628 ARSENIOUS ACID. 



this vapour is about 13,000 (Mitscherlich) ; one volume of 

 arsenious acid vapour, or the combining measure, contains 

 accordingly, one volume of arsenic vapour and three volumes 

 of oxygen gas. When slowly sublimed in a glass tube, it is 

 always obtained in distinct transparent crystals of adamantine 

 lustre, which are regular octohedrons. But arsenious acid is 

 dimorphous, and occurs sometimes, in the roasting of arsenical 

 ores, in thin, flexible prisms, of a pearly lustre, of which the 

 form does not belong to the regular system, (Wohler.) Arse- 

 nious acid dissolves very slowly in water, and the prismatic 

 crystals in particular require to be heated with it for a long 

 time. A concentrated solution prepared in this way may after- 

 wards be cooled without arsenious acid immediately crystallizing 

 from it. One hundred parts of boiling water dissolve 9.68 parts 

 of the vitreous acid, and 11.47 parts of the opaque acid ; and 

 when the solutions cool to 60, 1.78 parts remain in the first, 

 and 2.9 parts in the latter ; the first reddens litmus paper, the 

 second makes it blue, although feebly, if already red. When 

 the vitreous acid in powder, is covered with ammonia, it heats 

 a little ; no combination of the acid with ammonia takes place^ 

 for the latter may be completely removed by water, but the 

 washed powder is found to have passed into the condition of 

 the opaque acid. For these curious facts we are indebted to 

 M. Guibourt. The taste of powdered arsenious acid is scarcely 

 perceptible, but it is slightly sweet, and leaves a feeling of 

 acridity in the mouth. 



Arsenious acid dissolves in the solutions of many acids, par- 

 ticularly hydrochloric acid, to a greater extent than in water, 

 but without combining with these acids. It is dissolved, how- 

 ever, by the bitartrate of potash, with the formation of a crys- 

 tallizable salt, analogous to the potash-tartrate of antimony. 

 Arsenious acid is dissolved by potash, soda, and ammonia ; but 

 the salts which it forms with these bases do not crystallize. It 

 is also dissolved by alkaline carbonates, but is sometimes depo- 

 sited from these solutions in a free state ; so that it is doubtful 

 whether arsenious acid displaces carbonic acid in the humid way. 

 The arsenites of the earths and metallic oxides are insoluble in 

 water, but soluble in acids ; these precipitated arsenites usually 

 carry down an excess of arsenious acid, and are not easily ob- 

 tained in a definite state. 



Arsenic acid, AsO 5 , 144,0.1 or 115.34. This acid is obtained 



