76 ELECTROTYPE PROCESSES. 



has been shown that during the deposition of metal, say copper, in 

 electrotyping, the acid, when exhausted of the copper at the surface 

 of the medal, is transferred to the positive pole, and dissolves a por- 

 tion of copper ; but this portion is not transferred by the electric 

 current to the medal : hence it will be observed, that the solution 

 next the medal will become exhausted of copper, and will conse- 

 quently rise to the surface from its greater lightness. There is no 

 doubt a flow of stronger solution in a horizontal direction from the 

 positive pole to the medal, caused by the lighter portion ascending ; 

 but this does not mend the evil : the light portion is increasing on 

 the surface, and the whole solution soon becomes of different densi- 

 ties from the surface to the bottom of the medal ; and this constant 

 current of the solution flowing up the surface upon which the elec- 

 trotypist is depositing, causes the lines that are observed in deposits 

 under certain circumstances, and which are sometimes very annoy- 

 ing. If a small hollow be in the mould, or 

 even if a small portion of a plain surface re- 

 sist, the metal will accumulate round the 

 edge of the resisting portion, giving the de- 

 posit an appearance as if made in a flowing 

 stream, like a stone standing up in a current 

 of water. The black point in the centre re- 

 presents the resisting spot, around which the 

 deposit will thicken, causing a ridge of metal 

 37 - to radiate to a point immediately above the 



resisting portion. These disappointments are much more annoying 

 in solutions of gold and silver than in sulphate of copper, as will be 

 noticed when we come to treat of plating and gilding. A point of 

 grease or dirt, or small hole not cleaned out, hardly visible to the 

 naked eye, will give a very prominent effect upon the plain polished 

 surface of a piece of metal. 



From these observations, the reader will now be able to answer 

 the question What is the best position to place a medal in the 

 solution ? To make it still more apparent, take a glass jar, filled 

 with a solution of sulphate of copper ; place a piece of copper upon 

 the bottom of the jar, and suspend the medal at the top, having their 

 two faces parallel ; connect them with a battery ; in a short time 

 the solution round the medal becomes exhausted, and even colour- 

 less, the medal covered with a dirty brown powder, and no further 

 deposit will take place. But reverse the case : place the medal at 

 the bottom, and the copper positive electrode at the top ; the depo- 

 sition goes on constant and smooth ; the solution is maintained in 

 the same condition as it was at the first, there being a constant 

 transfer ; the acid is transferred by the current from the medal to 

 the copper pole : the sulphate of copper formed, descends by its 



