220 ELECTROLYSIS. 



metres, which corresponds to 8 centigrammes per square centi* 

 metre. The thickness was 09 millimetre, taking the density 

 of the precipitated copper at 8 89. 



Mr. Sprague used, as a generator of electricity, a Daniell 

 cell, and he varied the resistances so as to obtain a thickness of 

 0'09 millimetre in thirty hours at a maximum, or in forty-five 

 minutes at a minimum. 



This is the result of the experiment : 



hrs. min. mm. 



No. 1. In 30 thickness of .003 per hour. Excellent deposit. 



?. 15 '006 Copper of great tenacity. 



3. 50 



4. 2 30 



5. 1 15 



6. 45 



'018 Very good deposit. 



036 Good deposit. 



072 Sandy at the edges. 



122 Bad deposit. 



The first four deposits were practically equally regular and 

 homogeneous, but if the deposition is accelerated so as to exceed 

 a thickness of ' 036 millimetre per hour it becomes defective. 

 Mr. Sprague consequently recommends not to exceed that limit, 

 which corresponds to about 1 ampere per 33 square centimetres, 

 or 300 amperes per square metre of surface of anode. 



This electrolytic regimen cannot in practice be attained, for 

 the manufacturers do not proceed with the same care as Mr. 

 Sprague ; their baths often are only slightly homogeneous, the 

 anodes and cathodes irregularly disposed, and the deposit 

 becomes coarse and without adhesion much before that limit. 

 We only recommend it in one case, that is, in the production of 

 electrotypes for the reproduction of wood engravings, when a 

 prompt delivery is required ; and we would still insist that 

 the first coating should be slowly deposited, and that the rapid 

 deposition should only apply to the thickening of the coat- 

 ing. To obtain a good result 1 ampere per square decimetre, 

 or 100 amperes per square metre of surface of cathode should 

 not be exceeded when the anodes are of chemically pure 

 copper. 



In the experiments which we have described, M. Gramme, 

 placing himself at an entirely different point of view to that of 

 Mr. Sprague, and wishing to absolutely avoid the irregularities 

 in the quality of the depositions, so as not to arrive at any 



