A CENTURY OF CHEMISTRY. 129 



But difficulties soon gathered round this electro- 

 chemical theory. Even as early as 1834, Dumas 

 showed, in stating his " substitution " theory, that in 

 many organic compounds the positive element hydro- 

 gen may be replaced by the negative element chlorine 

 " without a fundamental alteration in the chemical 

 character of the resulting compound." This was 

 practically a deathblow to the theory of Berzelius. 



Faraday. About 1833, Faraday was led to con- 

 clude (a) that the chemical power of a current of 

 electricity is in direct proportion to the absolute quan- 

 tity of electricity which passes, and (&) that the 

 proportions of the bodies or ions evolved by an elec- 

 trolytic action (the electro-chemical equivalents of 

 the ions) are the same as their ordinary chemical 

 equivalents or combining proportions. And he re- 

 turned to the theory of Davy, saying that " the forces 

 termed chemical affinity and electricity are one and 

 the same." 



Sir Henry Roscoe points out that the great prin- 

 ciple of valency was foreshadowed from a physical 

 point of view in Faraday's law of electrolysis. 

 Faraday showed that the number of atoms electro- 

 lytically deposited is in the inverse ratio of their 

 valencies ; Helmholtz in his Faraday lecture ex- 

 plained this by the fact that " the quantity of elec- 

 tricity with which each atom is associated is directly 

 proportional to its valency." 



lonisaiion Theory. It does not seem possible, 

 at present, to be confident in affirming or denying the 

 idea that chemical combination is due to the union 

 of electrically charged atoms; but it is certain that 

 the question is not so simple as it appeared to Davy, 

 Berzelius, and Faraday. To make the matter in any 

 way clear it would be necessary to take account of 



