ARSENIC. 
555 
§ 734-] 
of opaque variety, 3*699 ; specific gravity of transparent variety, 3*7385. 
Composition in 100 parts, As 75*75, 0 24*25 ; therefore one part of 
metallic arsenic equals 1*32 of As 2 0 3 . It is entirely volatilised at a 
temperature of 204*4°. 
In analysis it is obtained in brilliant octahedral crystals as a sublimate 
on discs of glass, or within tubes, the result of heating a film of metallic 
arsenic with access of air. It is obtained in commerce on a very large 
scale from the roasting of arsenical pyrites. As thus derived, it is usually 
in the form of a white cake, the arsenious acid existing in four forms 
—an amorphous, a vitreous, and two crystalline—the cake being 
generally opaque externally, whilst in the centre it is transparent. 
According to Kruger, this change from the crystalline to the amorphous 
condition is dependent upon the absorption of moisture, no alteration 
taking place in dry air. The conditions under which three of the 
forms of arsenic are produced are well shown by an experiment of 
Debray’s ; a sealed tube of As 2 0 3 is heated in a sand bath in such a 
manner as to heat the lower part to 400° C., the upper to 200° C. On 
cooling, vitreous arsenic is found in the lower part, octahedral crystals 
in the upper part, and in the middle prismatic crystals. The varieties 
of arsenious anhydride are acid to test-paper. 
The solubility of arsenious acid is often a question involving forensic 
matters of great moment. Unfortunately, however, no precisely definite 
statement can be made on this point, the reason being that the 
varieties of arsenic occur in very different proportions in different samples. 
The amorphous and crystalline varieties having very unequal solubilities, 
every experimenter in succession has given a different series of figures, 
the only agreement amid the general discrepancy being that arsenic is 
very sparingly soluble in water. 
The statement of Taylor may, however, be accepted as very near the 
truth, viz. that an ounce of cold water dissolves from half a grain to a 
grain. According to M. L. A. Buchner, 1 one part of crystalline arsenious 
acid dissolves after twenty-four hours’ digestion in 355 parts of water at 
15° ; and the amorphous, under the same condition, in 108 of water. A 
boiling solution of the crystalline acid, left to stand for twenty-four 
hours, retains one part of acid in 46 of water ; a similar solution of the 
amorphous retains one of arsenic in 30 parts of water ; i.e. 100 parts of 
water dissolve from 2*01 to 3*3 parts of As 2 0 3 . 
Boiling water poured on the powdered substance retains on cooling 
a grain and a quarter to the ounce ; in other words, 100 parts of water 
retain T4. Lastly, arsenious acid boiled in water for an hour is dissolved 
in the proportion of 12 grains to the ounce ; i.e. 100 parts of water 
retain 2*5. Absolute alcohol dissolves 025 at 15° ; at 100°, 3*4 grms. 
(Girardin). 
1 Bull, de la Societe Chim. de Paris, xx. 10, 1873. 
