177 



was made with the assistance of Dr. Patterson, at the United States 

 Mint. For this purpose, a small globule of gold was placed on a 

 plate of sheet iron, and submitted to the heat of an assaying furnace; 

 but the experiment was unsuccessful j for, although the gold was 

 heated much above its melting point, it exhibited no signs of sinking 

 into the pores of the iron. The idea afterward suggested itself, that 

 a different result would have been obtained had the two metals been 

 made to adhere previous to heating, so that no oxide could have been 

 formed between the surfaces. In accordance with this view. Prof. 

 H. inquired of Mr. Cornelius, of Philadelphia, if, in the course of his 

 experience in working silver-plated copper, in his extensive manufac- 

 tory of lamps, he had ever observed the silver to disappear from the 

 copper when the metal was heated. The answer was, that the silver 

 always disappears when the plate is heated above a certain tempera- 

 ture, leaving a surface of copper exposed; and that it was generally 

 believed by the workmen, that the silver evaporates at this tempera- 

 ture. 



Professor H. suggested that the silver, instead of evaporating, 

 merely sunk into the pores of the copper, and that by carefully re- 

 moving the surface of the latter by the action of an acid, the silver 

 would reappear. To verify this by experiment, Mr. Cornelius heated 

 one end of a piece of thick plated copper to nearly the melting point 

 of the metal; the silver at this end disappeared, and when the metal 

 was cleaned by a solution of dilute sulphuric acid, the end which had 

 been heated presented a uniform surface of copper, whilst the other 

 end exhibited its proper coating of silver. The unsilvered end of the 

 plate was next placed, for a few minutes, in a solution of muriate of 

 zinc, by which the exterior surface of copper was removed, and the 

 surface of silver was again exposed. This method of recovering the 

 silver before the process of plating silver by galvanism came into use, 

 would have been of much value to manufacturers of plated ware, 

 since it often happened that valuable articles were spoiled, in the pro- 

 cess of soldering, by heating them to the degree at which silver dis- 

 appears. 



It is well known to the jeweller, that articles of copper, plated with 

 gold, lose their brilliancy after a time, and that this can be restored 

 by boiling them in ammonia; this effect is probably produced by the 

 ammonia acting on the copper, and dissolving off its surface, so as to 

 expose the gold, which, by diffusion, has entered into the copper. 



A slow diffusion of one metal through another probably takes place 



