This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Arsenious acid is prepared by sublimation from arsenical ores, and condenses in the cooler parts of the retort as a heavy powder, fine and white, like flour; in the hotter parts, it forms a vitreous mass, transparent and amorphous, which becomes, on exposure to air, opaque and crystalline, and is usually seen in smooth milk-white or yellowish pieces not unlike porcelain, and stratified in appearance according to the different opacity of its layers; the change from the amorphous to the crystalline form is accompanied with phosphorescence (one of several of its analogies with phosphorus). The two forms differ in density and in solubility, the transparent acid dissolving in about 100, the opaque in about 80 parts of water at 15° C.
The powder is not readily wetted by water, so that it is apt to remain floating on the surface, or adherent to the sides of a vessel. Organic products, milk or mucus, render it less, acids and alkalies more, soluble; oils and alcohol also dissolve it. It crystallizes from a saturated solution, or after slow sublimation in minute shining octahedra, or in rhombic prisms (like oxide of antimony, with which it is isomorphous): sprinkled on a red-hot surface, it evolves scarcely visible vapors of metallic arsenic, which have an odor like garlic, and, at a few inches from the hot surface, change to dense white odorless smoke, being the acid re-formed by oxidation. Arsenious acid itself has no smell: its taste is sharp and rather nauseating (Hirtz), but, in such small quantities as may be taken for trial, nothing more than a slight sweetness and grittiness will be detected (Christison).