350 METALS AND THEIR COMBINATIONS. 



exsiccated sodium arsenate in water, corresponding to 1.68 per cent, 

 of the crystallized salt. 



Lead arsenate, Pb 3 (AsO 4 ) 2 , is used for spraying plants to exterminate 

 moths. It is a white, fusible powder, insoluble in water, ammonia, and ammo- 

 nium salts. It is obtained by precipitation of basic lead acetate (subacetate) 

 with sodium arsenate, or lead nitrate with excess of sodium arsenate : 



3Pb(NO 3 ) 2 + 4Na 2 HAsO 4 = Pb 3 (AsO 4 ) 2 -f 6NaNO 3 + 2NaH 2 AsO 4 . 



Hydrogen arsenide, AsH 3 .(Arsine, Arsenetted or arseniuretted 

 hydrogen). This compound is formed always when either arsenous 

 or arsenic oxides or acids, or any of their salts, are brought in con- 

 tact with nascent hydrogen, for instance, with zinc and diluted 

 sulphuric acid, which evolve hydrogen : 



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

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

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



Hydrogen arsenide is a colorless, highly poisonous gas, having a 

 strong garlic odor. Ignited, it burns with a bluish flame, giving off 

 white clouds of arsenous oxide : 



2AsH 3 + 6O = As 2 O 3 + 3H 2 O. 



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

 arsenetted 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 explained 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. 



Arsenetted hydrogen, when heated to redness, is decomposed into 

 its elements ; by passing the gas through a glass tube heated to red- 

 ness, the liberated arsenic is deposited in the cooler part of the tube, 

 forming a bright metallic ring. 



Sulphides of arsenic. Three sulphides of arsenic are known. Two 

 have been mentioned above as the native disulphide or realgar, As 2 S 2 , 



