(b.) Arsenic (As).—­This metal occurs in considerable quantity in nature, chiefly combined with sulphur or metals.

Arsenic, in the metallic state, is of a whitish-grey color, high lustre, and is crystalline, of a foliated structure, and is so brittle that it can be pulverized.  It does not melt, but is volatilized at 356 deg..  Its vapor has a strong alliaceous odor.  Arsenic sublimes in irregular crystals.  By exposure to the air it soon tarnishes, and is coated black.  Being mixed with nitrate of potassa and inflamed, it detonates with vehemence.  Mixed with carbonate of potassa, it is inflamed by a stroke of the hammer, and detonates violently.

Heated in oxygen gas, it is inflamed, and burns with a pale blue flame to arsenious acid.

([beta].) Arsenious Acid (AsO^{3}).—­This acid crystallizes in octahedrons, or, when fused, forms a colorless glass, which finally becomes opaque and enamel-like, or forms a white powder.  It sublimes without change or decomposition.  When heated for a longer while below the temperature of sublimation, it melts into a transparent, colorless, tough glass.  The opaque acid is sparingly soluble in cold water, and still more soluble in hot water.  It is converted, by continued boiling, into the transparent acid, which is much more soluble in water.  Arsenious acid is easily dissolved by caustic potassa.  It is also soluble in hydrochloric acid.  This acid occurs associated with antimonious acid, protoxide of tin, protoxide of lead, and oxide of copper.  It occurs likewise in very small quantity in ferruginous mineral springs.

([gamma].)_Arsenic Acid_ (AsO^{5}) is a white mass, which readily absorbs moisture and dissolves.  It will not volatilize at a low red heat, nor will it decompose.  Exposed to a strong heat, it is decomposed, yielding oxygen, and passing into arsenious acid.

Reactions before the Blowpipe.

Metallic arsenic, heated in a glass tube closed at one end, yields a black sublimate of a metallic lustre, and at the same time gives out the characteristic alliaceous odor.  This is the case too with alloys of arsenic, if there is a maximum quantity of arsenic present.

When heated in a glass tube open at both ends, metallic arsenic is oxidized to arsenious acid, which appears as a white crystalline sublimate on the sides of the glass tube.  This deposit will occur at some distance from the assay, in consequence of the great volatility of the arsenic.  The sublimate can be driven from one place upon the tube to another, by a very low heat.  Alloys of arsenic are converted into basic arseniates of metal oxides, while surplus arsenic is converted into arsenious acid, which sublimes on the tube.  If too much arsenic is used for this experiment, a dark-brown incrustation will sublime upon the sides of the tube which will give an alliaceous smell.  If this sublimate should be deposited near the assay, then it resembles the white sublimate of arsenious acid.