216 Vegetation of Silver. >[Book VL 



hardens under the hammer, but very readily lofes that 

 hardnefs by heating. 



Silver, expofed to- the heat of the moft powerful 

 burning lenfes, is partly vitrified and partly volatilized 

 in fumes, which are found, when received on a plate 

 of gold, to be filver in the metallic ftate. It is like- 

 wife faid to have been partly calcined by twenty fuc- 

 cefiive expo fu res to the heat of the porcelain furnace 

 at Stives. This, however, may be doubted, as filver 

 does not undergo any degree of calcination by expo- 

 fuje to heat, even with the addition of nitre. Silver 

 melts in the firft degree of white heat, and appears in 

 the fire like the fineft quickfilver. When it is haftily 

 cooled, it exhibits a curious phenomenon, called vege- 

 tation j for we difcover from different parts of its 

 lurface ramifications and branches like thofe of trees 

 which fprout out with a item. The reafon of this 

 appearance feems to be the irregular contraction which 

 the filver undergoes in paffing from the fiuid to the 

 folid ftate. The melted filver fuffers the firft com- 

 mencement of congelation at its furface; by thefe 

 means a cruft is formed, which by its fudden contrac- 

 tion compreffes the fiuid filver within ; thus a protu- 

 berance is formed,, which, congealing in its turn, con- 

 trails and preffes the intermediate fiuid through its 

 cruft into branches. 



The air alters filver very little, unlefs it contains 

 fulphureous vapours, which it often does, from the 

 putrefacticm of animal fubftances or the exhalations of 

 drains, or of fulphureous mineral waters. This metal, 

 therefore, becomes fomewhat tarnifhed by long con- 

 tinued expofure to the atmofphere, and in time be- 

 comes covered with a -thin purple or black coating, 

 which, after a long feries of years, has been obfervcd 

 to fcale off from images of filver expofed in churches, 



and 



