180 METALS AND TIIEIK COMBINATIONS. 



Arseniuretted hydrogen, AsH 3 (Hydrogen arsenide]. This com- 

 pound is always formed when either arsenious or arsenic oxides 

 or acids, or any of their salts, are brought in contact with nascent 

 hydrogen, for instance, with zinc and diluted sulphuric acid, 

 which evolve hydrogen : 



As 2 O, + 12H = 2AsH 3 + 3H 2 O. 

 As 2 O 5 -f 16H = 2AsH 3 + 5H 2 O. 

 AsCl 3 + 6H = AsH 3 + 8HC1. 



Arseniuretted hydrogen is a colorless, highly poisonous gas, 

 having a strong garlic odor. Ignited, it burns with a bluish 

 flame, giving oft' white clouds of arsenious oxide : 



2AsH 3 + 60 = As 2 O 3 + 3H 2 O. 



When a cold plate (porcelain answers best) is held in the 

 flame of arseniuretted hydrogen, a dark deposit of metallic 

 arsenic (arsenic spots) is produced upon the plate (in a similar 

 manner as a deposit of carbon is produced by a common luminous 

 flame). The formation of this metallic deposit may be ex- 

 plained by the fact that the heat of the flame decomposes the 

 gas, and that, furthermore, of the two liberated elements, 

 arsenic and hydrogen, the latter has the greater affinity for 

 oxygen. In the centre of the flame, to which but a limited 

 amount of oxygen penetrates, the latter is taken up by the 

 hydrogen, arsenic being present in the metallic state until it 

 burns in the outer cone of the flame. It is this liberated arsenic 

 which is deposited upon a cold substance held in the flame. 



Arseniuretted hydrogen, when heated to redness, is decom- 

 posed into its elements; bypassing the gas through a glass tube 

 heated to redness : the liberated arsenic is deposited in the cooler 

 part of the tube, forming a bright metallic ring. 



Sulphides of arsenic. Two sulphides of arsenic are known 

 and have been mentioned above as the native disulphide or realgar, 

 As 2 S 2 , and the trisulphide or orpiment, As 2 S 3 . Disulphide 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 proportions. Trisulphide is a golden-yellow, fusible, 

 and volatile substance, which may also be obtained by fusing 

 the elements, or by precipitating an arsenic solution by hydro- 



