348 METALS AND THEIR COMBINATIONS. 



but it decomposes at once in water, giving arsenous acid. It exists in solution 

 only in the presence of excess of hydrochloric acid. When the solution is 

 evaporated to dryness, arsenous oxide remains. 



Arsenic trioxide, Arseni trioxidum, As 2 O 3 = 196.44 (Arsenous 

 oxide. White arsenic, 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 

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

 isolated condition, but is known in solution only : 



3H 2 = 2H 3 As0 3 . 



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

 s0 2 , is known in some salts, as, for instance, in sodium metarsenite, NaAsO* 

 which salt may be obtained by the action of arsenous oxide on the carbonate, 

 bicarbonate, or hydroxide of sodium : 



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



When heated to about 218 C. (424 F.) arsenous oxide is volatil- 

 ized without melting; the vapors, when condensed, form small, 

 shining^ eight-sided crystals; when heated on charcoal, it is deoxi- 

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



Arsenous oxide is frequently used in the arts and for manufacturing 

 purposes, as, for instance, in the manufacture of green colors, of 

 opaque white glass, in calico-printing, as a powerful antiseptic for 

 the preservation of organic objects of natural history, and, finally, as 

 the substance from which all arsenic compounds are obtained. 



