LECTURE IX.] HISTORY OF CHEMISTRY. 163 



influence upon Dumas. An extremely interesting discovery, 

 which he makes in 1839, obliges him to expound his views at 

 this time with respect to substitution ; and also to retract, 

 partially at least, the statements previously advanced and to 

 substitute in their place new ones of far greater significance. 

 In this way, from the empirical rules of substitution, the theory 

 of types arises. 



By the action of chlorine, in sunlight, upon acetic acid, Dumas 

 had obtained a crystalline substance whose composition could 

 be expressed by the formula C 4 CleH 2 O4, 25 and which could, 

 therefore, be regarded as acetic acid, C 4 H 8 O 4 , in which six 

 atoms or volumes of hydrogen were replaced by six atoms of 

 chlorine. 26 The interesting and important part of this reaction 

 lay in the properties of the new compound, which Dumas called 

 chloracetic acid. This acid had the same saturating capacity 

 as acetic acid, so that Dumas was able to assert that by the 

 entrance of chlorine in place of the hydrogen, the chief character 

 of the compound was not altered ; or, as he expresses himself, 

 " that in organic chemistry there are certain types, which persist 

 even when an equal volume of chlorine, bromine, or iodine is 

 introduced into them in place of the hydrogen which they 

 contain." 



It will thus be seen how Dumas, in consequence of his dis- 

 covery of chloracetic acid, is led to the same point of view 

 which had already been taken up by Laurent, but which the former 

 had at first put aside as extending beyond the limits of fact. 27 

 It is, however, an injustice to Dumas to represent his theory of 

 types as merely an application or perhaps an expansion of 

 Laurent's ideas. Laurent was a clever speculative thinker ; but 

 he did not hesitate to state a hypothesis for which a complete 

 scientific proof could not, at the time, be adduced, and this, I 

 think, was the case with respect to his views concerning sub- 

 stitution. That this, at least, was the impression made upon 

 his contemporaries is to be seen from Liebig's criticism of 



25 In the French papers Dumas retains the atomic weight C = 6. 

 26 Annalen. 32, 101. 27 Comptes Rendus. 6, 689. At that period Dumas 

 called Laurent's theory an extension of his ideas which did not concern him. 



