668 SILVER. 



by solutions of the hydrates of potash and soda. Its salts are 

 precipitated black by sulphuretted hydrogen, and afford, when 

 treated with hydrochloric acid or a soluble chloride, a white 

 curdy precipitate, the chloride of silver, which soon becomes 

 purple, if exposed, while humid, to the direct rays of the sun. 

 This precipitate is not dissolved by nitric acid, but is dissolved 

 by ammonia in common with most of the insoluble salts of 

 silver. 



Oxide of silver combines with ammonia and forms the fulmi- 

 nating ammoniaret of silver, a substance of a dangerous charac- 

 ter from the violence with which it explodes. The ammoniaret 

 may be formed by digesting newly precipitated oxide of silver in 

 strong ammonia, or more readily by dissolving nitrate of silver 

 in ammonia, and precipitating the liquor by potash in slight ex- 

 cess. If this substance be pressed by a hard body, while still 

 humid, it explodes with unequalled violence; when dry, the 

 touch of a feather is often sufficient to cause it to fulminate. 

 The explosion is obviously occasioned by the reduction of the 

 silver, from the combination of its oxygen with the hydrogen 

 of the ammonia, and the evolution of nitrogen gas. 



Sulphuret of silver, AgS, 1552.8 or } 24.43. Sulphur and 

 silver may be combined together by fusion ; the excess of sul- 

 phur escapes, and at a high temperature the sulphuret melts ; it 

 forms, on cooling, a crystalline mass. This compound has a lead- 

 grey colour and metallic lustre. It is so soft that it may be cut 

 by a knife, and is malleable. The sulphuret of silver is also re- 

 markable for conducting electricity, like a metal, when warmed. 

 The same compound occurs in nature, sometimes crystallized in 

 octohedrons with their secondary faces. This sulphuret is par- 

 ticularly interesting from being isomorphous with the subsul- 

 phuret of copper, AgS with Cu 2 S (page 144). These two 

 sulphurets replace each other in indeterminate proportions in 

 several double sulphurets of silver and other metals, as in poly- 

 basite andifahlerze., the composition of which may be expressed 

 by the following formulae, the symbols placed above each other, 

 representing constituents, of which either the one or the other 

 may be present : 



Polybasite . 9 



