114 



ELECTRO-PLATING. 



distinct salts. In the decomposition of the silver solution by the 

 electric current, the former, cyanide of silver, is alone affected : the 

 silver is deposited, and the cyanogen passes to the positive plate or 

 electrode. The cyanide of potassium is therefore set at liberty 

 upon the surface of the article receiving the silver deposition, and 

 its solution being specifically lighter than the general mass of the 



plating solution, rises to the top : this 

 causes a current to take place along 

 the face of the article being plated. 

 If the article has a flat surface, 

 suppose that of a waiter or tray, 

 upon which a prominence exists, as 

 a mounting round the edge, such 

 as a gadroon, see fig. 42, it will 

 cause lines and ridges from the bot- 

 tom to the top, as already described 

 at page 71. Newly-formed solutions 

 are most subject to produce this an- 

 noyance. 



Other Effects produced in Working. 



As the cyanogen combines with the 

 silver plate forming the positive elec- 

 trode, it is dissolved by the free cya- 

 nide of potassium, which the solution 

 must have ; and, being specifically 

 heavier, sinks to the bottom, by which 

 a current downwards is excited : this 



is of no greater annoyance than that .it renders the solution of 

 unequal density, which in its turn yields an unequal deposit, more 

 being laid upon the lower parts of the article than on the upper : 

 the silver plate also is destroyed more rapidly at the bottom than at 

 the top, except at the surface of the solution, if the silver be above 

 it, where the plate gets cut through. In a new solution, which 

 contained 1J ounces of silver to the gallon, we have found, just 

 before taking out the articles, that the top part of the solution 

 contained 200 grains of silver less, and the bottom part 200 

 grains more, per gallon, than when the articles were put into 

 it. These difficulties and annoyances may, however, be nearly 

 surmounted by keeping the articles in motion : agitating or stirring 

 the solution occasionally would also obviate these annoyances ; but 

 this is not advisable, for if the sediment (which always forms) were 

 stirred up it would settle upon the face of the articles and make 

 them rough. Where there is engine power it is an easy matter 

 to keep the articles in motion ; but where this power is., not avail- 

 able, a very simple apparatus, invented by Mr. Alex. Mitchell, of 



