SULPHUR 457 phur, and melt at 248. After a time the pris- matic crystals will be found to consist of ag- gregations of minute octahedral crystals. "When a saturated solution of sulphur in hot turpen- tine cools, the first crystals formed will be prismatic, while those which are deposited when the solution is comparatively cool will be octahedrons. Roll sulphur or brimstone is at first prismatic, but after keeping becomes octahedral, and the change of form is attended with the evolution of heat. The amorphous variety of soluble sulphur is precipitated as a greenish white emulsion on adding acids to dilute solutions of alkaline polysulphides. This amorphous sulphur changes after a time into a mass of octahedral crystals. Ordinary sub- limed sulphur (flowers of sulphur) belongs to this variety, but always contains small quanti- ties of one of the insoluble modifications. The principal modifications of the insoluble variety of sulphur are : 1, an amorphous modification, obtained as a soft pasty mass, or magma, by decomposing bisulphide of chlorine with water, or by adding dilute hydrochloric acid to a solu- tion of a hyposulphite ; 2, a plastic form, ob- tained by pouring viscid sulphur raised to near- ly 600 into cold water. The effect of heat upon sulphur is remarkable. It begins to melt at about 239, and between 248 and 284 it is yellow, transparent, and limpid. As the tem- perature rises to 356 it becomes brown, and at last nearly black and opaque and quite viscid. At this point the temperature becomes station- ary for a time, although the supply of heat is kept up, in consequence of a molecular change which is going on. Soon the temperature again rises, and when it has reached about 500 the mass becomes more liquid, but retains considerable viscosity. If it is now suddenly cooled by pouring it in a small stream into cold water, a brown tenacious mass is produced, which may be drawn out into elastic threads having a specific gravity of only 1-957. In a few hours it becomes yellow and opaque, and passes into the octahedral form. If the duc- tile sulphur is heated to 212, the change is sudden, with a further rise of heat, from con- densation, to 230. Compounds. Sulphur forms with oxygen an interesting series of compounds : two anhydrous oxides, or anhy- drides, sulphurous anhydride, SOa, and sul- phuric anhydride, SO 3 ; two acids, sulphurous and sulphuric, formed by the union of these anhydrides respectively with water, and a fur- ther series of acids which have no correspond- ing anhydrides. The constitution of all these bodies is remarkably illustrative of the law of multiple proportions. The formulas of the acids are as follows : Hyposulphurous acid . . H,SO a Sulphurous acid = SO 2 + H 2 O = H 2 SO 3 Sulphuric acid = SO 3 + H 2 = H 2 SO 4 Thiosulphuric (sometimes called hyposulphuric) acid H 2 S 2 O 3 Dithionic acid H 2 8 2 O 6 Trithionic acid H 2 8 3 O 6 Tetrathionic acid H 2 S 4 O 6 Pentathionic acid H 2 S 5 O 6 Thiosnlphuric acid (Gr. delov, sulphur) is so called because it has the constitution of sul- phuric acid with a molecule of oxygen re- placed by one of sulphur. The last four acids in the table are called polythionic acids, because they contain varying proportions of sulphur united with constant proportions of the other elements. Sulphurous anhydride, S0 2 , formerly called sulphurous acid, is the only product when sulphur is burned in dry air or oxygen gas. When the combustion takes place in pure oxygen, it is found that on returning to its former temperature the gase- ous product is doubled in weight, but that its volume is unchanged. It is in fact formed by the condensation of one volume of oxygen and half a volume of sulphur vapor into one vol- ume. When required pure, sulphurous acid is usually _ obtained by the partial reduction of sulphuric acid. This is conveniently effected by boiling strong oil of vitriol with copper turnings or mercury. The reaction is shown in the following equation : Cu -f 2H 2 S0 4 = CuSo 4 + 2H a O + SOs. It may also be obtained by passing the vapor of sulphuric acid over red- hot platinum foil or sponge, the product being sulphurous anhydride and oxygen. (See OXY- GEN, vol. xii., p. 769.) Sulphurous anhydride is a colorless gas, having a density of 2'21. When subjected to a pressure of three atmos- pheres at common temperatures, or if cooled to F. at the ordinary pressure, it is condensed to a colorless, transparent liquid, which solidi- fies to a crystalline mass at 105. The liquid anhydride may be obtained in large quantities by passing the gas from the generator first through a small quantity of water to wash it, then through a tube surrounded by ice to re- move moisture, then through a tube contain- ing pieces of calcium chloride to dry it com- pletely, and finally through a worm, or into a receiver immersed in a mixture of salt and ice. It may be preserved in sealed glass tubes, or corked and wired soda bottles. Sulphurous anhydride dissolves in water, forming a solu- tion of sulphurous acid, H 2 SO S , which again decomposes by the application of gentle heat into the anhydride and water. Water at 60 absorbs about 45 times its volume of the gas, the resulting liquid having a density of 1'04. By exposure to the air the solution slowly passes into sulphuric acid. By cooling a satu- rated aqueous solution to 32, Dopping ob- tained the pure acid, H 2 SO S , in cubical crystals. A crystalline hydrate, S0 2 8H 2 O, according to Pierre, may also be obtained at a low tempera- ture, which melts at 39, suffering decomposi- < tion. Sulphurous acid is a powerful reducing agent, instantly discoloring acid solutions of manganates and chromates, reducing the latter to green oxides of chromium. It reduces the salts of gold, precipitating the metal in the metallic state, and is capable of taking the sec- ond molecule of oxygen from almost any me- tallic binoxide. Brewers often employ a solu- tion of sulphurous acid to wash out their beer
