Scientific InlelUgence — Geology. 391 



Professor Henry gave an account of some observations he had made 

 on capilhirity, in addition to those he had before communicated to the 

 Society on the same subject. 



In 1839, he presented the results of some experiments on the 

 permeability of lead to mercury ; and subsequent observation had led 

 him to believe that the same property was possessed by other metals 

 in reference to each other. The first attempt to verify this conjec- 

 ture was made, with the assistance of Dr Patterson, at the United 

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

 a plate of sheet-iron, and submitted to the heat of an assaying fur- 

 nace ; but the experiment was unsuccessful ; 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, Professor Henry inquired of Mr Cornelius, of Philadelphia, if, 

 in the course of his experience in working silver-plated copper, in 

 his extensive manufactory 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 temperature, leaving a surface of copper exposed ; 

 and, that it was generally believed by the workmen, that the silver 

 evaporates at this temperature. Professor Henry suggested that the 

 silver, instead of evaporating, merely sunk into the pores of the cop- 

 per ; and that, by carefully removing the surface of the latter by the 

 action of an acid, the silver would re-appear. To verify this by ex- 

 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 unsilvered end of the plate was next placed, 

 for a few minutes, in a solution of muriate of zinc, by which the ex- 

 terior surface of copper was removed, and the surface of silver was 

 again exposed. This method of recovering the silver before the pro- 

 cess of plating silver by galvanism came into use, would have been 

 of much value to manufacturers of plated ware, since it often hap- 

 pened that valuable articles were spoiled, in the process of soldering, 

 by heating them to the 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 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 in cases of alloys. Silver coins, after having lain long in the 



