110 GALVANIC ELECTRICITY. 



solution, when it goes to the positive pole, as before ex- 

 plained, combines with silver, forming new cyanide of 

 silver, which dissolves and mixes with the rest. Thus, 

 the strength of the solution is always maintained. The 

 coin is attached to the negative wire, by flattening the 

 latter, laying it on the back of the coin, and covering 

 the whole with sealing wax ; the coin and wire should 

 be previously slightly warmed, and the wax used at a 

 moderate heat, so that it shall not run between the wire 

 and the coin, and prevent their perfect contact. 



How are med- 265. COPYING OF MEDALS. If it IS de- 



ais copied? sired to copy the face of a medal or a coin, 

 the same apparatus suffices. The reverse and edges of 

 the coin are very slightly oiled, to prevent the adhesion 

 of the copy about to be made. It is then placed in the 

 solution. The metal deposits upon it, copying perfectly 

 every elevation and depression. When the crust is suffi- 

 ciently thick, which will be after the lapse of twelve 

 hours, the coin, with its shell of metal, is removed, and 

 the whole process repeated with the mould. The de- 

 posit which now forms in the shell, is an exact copy of 

 the face of the original coin. Moulds are also made by 

 stamping the coin into soft metal, and using the impres- 

 sion thus produced instead of the copper shell. Copper 

 plates, for engravings, may be copied so perfectly by 

 this method, as to be fully equal to the original. 

 How are wood 266. COPYING OF WOOD CUTS. The diffi- 

 cuts copied? culty of copying other than metallic ob- 

 jects, by the processes, that they are not generally good 

 conductors. Thus, when a wood cut is attached to 

 the negative wire, it does not itself receive a nega- 



