184 METALS AND THEIR COMBINATIONS. 



arsenious 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, quin- 

 quivalent in others. 



Arsenious oxide, Acidum arseniosum, As 2 3 = 197.8 ( White arsenic, 

 Arsenic trioxide, Arsenious anhydride, improperly Arsenious acid). 

 This compound is frequently obtained as a by-product in metal- 

 lurgical 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 arsenious oxide, which at 

 that temperature is volatilized and condensed in chambers or 

 long flues. 



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

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

 transparent amorphous masses which frequently show a stratified 

 appearance; recently sublimed, arsenious oxide exists as the 

 semi-transparent glassy mass, known as vitreous arsenious oxide, 

 which gradually becomes opaque and ultimately resembles 

 porcelain. 



These two modifications of arsenious oxide differ in their 

 solubility in water, the amorphous or glassy variety dissolving 

 more freely than the crystallized. One part of arsenious oxide 

 is dissolved in from 30 to 100 parts of cold and in 15 parts of 

 boiling water, the solution having at first a faint acrid and 

 metallic, and afterwards a sweetish" taste. This solution con- 

 tains the arsenious oxide not as such, but as arsenious acid, 

 H 3 As0 3 , which compound, however, cannot be obtained in an 

 isolated condition, but is only known in solution : 



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



The salts of arsenious acid are known as arsenites. 



When heated to about 220, arsenious oxide is volatilized 

 without fusion ; the vapors, when condensed, form small, shining, 

 eight-sided crystals; when heated on charcoal, it is deoxidized, 

 giving off" at the same time an odor resembling that of garlic. 



