210 METALS AND THEIR COMBINATIONS. 



Silver arsenite dissolved in water of ammonia and boiled forms silver 

 arsenate and metallic silver. 



4. Ammonio-sulphate of copper (made similarly to ammonio- 

 nitrate of silver from cupric sulphate) added to neutral arsenous 

 solutions produces a green precipitate of cupric arsenite (CuHAsO 3 ) 

 known as Scheele's green (Plate V., 2). (Arsenite of copper mixed 

 with cupric acetate is known as Schweinfurth green). The same 

 reagent produces in neutral solutions of an arsenate a similar green 

 precipitate of cupric arsenate, CuHAsO 4 . Cupric arsenite boiled 

 with potassium hydroxide forms potassium arsenate and red cuprous 

 oxide. 



Instead of using for the above tests the ammonio salts, silver 

 nitrate or cupric sulphate may be added to the acid (or neutral) solu- 

 tion of arsenic, then adding water of ammonia carefully in small 

 quantities until a neutral reaction has been obtained, when the pre- 

 cipitate is formed. 



5. Soluble arsenates give white precipitates with soluble salts of 

 barium, calcium, magnesium, zinc, and some other metals; soluble 

 arsenites do not. Arsenates give, on heating with ammonium molyb- 

 date, a yellow precipitate of ammonium arseno-molybdate, (NH 4 ) 3 

 AsO 4 .10MoO 3 . 



6. Heat any dry arsenic compound, after being mixed with some 

 charcoal and dry potassium carbonate in a very narrow test-tube (or, 



FIG. 13. 



better, in a drawn-out glass tube having a small bulb on the end) : 

 the arsenic compound is decomposed and the metallic arsenic deposited 

 as a metallic ring in the upper part of the contraction. (Fig. 13.) 



