ON ELECTROLYSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 319 



Further, it is well known that a copper plate when immersed in a solution of 

 copper sulphate, without any current, pives rise to some chemical action which 

 causes the copper plate to alter iu weight. The alteration may he in the direc- 

 tion either of increase or of decrease of the weight, the effect depending upon the 

 state of the plate and the state of the solution. 



In considering the increase in weight of a copper cathode in an electrolytic cell 

 we cannot without definite experimental reasons neglect to consider the possibility 

 of both these actions occm-ring together, and, moreover, the passage of the current 

 may produce complicated results which cannot be inferred from the action of the 

 solution in the cell under other conditions. Dr. Gore has made very numerous 

 experiments upon this subject, without, however, arriving at any very satisfactory 

 result. 



Tn view of the importance of the subject, a very large number of experiments 

 have been made at the Cavendish Laboratory at Cambridge under my direction 

 during the past two years. They may be divided into two groups. (A). Experi- 

 ments to ascertain whether two copper cells give the same amount of deposit for 

 the same current; and (B). Experiments to determine whether a copper cell can 

 be arranged so that, on comparison with a silver cell, within sufficiently wide 

 limits of current it will give a value of the chemical equivalent of copper agree- 

 ing with that obtained by chemical methods. 



A. Experiments on Copper Cells only. 



These experiments consisted in arranging pairs of copper cells with various 

 electrodes, as copper plates, copper wires, copper cylinders having wire connections, 

 platinum wires, in order to ascertain whether the disturbing actions were limited 

 to the surface. The results were very irregular and disappointing. Errors 

 amounting frequently to more than 1 per cent, occurred without any assignable 

 cause. 



The same is all that can be said about experiments with different solutions of 

 sulphate, including solutions from which air had been expelled by boiling, the experi- 

 ments being then conducted under the receiver of an air-pump. Endeavours to 

 allow for the differences of the cathode weighings, by supplementary experiments 

 on the effect of the solution upon plates not connected with the electrodes, have 

 likewise proved fruitless. The enumeration of these experiments would serve no 

 practical pui-pose, as they lead to no generalisation ; I shall therefore pass over them. 



B. Experiments on the Comparison of Silver and Copper Cells. 



One of the most disagreeable features of the experiments refeiTed to in the 

 preceding section was that when two cells were compared, and^ a difference in the 

 increase of weight of the cathodes was obtained, it was impossible to say whether 

 either or both of the two cells was at fault, and it was therefore decided to employ 

 a silver cell as a standard. Lord Rayleigh was at the time engaged on his experi- 

 ments upon absolute value of the electro-chemical equivalent of silver, and had 

 not yet arrived at a result. Experiments were therefore made to obtain a silver 

 cell that might be fairly regarded as a standard. The only difficulty in the way 

 was that the deposit of silver from pure nitrate is crystalline and very rough. The 

 deposit was taken upon a platinum crucible, and it was feared that error might 

 arise, either from mechanical loss of the silver during the operation of drying, or 

 from the retention by the deposit of water, or salt in solution. Attempts were 

 therefore made to use as electrolyte (1) a solution of chloride of silver in hypo- 

 sulphite of silver, (2) a solution of nitrate of silver containing glycerine, (3) a 

 solution of nitrate of silver containing acetate of silver. The deposits obtained with 

 all these are much closer and harder, but the hyposulphite solution is very unstable, 

 and a current beyond a certain density precipitates a black sulphide which destroys 

 the experiment. The other solutions were also discarded after consultation with 

 Lord Rayleigh, who had then shown that a 15 per cent, solution of pure nitrate 

 gave a perfectly satisfactory silver cell if proper precautions were taken with the 



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