_Compounds._--Arsenic forms two hydrides:--The _dihydride_, As2H2, is a brown velvety powder formed when sodium or potassium arsenide is decomposed by water. It is a somewhat unstable substance, decomposing on being heated, with liberation of hydrogen. Arsenic _trihydride_ (arsine or arseniuretted hydrogen), AsH3, is formed by decomposing zinc arsenide with dilute sulphuric acid; by the action of nascent hydrogen on arsenious compounds, and by the electrolysis of solutions of arsenious and arsenic acids; it is also a product of the action of organic matter on many arsenic compounds. 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 liquefies at -40 deg. C. and becomes solid at -118.9 deg. 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, the action being most violent in the case of chlorine. _Arsenic tribromide_, AsBr3, is formed by the direct union of arsenic and bromine, and subsequent distillation from the excess of arsenic; it forms colourless deliquescent prisms which melt at 20 deg.-25 deg. C., and boil at 220 deg. C. Water decomposes it, a small quantity of water leading to the formation of the _oxybromide_, AsOBr, whilst a large excess of water gives arsenious oxide, As4O6. Arsenic certainly forms two, or possibly three iodides. The _di-iodide_, As2I4 or AsI2, which is prepared by heating one part of arsenic with two parts of iodine, in a sealed tube to 230 deg. C., forms dark cherry-red prisms, which are easily oxidized, and are readily decomposed by water. The _tri-iodide_, AsI3, prepared by subliming arsenic and iodine together in a retort, by leading arsine into an alcoholic iodine solution, or by boiling powdered arsenic and iodine with water, filtering and evaporating, forms brick-red hexagonal tables, of specific gravity 4.39, soluble in alcohol, ether and benzene, and in a large excess of water; in the presence of a small quantity of water, it is decomposed with formation of hydriodic acid and an insoluble basic salt of the composition 4AsOI.3As4O6.24H2O. It combines with alkaline iodides to form very unstable compounds. The _pentaiodide_, AsI5, appears to be formed when a mixture of one part of arsenic and seven parts of iodine is heated to 190 deg. C., but on dissolving the resulting product in carbon bisulphide and crystallizing from this solvent, only the tri-iodide is obtained.