630 ARSENIC. 



contains, and the solution is highly concentrated, the salt 

 named binarseniate of soda crystallizes at a low temperature. 

 This salt contains 1 eq. of soda, and 2 eq. of water as bases, 

 with 2 eq. of water of crystallization, and corresponds with the 

 biphosphate of soda. Its formula is NaO, 2HO, AsO 5 + 2HO. 

 The binarseniate of potash, which is analogous in composition, 

 is a highly crystallizable salt. It is sometimes prepared by de- 

 flagrating arsenious acid, with an equal weight of nitrate of 

 potash. These arseniates of the alkalies, which contain water 

 as base, all lose that element at a red heat, but unlike the phos- 

 phates they recover it, when again dissolved in water. Arsenic 

 acid, therefore, forms only one, and that a tribasic class of 

 salts. The arseniates of earths and other metallic oxides are 

 insoluble in water, but soluble in acids. The arseniate of silver 

 (3AgO, AsO 5 ) falls as a precipitate, of a chocolate brown colour, 

 when nitrate of silver is added to the solution of an arseniate, 

 and affords an indication of the presence of arsenic acid. 



Sulphurets of arsenic. When the following sulphuret, real- 

 gar, is digested in caustic potash, it loses sulphur and leaves a 

 brownish black powder, having some resemblance to peroxide of 

 lead, which is the sulphuret SAs 6 , according to Berzelius. 

 Bisulphuret of arsenic, As S 2 , is obtained by fusing sulphur 

 with an excess of arsenic or arsenious acid. It is transparent 

 and of a fine ruby colour after cooling, and may be distilled 

 without decomposition. It forms the crystalline mineral realgar. 

 Sulpharsenious acid or orpiment, As S 3 , also occurs native. It 

 may be prepared by decomposing a solution of arsenious acid in 

 hydrochloric acid, by sulphuretted hydrogen, or by an alkaline 

 sulphuret. This sulphuret has a rich yellow colour, and is the 

 basis of the pigment king's yellow. It is insoluble in acids, 

 but is soluble to a small extent in water, containing sulphu- 

 retted hydrogen, and also in pure water, but is precipitated by 

 ebullition with a little hydrochloric acid. When heated it fuses 

 readily, and becomes crystalline on cooling. It is readily dis- 

 solved by ammonia and solutions of the fixed alkalies, and is 

 indeed a powerful sulphur acid. Sulpharsenic acid, As S 5 , falls 

 as a yellow precipitate, having very much the appearance of 

 orpiment, when a solution of arsenic acid, somewhat concen- 

 trated, is decomposed by sulphuretted hydrogen. It may be 

 sublimed without change ; and gives a mass after cooling, which 

 is not crystalline. 



