132 MERCURY. 



powers of light, and, further, as giving us the found- 

 ation of that very beautiful art of producing pic- 

 tures by means of light — the Daguerreotype. The 

 salts of gold, also, and some of the compounds of 

 mercury are very easily decomposed, vv'hen exposed 

 to the action of light. 



296. Silver has a very strong alBnity for chlorine, 

 and the chloride of silver is quite insoluble in water; 

 hence silver cannot be dissolved in muriatic acid ; and 

 if muriatic acid, or any solution containing chlorine, 

 either free or in combination, be added to a solution 

 of silver, the whole of the metal will be thrown down 

 as insoluble white chloride. A solution of nitrate of 

 silver is, therefore, a very useful test to ascertain the 

 presence of chlorine in any solution. 



297. Silver has also a strong affinity for sulphur, 

 combining eagerly with it and forming a shining gray 

 brittle substance. The tarnish which we see on old 

 silver is a thin coat of sulphuret, formed by the sul- 

 phuretted hydrogen which generally exists in the air 

 of towns. Gold and silver are frequently called no- 

 ble metals, from their having no tendency to oxidisie 

 when exposed to the air. 



298. Mercury, or quicksilver, likewise, is some- 

 times found native in the metallic state, but by far 

 most commonly as a sulphuret ; it is obtained from 

 its ore by heating it with a mixture of iron filings 

 and lime; these substances combine with the sulphur; 

 and the mercury, being a volatile metal, is obtained 

 by a process of distillation. 



