34. HISTORY OF GALVANISM. 



tion of two fluids that bore a different chemical re- 

 lation to them. 



Pile win Sir H. Davy also made some very important 

 with pure observations on the nature of the fluid interposed 

 between the plates of the pile, and the nature of 

 the changes which it experiences. If the water 

 that is employed be perfectly pure, containing no 

 acid, salt, or gas, the apparatus is incapable of 

 acting. He found that its energy was in pro- 

 portion to the rapidity with which the oxidation 

 of the metal advances, and consequently was most 

 energetic when nitric acid was made use of. 

 Strong sulphuric acid, on the contrary, had little 

 effect, because zinc has not the power of decom- 

 posing it ; but if the acid be diluted, the metal was 

 oxidated, and the electricity was evolved. Upon 

 the same principle it appeared, that the sulphu- 

 rets, which cannot act upon the metallic surface, 

 had not the power of exciting the energy of the 

 pile. In pursuance of this opinion, he discovered, 

 that the pile can act in vacuo, if an acid be in- 

 terposed between the plates.* 



Remarks Sir H. Davy's very curious discovery, that the 



Davy's ex. two constituents of water might be obtained sepa- 



nts ' rate from two portions of the /fluid contained in 



different glasses, seemed at variance with all 



former opinions of chemical composition, and gave 



rise to a number of speculations, which although 



* Nicholson's Journ. 4to. iv. 275, 326, 337, 380, 394- ; 

 v. 78, 341 ; and 8vo. i. 144. 



