48 CASSEL, ELECTROLYTICALLY DEPOSITEU METALS. 



cupric snlphate and received brass on the cathode. By elevating 

 the temperature more copper was deposited; by lowering the 

 temperature more zinc was received. 



Walenn^) states, that a deposite of nearly pure zinc may 

 be had from the above named brass Solution, if the temperature 

 is kept by about zero. 



Tt is not very difficult to explain the difference between 

 the two classes in this respect: a higher temperature favours the 

 chemical and secundary processes, and increases the difFusion of 

 the liqvid, hence it is necessary td have a higher temperature 

 when depositing metals by Substitution. 



But when a metal has to be deposited, which is more posi- 

 tive than hydrogen, a iower temperature must be advantagous, 

 in as much as the difFusion of the liquid is lessened and the 

 deposit prevented from uniting witli the liberated hydrogen and 

 the acid. 



in. Condition of the liquid. 



We have already referred to the detrimental influences of 

 hydrogen on electro-deposited metals. It is natural that the 

 amount of liberated gas increases, when free acid is added to 

 the eiectrolyte, and that metal more positive than hydrogen is 

 more corroded. 



The class of metals represented by copper, can be deposited 

 from an acidulated Solution, while the other metals are very 

 sensitive to the presence of free acid; in fact, a very small trace 

 is sufticient to spoil the deposits. Too great a qvantity is 

 however detrimental to all metals. 



In order to avoid free acid, when it is necessary, various 

 method can be used. In the art of electro-plating this is accom- 

 plished by using such electrolytes as zincate of potash, plumbate 

 of potash etc., in which cases the presence of all acids is avoided. 



') Walknn, Chem. News., vol. XXXV, s. 154; Watt, Dictionary of chemistry, 

 vol. VII, 8. 382. 



