187. Occurrence and Purification.--Sulphur occurs both free and combined, and is a very common element. It is found free in all volcanic regions, but Sicily furnishes most of it. Great quantities are thrown up from the interior of the earth during an eruption. The heat of volcanic action probably separates it from its compound, which may be CaSO4. Vast quantities of the poisonous SO2 gas are also liberated during an eruption, this being, in volume of gases evolved, next to H2O. S is crudely separated from its earthy impurities in Sicily by piling it into heaps, covering to prevent access of air, and igniting, when some of the S burns, and the rest melts and is collected. After removal from the island it is further purified by distilling in retorts connected with large chambers where it sublimes on the sides as flowers of sulphur (Fig. 43). This is melted and run into molds, forming roll brimstone. S also occurs as a constituent of animal and vegetable compounds, as in mustard, hair, eggs, etc. The tarnishing of silver spoons by eggs is due to the formation of silver sulphide, Ag2S. The yellow color of eggs, however, is due to oils, not to S. The main compounds of S are sulphides and sulphates. What acids do they respectively represent? Metallic sulphides are as common as oxides; e.g. FeS2, or pyrite, PbS, or galenite, ZnS, or sphalerite, CuFeS2, or chalcopyrite, etc. The most abundant sulphate is CaSO4, or gypsum. BaSO4, or barite, and Na2SO4, or Glauber's salt, are others. The only one of these compounds that is utilized for its S is FeS2. In Europe this furnishes a great deal of the S for H2SO4. S is obtained by roasting FeS2. 3 FeS2 = Fe3S4 + 2 S. 188. Uses. -The greatest use of S is in the manufacture of H2SO4. A great deal is used in making gunpowder, matches, vulcanized rubber, and the artificial sulphides, like HgS, H2S, CS2, etc. The last is a very volatile, ill- smelling liquid, made by the combination of two solids, S being passed over red-hot charcoal. It dissolves S, P, rubber, gums, and many other substances insoluble in H2O. 189. Sulphur Dioxide, SO2, has been made in many experiments. It is a bleaching agent, a disinfectant, and a very active compound, having great affinity for water, but it will not support combustion. Like most disinfectants, it is very injurious to the system. It is used to bleach silk and wool--animal substances-- and straw goods, which Cl would injure; but the color can be restored, as the coloring molecule seems not to be broken up, but to combine with SO2, which is again separated by reagents. Goods bleached with SO2 often turn yellow after a time. 190. SO2 a Bleacher. Experiment 107.-Test its bleaching power by burning S under a receiver under which a wet rose or a green leaf is also placed. Chapter XXXVIII. HYDROGEN SULPHIDE. Examine ferrous sulphide, natural and artificial. 191. Preparation.