CRYSTALLIZATION OF GOLD, SILVER, ETC. 439 



the salt constantly change places and enter into fresh combination, one 

 consequence of which would be a gradual passage of the nitric element 

 from the growing silver to the copper plate. This actually takes place, 

 and there is a diminution of the salt at the ends of the silver branches, 

 giving rise to an upward current and a condensation of nitrate of cop- 

 per against the copper plate, which gives rise to a downward current. 

 These two currents are seen in every reaction of this nature. In the 

 case of silver and copper, however, it has been proved that the crowd- 

 ing of the salt toward the copper plate is more rapid than would follow 

 in the usual " polarization theory." The instrument used in determin- 

 ing this point was a divided cell in which two plates, one of silver and 

 one of copper, connected with a wire, are each immersed in a solution 

 of its own nitrate, contained in each division of the cell, and separated 

 merely by parchment paper. The crystals of silver deposited on the 

 silver plate in this experiment are very brilliant. There are indica- 

 tions of the liquid being put into a special condition by the presence 

 of two metals which touch one another. Zinc alone is incapable of 

 decomposing pure water, but if copper or platinum be deposited on 

 the zinc in such manner that the water can have free access to the junc- 

 tion of the two metals, a decomposition is effected ; oxide of zinc is 

 formed and hydrogen gas is evolved. At ordinary temperature the 

 bubbles of gas rise slowly through the liquid, but, if the whole be 

 placed in a flask and heated, pure hydrogen is given off in large quan- 

 tity. We have also found that iron or lead similarly brought into inti- 

 mate union with a more electro-negative metal, and well washed, will 

 decompose in pure water. 



As might be expected, the action of magnesium on water may be 

 greatly enhanced by this method ; and a pretty and instructive experi- 

 ment may be made by placing a coil of magnesium in pure water at 

 the ordinary temperature, when there will be scarcely any visible effect, 

 and, then adding a solution of sulphate of copper, the magnesium is 

 instantly covered with a growth of the other metals, and at the same 

 time the liquid seems to boil with the rapid evolution of the hydrogen- 

 bubbles from the decomposed water. 



When, however, the force of the two metals in contact has to trav- 

 erse a layer of water, the resistence offered by the fluid prevents its 

 decomposition. This must also be an important element in the decom- 

 position of a metallic salt dissolved in water — and, in fact, we have 

 found that the addition of some neutral salt, such as nitrate of potas- 

 sium, increases the action, apparently by diminishing the resistance of 

 the liquid. If, too, we increase the quantity of the dissolved salt, we 

 get more than a proportional increase of deposited metal. Thus, in an 

 experiment made with different strengths of nitrate of silver, the follow- 

 ing results were obtained in ten minutes, all the circumstances being 

 the same except the strength of the solution : 



