_Arsenic trichloride_, AsCl3, is prepared by distilling white arsenic with concentrated sulphuric acid and common salt, or by the direct union of arsenic with chlorine, or from the action of phosphorus pentachloride on white arsenic. It is a colourless oily heavy liquid of specific gravity 2.205 (0 deg. C.), which, when pure and free from chlorine, solidifies at -18 deg. C., and boils at 132 deg. C. It is very poisonous and decomposes in moist air with evolution of white fumes. With a little water it forms arsenic oxychloride, AsOCl, and with excess of water it is completely decomposed into hydrochloric acid and white arsenic. It combines directly with ammonia to form a solid compound variously given as AsCl3.3NH3, or 2AsCl3.7NH3, or AsCl3.4NH3. _Arsenic trifiuoride_, AsF3, is prepared by distilling white arsenic with fluorspar and sulphuric acid, or by heating arsenic tribromide with ammonium fluoride; it is a colourless liquid of specific gravity 2.73, boiling at 63 deg. C.; it fumes in air, and in contact with the skin produces painful wounds. It is decomposed by water into arsenious and hydrofluoric acids, and absorbs ammonia forming the compound 2AsF3.5NH3. By the action of gaseous ammonia on arsenious halides at -30 deg. C. to -40 deg. C., _arsenamide_, As(NH2)3, is formed. Water decomposes it into arsenious oxide and ammonia, and when heated to 60 deg. it loses ammonia and forms _arsenimide_, As2(NH)3 (C. Hugot, _Compt. rend._ 1904, 139, p. 54). For AsF5, see _Ber_., 1906, 39, p. 67.