1845] WRITINGS OF JOSEPH HENRY. 229 



periment, 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 un- 

 silvered 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 process of soldering, by heating them to a degree at 

 which silver disappears. 



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

 plated with gold, lose their brilliancy after a time, and that 

 it 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 in cases of alloys. Silver coins, after having lain 

 long in the earth, have been found covered with a salt of 

 copper. This may be explained by supposing that the alloy 

 of copper, at the surface of the coin, enters into combination 

 with the carbonic acid of the soil, and being thus removed, 

 its place is supplied by a diffusion from within; and in this 

 way it is not improbable that a considerable portion of the 

 alloy may be exhausted in the process of time; and the 

 purity of the coin be considerably increased. 



Perhaps also the phenomenon of what is called segregation, 

 or the formation of nodules of flint in masses of carbonated 

 lime, and of indurated marl in beds of clay, may be explained 

 on the same principle. In breaking up these masses it is 

 almost always observed that a piece of shell, or some extra- 

 neous matter, occupies the middle, and probably formed the 

 nucleus around which the matter was accumulated bv attrac- 



