82 



Mr. W. Gannon. 



Table XII includes three experiments at a density of 0'013 ampere 

 per square centimetre. The deposits show no difference in weight. 



It would thus appear that, at current densities above O'Ol amperr per 

 square centimetre, there is practically no difference in weight of the 

 deposits obtained from solutions of copper sulphate (made acid), one 

 of which is under ordinary atmospheric pressure, and the other under 

 reduced pressure ; but at densities below this there is a very appre- 

 ciable difference, the deposit in a partial vacuum being always 

 greater than the corresponding deposit in air. No experiment with 

 acid solutions ever gave the vacuum deposit less than the air deposit. 

 Why this difference should exist at certain densities and not at 

 others is not very clear. The chemical corrosion above referred to 

 will be more marked in the air solutions at weak current densities, 

 where the duration of the experiments is three to four hours, but it 

 would not be expected that it would be altogether missing at densi- 

 ties where the duration of the experiments is less than forty minutes 

 and less. 



My experiments do not completely reveal the cause of the relations 

 existing between the deposit and current density. They clearly 

 show, however, that the oxygen present in the solution has a certain 

 diminutive action on the weight of the deposit, and that this action 

 is prevented (partly or wholly) by conducting the electrolysis in 

 vacuo. The removal of the oxygen of the solution prevents the 

 copper plates, and more particularly the copper deposit, from dis- 

 solving in the solution ; but it does not prevent (apparently) what 

 some physicists speak of as an electrical corrosion, which occurs only 

 when the current is passing through the solution. 



It is of interest to draw a curve representing the deposits obtained 

 in vacuo with different current densities. This I have done by 



\ri of plate in square centimetres per ampere 



