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THE ROYAL SOCIETY OF CANADA 



by adding weighed amounts of zinc oxide from time to time. The 

 results are given in the following table. 



4 • 4 per cent Zn ; 2 • 4-2 • 6 per cent acid. 



It appears that a current density of 40 amperes is quite satis- 

 factory when the solution contains a considerable amount of acid. 

 Changes in the design of the tank rooms, permitting a higher current 

 density to be used on the cells containing large amounts of acids, 

 might prove economical. 



From these experiments, as well as others made on solutions 

 containing colloids, the following conditions for good deposits may be 

 formulated: 



1. The solution should be absolutely clear and free from colloids. 



2. Iron should be low. Arsenic, antimony, copper, cobalt, 

 nickel, and in general metals more electro-positive than zinc must 

 be absent. 



3. The solution, particularly in the tanks containing large amounts 

 of acid, should be cold. The zinc deposits in a semi-passive form; 

 but when it begins to dissolve, solution cannot be stopped in any 

 simple way. 



4. The zinc concentration should be as high as possible (6-7 

 per cent), and no attempt should be made to electrolyse solutions 

 containing less than 2 -0-1 -5 per cent of zinc. 



5. With the ordinary cascade system the optimum current 

 density is 25-30 amperes per square foot. 



