ARSENIC. 351 



and the bisulphide or orpiment, As 2 S 3 . Bisulphide of arsenic is an 

 orange-red, fusible, and volatile substance, used as a pigment; it 

 may be made by fusing together the elements in the proper propor- 

 tions. Trisulphide is a golden-yellow, fusible, and volatile substance, 

 which also may be obtained by fusing the elements, or by precipitating 

 an arsenic solution by hydrogen sulphide (Plate V., 1). 



The pentasulphide, As 2 S 5 , has the same color as the trisulphide, and 

 is most readily obtained by acidifying a solution of a sulph-arsenate : 



2(NH 4 ) 3 AsS 4 4- 6HC1 2H 3 AsS 4 + 6NH 4 C1. 



2H 3 AsS 4 = 3H 2 S + As 2 S 5 . 



The tri- and pentasulphide of arsenic have acid properties, similar to 

 the corresponding oxides. They unite with alkali sulphides to form 

 soluble meta-sulph-arsenites and sulph-arsenates : 



As. 2 S 3 + (NH 4 ) 2 S = 2NH 4 AsS 2 . 

 As 2 S 5 + 3(NH 4 ) 2 S = 2(NH 4 ) 3 AsS 4 . 



When the trisulphide is dissolved in a solution of a polysulphide 

 (yellow ammonium sulphide), a sulph-arsenate is formed, 



As 2 S 3 4- 3(NH 4 ) 2 S 4- S, = 2(NH 4 ) 3 AsS 4 , 



from which acids precipitate the pentasulphide. Both sulphides are 

 also soluble in solutions of alkali hydroxides or carbonates, forming a 

 mixture of met-arsenite and meta-sulph-arsenite, and arsenate and 

 sulph-arsenate respectively. 



Arsenous iodide, Arseni iodidum, AsI 3 = 454.5 (Iodide of arsenic), may 

 be obtained by direct combination of the elements, and forms orange-red crys- 

 talline masses, soluble in water and alcohol, but decomposed by boiling with 

 either of these liquids. It is used in the official preparation, Solution of arsenous 

 and mercuric iodides, Donovan's solution, which is made by dissolving one part 

 each of arsenous iodide and mercuric iodide in 98 parts of water. 



Tests for arsenic. 



(For arsenous compound, use a solution of arsenous oxide made by dissolving 0.5 

 gramme in 100 c.c. of hot water and allowing to cool ; for arsenic compound, use a 5 

 per cent, solution of sodium arsenate.) 



1. Hydrogen sulphide produces in the solution of arsenous acid a 

 yellowish coloration, but no precipitate. This is due to the fact that 

 the arsenic trisulphide remains in solution in the colloidal state and is 

 precipitated only after a long time. Addition of some hydrochloric 

 acid causes precipitation immediately of the yellow trisulphide (Plate 



V, 1): 



2H 3 AsO 3 4- 3H 8 S ~ 6H.p 4- As 2 S 3 . 



