H_{4}As_{2}O_{3} pyroarsenic acid. HAsO_{3} metarsenic acid. Several other acids of arsenic are also known. Not all of these can be obtained as free acids, since they tend to lose water and form the oxides. Thus, instead of obtaining arsenious acid (H_{3}AsO_{3}), the oxide As_{2}O_{3} is obtained: 2H_{3}AsO_{3} = As_{2}O_{3} + 3H_{2}O. Salts of all the acids are known, however, and some of them have commercial value. Most of them are insoluble, and some of the copper salts, which are green, are used as pigments. Paris green, which has a complicated formula, is a well-known insecticide. ~Antidote for arsenical poisoning.~ The most efficient antidote for arsenic poisoning is ferric hydroxide. It is prepared as needed, according to the equation Fe_{2}(SO_{4})_{3} + 3Mg(OH)_{2} = 2Fe(OH)_{3} + 3MgSO_{4}. ~Sulphides of arsenic.~ When hydrogen sulphide is passed into an acidified solution containing an arsenic compound the arsenic is precipitated as a bright yellow sulphide, thus: 2H_{3}AsO_{3} + 3H_{2}S = As_{2}S_{3} + 6H_{2}O, 2H_{3}AsO_{4} + 5H_{2}S = As_{2}S_{5} + 8H_{2}O. In this respect arsenic resembles the metallic elements, many of which produce sulphides under similar conditions. The sulphides of arsenic, both those produced artificially and those found in nature, are used as yellow pigments. ANTIMONY ~Occurrence.~ Antimony occurs in nature chiefly as the sulphide (Sb_{2}S_{3}), called stibnite, though it is also found as oxide and as a constituent of many complex minerals. ~Preparation.~ Antimony is prepared from the sulphide in a very simple manner. The sulphide is melted with scrap iron in a furnace, when the iron combines with the sulphur to form a slag, or liquid layer of melted iron sulphide, while the heavier liquid, antimony, settles to the bottom and is drawn off from time to time. The reaction involved is represented by the equation Sb_{2}S_{3} + 3Fe = 2Sb + 3FeS. ~Physical properties.~ Antimony is a bluish-white, metallic-looking substance whose density is 6.7. It is highly crystalline, hard, and very brittle. It has a rather low melting point (432 deg.) and expands very noticeably on solidifying. ~Chemical properties.~ In chemical properties antimony resembles arsenic in many particulars. It forms the oxides Sb_{2}O_{3} and Sb_{2}O_{5}, and in addition Sb_{2}O_{4}. It combines with the halogen elements with great energy, burning brilliantly in chlorine to form antimony trichloride (SbCl_{3}). When heated on charcoal with the blowpipe it is oxidized and forms a coating of antimony oxide on the charcoal which has a characteristic bluish-white color. ~Stibine~ (SbH_{3}). The gas stibine (SbH_{3}) is formed under conditions which are very similar to those which produce arsine, and it closely resembles the latter compound, though it is still less stable. It is very poisonous.