896 Mr. T. Gray on the Electrolysis 



that above described for the silver anodes, with the exception 

 that they show no tendency to become soft and inelastic. If 

 the current-density at the anode exceed the fortieth of an 

 ampere per square centimetre, the current is apt to be variable 

 and may almost stop completely, even when an electromotive 

 force of 25 volts is used to produce it*. This is due to excessive 

 resistance at the surface of the anode. In some cases the 

 current will, after a few minutes, resume nearly its former 

 strength, and gases are then freely gi^en off at the anode. 

 When the current-densityis small, from the two-hundredth of 

 an ampere per square centimetre downwards, the plate can 

 generally be washed, and used to show the loss of copper during 

 the passage of the current. This loss does not in any case agree 

 with the gain on the cathode, in consequence of the solution 

 taking up copper during the experiment ; some examples of the 

 results which have been obtained, and some further remarks on 

 this subject are given below. In almost all cases there remains 

 on the surface of the anode more or less of a very fine brown 

 powder, which increases in amount as the current-density is 

 increased, and which also seems to depend somewhat on the 

 nature of the plate. If, for example, a plate of electrotype 

 copper be used, the surface will be found to have become a 

 dark red when the plate is removed from the cell, but no loose 

 copper will be found. That is certainly the case at least so 

 long as the current-density does not exceed the one hundred 

 and fiftieth of an ampere per square centimetre. Currents of 

 greater density were not tried with electrotype copper. This 

 result seems to point either to the mechanical state of the 

 copper in the rolled sheet, 01 the presence of a somewhat in- 

 soluble oxide, as the cause of the loose powder being left on 

 the plate. Local action, due to unequal quality of the plate 

 itself, may have something to do with it ; it is certainly greatly 

 increased by frequent reversals of the current in the cell. 



The effect of varying the density of the solution of copper 

 sulphate is not great until the density falls to about 1*05, when 

 the deposit begins to be less adherent. There is, however, 

 greater danger of error due to oxidation of the deposit in 

 weak than in strong solutions, when no acid is added, owing 

 to the fact that the solution becomes more quickly saturated 

 with copper. Any density between 1 and 1*18 will be found 

 to answer perfectly, but it is not advisable to use a saturated 

 solution because there is then a risk of crystals forming on 

 the plates. 



Preparation of Plates. — In all experiments on electrolysis 

 the proper treatment of the plates previous to their immersion 

 in the liquid is a most important consideration. The treat* 

 * See note on page 413. 



