It is known that the neutral sulphate of 

 the protoxide of iron (ferrous sulphate) contains, for the same 



56 Ann. Chim. [3] 18, 296 ; also Comptes rendus des travaux chimiques 

 par Laurent et Gerhardt 1849, 257. 57 Ibid. 1849, l etc. 68 Compare 

 p. 103. 



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