of Silver and of Copper. 391 



deposit in nitric acid and the satisfactory washing of a bowl 

 generally is troublesome ; indeed the time spent in this ope- 

 ration is often more valuable than if the gain plate, deposit, 

 and all were thrown away ; while, since the deposit is pure 

 silver, the depreciation in value due to cutting up the sheet 

 is small. When the deposit is good it may be rolled, beaten, 

 or burnished down sufficiently to allow the plate to be again 

 used, and thus the operation simply means a transfer of silver 

 from the anode to the cathode. Another and perhaps more 

 important reason for preferring the vertical plates is the fact 

 that, if the plates be properly proportioned as to size and 

 properly prepared, the loss plate can be used with perfect ease 

 as a check on the result of the gain plate. This is a point of 

 some importance in the case of silver, where the deposit is apt 

 to be of such a nature that there is considerable risk of loss in 

 the operation of washing. There is of course the objection to 

 the use of vertical plates, that the density of the solution is 

 apt to decrease at the cathode and increase at the anode. 

 Such an action does take place, and the result is a slightly 

 thicker deposit on the lower part of the plate, thus changing 

 to a small extent the effective area ; but no difficulty has been 

 experienced from this cause, even when the current is allowed 

 to flow for three or four hours, when the current and the 

 solution have the densities stated below. 



In the later experiments copper has been almost exclusively 

 used. It is found to be very much more easily managed than 

 silver, and gives, over a wide range of size of plate and den- 

 sity of solution, deposits which are perfectly adherent and 

 homogeneous, thus rendering it more generally convenient 

 where currents of a considerable variety of strength are used. 

 Besides, for large currents such as from 10 to 100 amperes 

 and upwards, the use of silver is almost excluded on account 

 of the expense of the necessary materials. When the highest 

 accuracy is required, however, and when it is used in experi- 

 enced hands, silver is decidedly superior to copper. It pre- 

 sents hardly any of the uncertainties which, as will be pointed 

 out below, it is absolutely necessary to guard against in the 

 case of copper ; but the fact must not be lost sight of that an 

 expertness in manipulation and a degree of care are required 

 which cannot always be obtained. 



The objections which have been brought against the use of 

 copper are its readiness to oxidize in the air, especially if 

 moisture be present, and the fact that it loses weight in the 

 liquid*. Both of these are of course legitimate objections, 



* See a paper by G. Gore, LL.D., F.R.S., « Nature,' March 16, 1882 ; 

 and Lord Rayleigh and Mrs. Sidgwick, Trans. Roy. Soc. 1884, pp. 411-460. 



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