206 METALS AND THEIR COMBINATIONS. 



(356 F.), without access of air. Heated in air, it burns with a 

 bluish-white light, forming arsenous oxide. Although insoluble in 

 water, yet water digested with arsenic soon contains some arsenous 

 acid in solution, the oxide of arsenic being formed by oxidation of 

 the metal by the oxygen absorbed in the water. 



Arsenic is used in the metallic state as fly-poison, and in some 

 alloys, chiefly in shot, an alloy of lead and arsenic. 



The molecule of arsenic contains four atoms, and not two, like 

 most elements. It is trivalent in some compounds, quinquivalent in 

 others. 



Arsenous oxide, Acidum arsenosum, As 2 O 3 = 197.8 (Arsenious 

 oxide, White arsenic, Arsenic trioxide, Arsenous anhydride, improperly 

 Arsenous acid). This compound is frequently obtained as a by- 

 product in metallurgical operations during the manufacture of metals 

 from ores containing arsenic. Such ores are roasted (heated in a 

 current of air), when arsenic is converted into arsenous oxide, which, 

 at that temperature, is volatilized and afterward condensed in 

 chambers or long flues. 



Arsenous oxide is a heavy, white solid, occurring either as an 

 opaque, slightly crystalline powder, or in transparent or semi-trans- 

 parent masses which frequently show a stratified appearance; 

 recently sublimed arsenous oxide exists as the amorphous semi- 

 transparent glassy mass known as vitreous arsenous oxide, which 

 gradually becomes opaque and ultimately resembles porcelain. This 

 change is due to a rearrangement of the molecules into crystals which 

 can be seen under the microscope. 



The two modifications of arsenous oxide differ in their solubility 

 in water, the amorphous or glassy variety dissolving more freely than 

 the crystallized. One part of arsenous oxide dissolves in from 30 to 

 80 parts of cold and in 15 parts of boiling water, the solution having 

 at first a faint acrid and metallic, and afterward a sweetish taste. 

 This solution contains the arsenous oxide not as such, but as arsenous 

 acid, H 3 AsO 3 , which compound, however, cannot be obtained in an 

 isolated condition, but is known in solution only : 



As 2 O 3 + 3H 2 O = 2H 3 AsO 3 . 



A second arsenous acid, termed met-arsenous acid or meta-arsenous 



acid, HAsO 2 , is known in some salts, as, for instance, in sodium met- 



arsenite, NaAsO 2 , which salt may be obtained by the action of 



arsenous oxide on the carbonate, bicarbonate, or hydroxide of sodium : 



As 2 O 3 + 2NaOH = 2NaAsO 2 + H 2 O. 



