ELECTROLYTIC LAWS. 27 



immersed in dilute sulphuric acid, does not decompose water, 

 according to the theory, if the two electrodes are of platinum ; 

 2. In order to effect the decomposition of water, the positive 

 electrode must be constituted of such a substance that when 

 submitted to the influence of an electric current it should be 

 capable of combining itself with the oxygen of the water. 



The explanation of the results obtained by M. Tommasi is 

 exceedingly simple. M. Tommasi assumed that he was elec- 

 trolysing water by means of a single cell, whereas he was really 

 using two cells joined in series, and placed one externally to and 

 the other in the interior itself of the voltameter. 



The zinc-copper cell was giving off 19 calories, when at the 

 same time the copper at the positive electrode was getting 

 oxydised, being transformed into sulphate, and giving off 28 2 

 calories. Instead of requiring 34 5 calories the dissolution would 

 then only require 31*5 28 2 = 6 3 calories, a number greatly 

 inferior to the 19 calories supplied by the zinc-copper cell. 



