36 ELECTROLYSIS. 



step M. Berthelot in the calculation of the useful and secondary 

 effects. 



When a bath of sulphate of copper is traversed by an electric 

 current, one equivalent of copper is deposited on the negative 

 pole, whereas the other constituents, such as the oxygen which is 

 liberated and the dilute sulphuric acid which gradually accumu- 

 lates, are transported to the positive pole. Instead of liberating 

 the elementary constituents, as was the case with chloride of zinc, 

 the reaction liberates the metal on the one part, and the system 

 S0 4 on the other part : 



S0 4 is gradually resolved into oxygen which is liberated, and 

 into sulphuric acid, which remains dissolved in the excess of 

 water. 



Such is the normal type of all decompositions of a ternary, 

 or even of a more complicated salt. 



The chemical work performed in the course of this de- 

 composition is measured by the sum of the quantities of heat 

 developed firstly when the copper combines with the oxygen and 

 then when the oxide of copper and the dilute sulphuric acid 

 combine together, or : 



19 + 9-2 = 28 -2 calories. 



M. Berthelot remarks, with reason, that an electric current 

 produces besides, the heating of the liquors to a certain extent, 

 and causes a certain displacement of substances comprising not 

 only sulphuric acid, oxygen, and copper, but also a portion of 

 the mass of the sulphate of copper ; the relative proportion and 

 the laws of such accessory work are not well known. 



The electric action is such as we have described it, at the 

 beginning of the operation ; but as soon as the latter has begun, 

 the liquor contains a certain amount of dilute sulphuric acid, 

 and particularly in the vicinity of the positive pole. From, that 

 moment the acid becomes electrolysed at the same time that 

 the sulphate of copper, that is to say hydrogen is liberated at 

 the negative pole. One portion of that hydrogen is freed ; 



