~Occurrence.~ Arsenic occurs in considerable quantities in nature as the native element, as the sulphides realgar (As_{2}S_{2}) and orpiment (As_{2}S_{3}), as oxide (As_{2}O_{3}), and as a constituent of many metallic sulphides, such as arsenopyrite (FeAsS). ~Preparation.~ The element is prepared by purifying the native arsenic, or by heating the arsenopyrite in iron tubes, out of contact with air, when the reaction expressed by the following equation occurs: FeAsS = FeS + As. The arsenic, being volatile, condenses in chambers connected with the heated tubes. It is also made from the oxide by reduction with carbon: 2As_{2}O_{3} + 3C = 4As + 3CO_{2}. ~Properties.~ Arsenic is a steel-gray, metallic-looking substance of density 5.73. Though resembling metals in appearance, it is quite brittle, being easily powdered in a mortar. When strongly heated it sublimes, that is, it passes into a vapor without melting, and condenses again to a crystalline solid when the vapor is cooled. Like phosphorus it can be obtained in several allotropic forms. It alloys readily with some of the metals, and finds its chief use as an alloy with lead, which is used for making shot, the alloy being harder than pure lead. When heated on charcoal with the blowpipe it is converted into an oxide which volatilizes, leaving the charcoal unstained by any oxide coating. It burns readily in chlorine gas, forming arsenic trichloride,-- As + 3Cl = AsCl_{3}. Unlike most of its compounds, the element itself is not poisonous. ~Arsine~ (AsH_{3}). When any compound containing arsenic is brought into the presence of nascent hydrogen, arsine (AsH_{3}), corresponding to phosphine and ammonia, is formed. The reaction when oxide of arsenic is so treated is As_{2}O_{3} + 12H = 2AsH_{3} + 3H_{2}O. Arsine is a gas with a peculiar garlic-like odor, and is intensely poisonous. A single bubble of pure gas has been known to prove fatal. It is an unstable compound, decomposing into its elements when heated to a moderate temperature. It is combustible, burning with a pale bluish-white flame to form arsenic trioxide and water when air is in excess: 2AsH_{3} + 6O = As_{2}O_{3} + 3H_{2}O. When the supply of air is deficient water and metallic arsenic are formed: 2AsH_{3} + 3O = 3H_{2}O + 2As. These reactions make the detection of even minute quantities of arsenic a very easy problem. [Illustration: Fig. 72]