Many organic arsenic compounds are known, analogous to those of nitrogen and phosphorus, but apparently the primary and secondary arsines, AsH2.CH3 and AsH(CH3)2, do not exist, although the corresponding chlorine derivatives, AsCl2.CH3, methyl arsine chloride, and AsCl(CH4)2, dimethyl arsine chloride, are known. The tertiary arsines, such as As(CH3)3, trimethyl arsine, and the quaternary arsonium iodides and hydroxides, (CH3)4AsI and (CH3)4As.OH, tetramethyl arsonium iodide and hydroxide, have been obtained. The arsines and arsine chlorides are liquids of overpowering smell, and in some cases exert an extremely irritating action on the mucous membrane. They do not possess basic properties; the halogen in the chlorine compounds is readily replaced by oxygen, and the oxides produced behave like basic oxides. The chlorides AsCl2.CH3 and AsCl(CH3)2 as well as As(CH3)3 are capable of combining with two atoms of chlorine, the arsenic atom apparently changing from the tri- to the penta-valent condition, and the corresponding oxygen compounds can also be oxidized to compounds containing one oxygen atom or two hydroxyl groups more, forming acids or oxides. The compounds of the type AsX5, e.g. AsCl4.CH3, AsCl3(CH3)2, on heating break down, with separation of methyl chloride and formation of compounds of the type AsX3; the breaking down taking place more readily the fewer the number of methyl groups in the compound. The dimethyl arsine (or cacodyl) compounds have been most studied. On distillation of equal parts of dry potassium acetate and arsenious oxide, a colourless liquid of unbearable smell passes over, which is spontaneously inflammable and excessively poisonous. It is sometimes called Cadet's fuming liquid, and its composition was determined by R. Bunsen, who gave it the name cacodyl oxide ([Greek: kakodes], stinking); its formation may be shown thus: As4O6 + 8CH3CO2K = 2[(CH3)2As]2O + 4K2CO3 + 4CO2. The liquid is spontaneously inflammable owing to the presence of free cacodyl, As2(CH3)4, which is also obtained by heating the oxide with zinc clippings in an atmosphere of carbon dioxide; it is a liquid of overpowering odour, and boils at 170 deg.C. Cacodyl oxide boils at 150 deg. C., and on exposure to air takes up oxygen and water and passes over into the crystalline cacodylic acid, thus: [(CH3)2As]2O + H2O + O2 = 2(CH3)2As.O.OH.