2l8 HISTORY OF CHEMISTRY. [LECTURE XI. 



it in 1839. Radicals were residues of compounds, i.e., they 

 were atomic groups which, in certain reactions, could be trans- 

 ferred, undecomposed, from one substance to another ; they did 

 not, however, on this account at all require to exist indepen- 

 dently, and they were only intended to express the relation in 

 which elements or atomic groups are replaced. 613 



The formulae which are thus obtained for compounds, do 

 not indicate the arrangement of the atoms ; they are merely 

 reaction formulae, which recall a series of analogies. It can 

 thus be understood how Gerhardt could imagine several radi- 

 cals and several rational formulae in the case of the same sub- 

 stance. The determination of the true constitution of substances 

 appeared to him a task not capable of accomplishment, since 

 modes of formation and of decomposition can alone lead to a 

 judgment respecting it, and the multiplicity of these does not 

 permit of any conclusion as to the arrangement of the atoms. 

 Thus, for example, barium sulphate is formed from sulphuric 

 acid and baryta, from sulphurous acid and barium peroxide, and 

 besides, from barium sulphide and oxygen. The constitution 

 of the salt might, therefore, be represented symbolically by 

 means of the three formulae : 



Ba 2 O + SO 3 , Ba 2 O. 2 + S(X, Ba. 2 S + O 4 67 



(0=i6,S- 3 2, Ba = 68. 5 ). 



By means of this single example Gerhardt considered himself 

 able to prove that all endeavours directed towards the repre- 

 sentation of the arrangement of the atoms by means of symbols, 

 must lead to nothing. 



With Gerhardt, reactions are double decompositions ; and 

 here the contrast is seen between his system and the dualistic 

 system, in which all compounds are conceived as formed by 

 additions. Gerhardt goes so far as even to assume a double 

 decomposition, or as he calls it, a typical reaction, when two 

 molecules unite to form a single one. Thus ethylene chloride, 

 according to him, is produced from olefiant gas in consequence 

 of the substituting action of chlorine. The chloride C 2 H 3 C1 is 



66 Gerhardt, Traite de Chimie organique. 4, 569. 67 Compare p. 177. 



