of Silver and of Copper. 395 



that case the surface of the plate remains bright and moder- 

 ately hard, so that the plate can be washed and weighed if 

 that be thought necessary. The density of the current at the 

 anode should not exceed one ampere to 400 square centimetres 

 of surface, and should be even less if the plates are to be weighed. 

 The action of the current in causing the solution of the anode 

 is somewhat curious, especially if the plates be made of rolled 

 sheet. If the plates be simply washed and placed in the cell 

 with the bright polished surface still on them, it is seldom that 

 the outside skin will be dissolved. The silver is taken from 

 the interior of the plate, leaving a very thin skin lying loose 

 on the surface, which is ready to fall off either when the plates 

 are lifted out of the cell or when they are placed in water for 

 the purpose of washing. Whether this is due to the mecha- 

 nical state of the silver on the surface or to a peculiar difficulty 

 in so cleaning the surface of the plate that it is properly 

 wetted by the liquid, is not yet quite clear. A plate of silver 

 when repeatedly used for an anode becomes soft and almost 

 devoid of elasticity, due to the solvent action taking place 

 deep into the interior of the plate ; and it is well to heat the 

 plates to about a red heat in a spirit flame between different 

 experiments, so as to keep it hard and prevent loss of silver. 



In the case of copper and copper sulphate, the size of the 

 plate may be almost anything from twenty square centimetres- 

 to the ampere upwards; but for experiments which are to be 

 continued for two or three hours, the cathode-plate should 

 present about fifty square centimetres of surface per ampere 

 of current. With plates of from this size upwards the cur- 

 rent may be allowed to flow for almost any length of time- 

 without introducing the least difficulty as to loss of copper in 

 the subsequent washing. At the higher limit of current- 

 density here mentioned (one fiftieth of an ampere per square 

 centimetre), there is a slight tendency for the deposit to thicken 

 at the edges of the plate and become rough, but as the current- 

 density diminishes this becomes less and less marked. It may 

 be taken as certain that the aggregation of the deposit at the 

 edges of the plate is due to the current- density becoming too 

 great at the sharp edges, and thus causing the formation of 

 sharp crystals, which in their turn intensify the action. If 

 the plates are made large enough, this critical current-density 

 is never reached, and the deposit is as smooth on the edges as 

 elsewhere. It is mainly in the ease with which a perfectly 

 uniform and solid deposit can be obtained that the great 

 advantage of copper over silver for ordinary use in electro- 

 lytic measurements lies. 



The anodes in the copper cell behave in a similar manner to 



