Tin

Tin (Sn = 118) is one of the metals known to the ancients, and of which mention is made in the earliest writings. It is, however, not widely distributed, the most abundant source of the ore being Cornwall, and there seems to be some evidence that the Phoenicians obtained the metal from tin-mines of that district. The chief ore - indeed, the only one employed for the extraction of the metal - is the dioxide SnO₂, known as tinstone, which also occurs in Mexico and Australia. For the purpose of obtaining the metal, the ore is first crushed and then washed, in order to get rid of the lighter earthy impurities. It is next roasted in a reverberatory furnace, and by this operation sulphur and arsenic, which are usually present, are expelled. It is again washed to dissolve soluble matters, chiefly copper sulphate, and finally reduced by heating it together with lime and coal-slack in a reverberatory furnace, the heat being raised very gradually. A slag is formed by the union of the lime with siliceous matters, which rises to the top, while the reduced metal sinks to the bottom, and is run off into pans or ingots. The metal is purified by heating slowly and pouring away the more easily fusible portion, which is further purified by melting in a large pot and stirring with a wooden pole. When pure, it is a bright white soft metal, very malleable, so that it may be beaten out into thin leaves (tinfoil) Just below its me1ting~point, however - 230° C. - it becomes very brittle. It has a crystalline structure, and a rod of the metal emits a peculiar crackling sound when bent. It does not oxidise on exposure to air, but, if very strongly heated, it burns with a bright white light. Dilute nitric acid oxidises it to a white oxide, and it is slowly attacked, by hydrochloric and sulphuric acids. It forms two series of salts, the stannous and stannic salts, and of these the most important are the chlorides. Stannous chloride (SnCl₂) is known commercially, as salts of tin. It forms bright prismatic hydrated crystals, which are soluble in water, but decomposed by excess. It is a strong reducing agent, and is largely employed as a mordant in dyeing and calico-printing. Stannic chloride (SnC1₄) is also used in a crude and impure state by dyers, and known as composition, while a double salt with salammoniac is also employed under the name of pink salt. The artificial stannic sulphide (SnS₂), called mosaic gold, is employed as a bronzing powder; while the dioxide (SnO₂) finds application as a polishing agent for stones, etc., and is extensively used by lapidaries, to whom it is known as putty powder. Tin unites very readily with other metals, e.g. 1ead or copper, and the alloys are in many cases most important products. Thus, Britannia-metal, pewter, Queen's-metal, and the various varieties of solder, are alloys, consisting chiefly of tin and lead. Speculum~metal, employed largely for optical purposes, is an alloy of tin and copper; while bronze, gun-metal and bell-metal contain also essentially the same constituents. The chief use of the metal is, however, for the manufacture of tin-plates. Such tin~plates are used for innumerable purposes, as e.g. the manufacture of basins, jugs, biscuit-tins, and the tin vessels for the preserved fruits, meats, etc, Chemically, tin is usually recognised by its sulphides, or by reduction before the blowpipe, and is quantitatively estimated in the form of the dioxide,