EFFECTS PRODUCED IN WORKING. 117 



was hardly any perceptible difference from the original weight ; in 

 short, there had been no material deposit. These phenomena will 

 be found to occur with the greatest frequency when the solutions 

 and the batteries are in the best condition for working, and when 

 the article upon which the deposit is going on, and the pole, or plate, 

 of metal forming the positive electrode, are at a considerable distance 

 from each other. But, before explaining what we consider to be 

 the cause of these annoyances, we will refer to another phenomenon 

 connected with them. 



Opposite Currents of Electricity from Vals If, under the circum- 



stances referred to, and when the deposit has gone on for some 

 time, the wires connecting the battery with the electrodes in the 

 depositing solution be disconnected from the battery, and their two 

 ends be joined together, a current of electricity nearly as strong as 

 that from the battery will pass through the wires, but in the oppo- 

 site direction from that which was obtained by the battery; and if 

 two pieces of metal were attached to these wires and put into a solu- 

 tion of copper, or any metal, a deposition would occur, the original 

 electrodes now constituting a battery in relation to this second 

 decomposition cell: the current, however, would gradually weaken 

 until it ceased. The cause of all these actions and reactions is this : 

 the article being plated with silver in connection with the battery, 

 exhausts the solution of silver around it, leaving free cyanide of 

 potassium in solution, while around the sheet of silver which is serv- 

 ing as the positive electrode the solution is on the contrary becoming 

 saturated with silver, so that we have alf the conditions necessary to 

 constitute a battery, having silver in two kinds of solution the one 

 capable of dissolving silver, the other not. In these conditions lie the 

 source of the annoyances described above. From the moment the 

 deposition of metal begins, there also arises an opposite current of 

 electricity, tending to neutralize the effects of the battery, which 

 current goes on increasing in quantity until the two currents neu- 

 tralize each other, or it may be until the current from the trough 

 overpowers that from the battery. In the latter case, as we have 

 said, there may, at the termination of the ordinary period, be little 

 or no silver deposited on the articles intended to be plated. Motion 

 in the silver or depositing solution will prevent all these annoyances ; 

 and this being now generally adopted, these phenomena are not now 

 observed, but the effects take place less or more in every solution. 



Test for the Quantity of Free Cyanide of Potassium in Solutions. It 



has been already mentioned, that the cyanide of silver, as it forms 

 upon the surface of the silver plate, is dissolved by the cyanide of 

 potassium : this renders it necessary to have always in the solution 

 free cyanide of potassium. Were we to use the pure crystalline 

 salt of cyanide of potassium and silver, dissolved in water, without 



