INORGANIC COMPOUNDS Arsenic Hydrides.—The dihydride is a brown velvety unstable powder formed when sodium or potassium arsenide is decomposed by water. Arsenic trihydride (arsine or arseniuretted hydrogen) is formed by decomposing zinc arsenide with dilute sulphuric acid, by the action of nascent hydrogen on ar senious compounds, and by the electrolysis of solutions of arse nious and arsenic acids; it is also a product of the action of organic matter on many arsenic compounds. It is conveniently prepared by dropping water on to aluminium arsenide, obtained by a modi fied thermite process. It is a colourless gas of unpleasant smell, excessively poisonous, very slightly soluble in water. It easily burns, forming arsenious oxide if the combustion proceeds in an excess of air, or arsenic if the supply of air is limited; it is also decomposed into its constituent elements when heated. It lique fies at —40°C and becomes solid at —118.9°C (K. Olszewski). Metals such as tin, potassium and sodium when heated in the gas form arsenides with liberation of hydrogen ; and solutions of gold and silver salts are reduced by the gas with precipitation of metallic gold and silver. Chlorine, bromine and iodine decompose arsine readily.
Arsenious oxide is used in glass-making as an oxidizing agent to remove the colour produced by the lower oxides of iron. It is employed in the manufacture of arsenic acid, pigments, enamels, sheep dips and arsenical soaps. It enters into the composition of germicides, insecticides and rat poisons. It also finds employment in calico printing, in the fixation of aniline colours, and as Fowler's solution (potassium arsenite) in medicine. Arsenious oxide arises as a volatile by-product in many smelting operations for the ex traction of metals, and in this form has been known from the earliest times, being called Huttenrauch (furnace smoke) by Basil Valentino. It occurs as an impurity or adulterant in many com mercial products, having been found in caustic soda, glucose, cos metics and in certain wines. In 190o an outbreak of "peripheral neuritis" with various skin affections occurred in Lancashire and was traced to beer made from glucose and invert sugar, in the preparation of which arsenical sulphuric acid had been used.
On concentration, an aqueous solution of arsenious oxide de posits the anhydrous oxide and not arsenious acid, But although this acid has not been isolated its salts are well known. Sodium arsenite and the other alkali arsenites are soluble in water. Silver arsenite, is a yellow precipitate soluble in acid or ammonia. Cupric arsenite, Scheele's green, is a green powder formerly used extensively for wallpapers and for calico printing. Schweinfurt green, also known as imperial green or emerald green, is a mixed copper metarsenite and acetate, employed as a pigment for wallpapers. By the combined action of damp and moulds,paper tinted with this pigment evolves a peculiar odour said to be due to diethylcacodyl oxide (Biginelli, 1900).
Arsenic pentoxide, is manufactured by oxidizing white arsenic (arsenious oxide) with nitric acid (sp. gravity 1.35). The solution on concentration yields crystalline ortho-arsenic acid H;,As04, At 16o° C this substance loses water and passes into the pentoxide, a deliquescent crystalline solid which at red heat evolves oxygen and reverts to the lower oxide. Arsenic acid was formerly employed in the manufacture of magenta (see