It conducts electricity nearly as well as silver, and unlike silver is not tarnished by hydric sulphide. Nitric and sulphuric acids, when cold, do not act upon this metal. It dissolves in hydric chloride, forming aluminic chloride, and in potassic or sodic hydrate solutions with production of corresponding aluminates. At present, aluminium is principally employed in the manufacture of aluminium bronze, small weights, and optical instruments.

469. Aluminic Oxide, Al,O,, Alumina.-This compound is found native. Crystallized and colored by chromic oxide it forms the ruby and sapphire, which rank next to the diamond in hardness and value. In a pure massive form it is known as corundum, and this when ground constitutes emery. It may be artificially prepared by heating aluminic hydrate. It is a white powder, insoluble in water, inodorous and tasteless.

470. Aluminic Silicates.-These bodies form the chief constituents of clays, which result from the decomposition of feldspathic and silicious rocks, and are the basis of all kinds of pottery. Their adaptation for this purpose depends upon their plasticity when mixed with water, the readiness with which they may be moulded, and also upon their capability of solidifying when exposed to a high heat. After burning, the ware, though hard, is porous, and absorbs water with avidity, even allowing it to filter through. To prevent this, the ware is covered with a glassy coating, or glazed.

471. Porcelain consists of a mixture of decomposed feldspar (called kaolin), silica, and a small proportion of lime, the ingredients being carefully selected, and thoroughly ground and incorporated. When moulded into the proper form, the articles are dried and subjected to a high heat in a furnace, in which state the ware is called biscuit. They are then glazed by dipping them into a solution of powdered quartz and feldspar, which, when

heated, fuses into the ware, giving it a vitreous coating which adds to its compactness and strength. The partial fusion of the materials gives porcelain the beautiful semitransparency which distinguishes it from earthen-ware. In coloring porcelain, the patterns are printed on paper which is applied to the biscuit while the color is still moist. When the color is absorbed, the porcelain is subjected to another baking, which fixes the tint. In the finer kinds of porcelain the colors are mixed with a fusible glaze, and applied with a hair pencil. Common red pottery ware owes its color to ferric oxide (Fe,O,), and is glazed with a preparation of clay and plumbic oxide. Vessels thus coated are objectionable for domestic use, as the lead-glaze is sometimes dissolved by acids, producing poisonous effects. Bricks are unglazed. Stone-ware is a coarse kind of porcelain glazed with salt. Fire-bricks, muffles, and Hessian crucibles, are made of a pure, infusible clay, containing a large amount of silica. The beautiful blue pigment ultramarine is one of the aluminic silicates supposed to be colored by a sodic sulpho ferrate. A variety of clay known as fuller's-earth is also used to remove grease from woolen cloths.

I IV

472. Potassio-Aluminic Sulphate. KAIS,O,+12H,O, Alum.—Small quantities of this important salt are found native, but for commercial purposes it is prepared artificially by several different methods. In this country it is formed by treating alumina or clay with sulphuric acid, and, after the lapse of a few months, adding potassic sulphate or carbonate. The whole is then leached, and the alum separated from the solution by crystallization. Alum is used largely for purifying and preserving skins, for mordants in dyeing and calico-printing, for glazing paper, for hardening and whitening tallow, clarifying liquors, and in medicine as an astringent and caustic. Wood impregnated with it is almost incombustible. Alum has a sweetish, styptic taste, and is soluble in 18 parts of cold water, or in

its own weight of boiling water, the solution having an acid reaction. When heated, alum swells up into a light, puffy condition, at the same time giving off its water of crystallization, and leaving a white, anhydrous, infusible mass known as burni alum.

§ 3. The Platinum Group.

PLATINUM.-Symbol, Pt. Atomic Weight, 197; Quantivalence, II. and IV.; Specific Gravity, 21.5.

473. This rare metal is always found native, and usually associated with palladium, rhodium, and iridium. It also occurs alloyed with gold, copper, iron, and lead. Its chief sources are the mines of Mexico, Brazil, and the Ural Mountains. Platinum is of a grayish-white color, and closely resembles silver in appearance. It is one of the heaviest of metals, and when pure it scarcely yields in malleability to gold and silver; is very ductile, and takes a good polish. But the qualities which render it so useful, and in some cases indispensable to the chemist, are its extreme difficulty of fusion (being unaffected by any furnace heat), and the perfect manner with which it resists the action of almost all acids. It does not oxidize in the air at any temperature, and is not acted upon by simple acids. It is slowly dissolved by aqua regia. We have already alluded (259) to the power possessed by spongy platinum of condensing gases and causing the union of oxygen and hydrogen. Platinum-black is a preparation of the metal in a still more minute state of subdivision, and has the property of effecting chemical changes more energetically than platinum sponge. It may be produced by electrolyzing a dilute solution of the metal.

474. Platinic Tetrachloride, PtCl,, is obtained by dissolving platinum in aqua regia and evaporating the solution over the water-bath. It is a brownish-red substance, soluble in water and alcohol, forming a reddish-yellow solution.

It is a valuable reagent for potassic, rubidic, and cæsic compounds.

Rhodium, Ruthenium, Palladium, Iridium, and Osmium, are rare and generally found associated with platinum, which they resemble both in appearance and properties.

CHAPTER XXIII.

TIN, SILICON.

§ 1. Tin and its Compounds.

TIN.-Symbol, Sn. (Stannum) Atomic Weight, 118; Quantivalence, II. and IV.; Molecular Weight 236 (?); Specific Gravity, 7.29.

475. History and Occurrence.-Tin is a brilliant, silverwhite metal, which has been found native only in small quantities, and in few localities. It is obtained on a large scale by the decomposition in furnaces of various ores, of which the mineral cassiterite is the most important. It is softer than gold, slightly ductile and very malleable, and may be beaten into leaves one-fortieth of a millimetre thick. It melts at 442° F. The peculiar crackling sound given by tin when bent is due to a disturbance of its crystalline structure. It tarnishes but slightly on exposure to the air or moisture, and is therefore very valuable for domestic utensils. This property also renders it useful for coating other metals to prevent them from oxidizing. Sheet-iron coated with tin, with which it forms an alloy, constitutes common tin-ware.

476. Stannic Dioxide, SnO2, Oxide of Tin.-This compound exists in several modifications. It is found native, as the mineral cassiterite, in broad square prismatic crystals. More or less rounded by attrition, these crystals are met with in the alluvial deposits of rivers forming "stream-tin,"

from which metallic tin is obtained. Another modification may be obtained in the form of colorless prisms of the trimetric system, which are very hard and brilliant. Stannic dioxide is much used in the manufacture of enamels and opaque glasses. Britannia metal is a white alloy much resembling German-silver in appearance. It consists chiefly of tin and antimony in the proportion of 9 parts of the former to 1 part of the latter.

Titanium, Zirconium, Thorium, are but little known and comparatively unimportant. They are allied to tin by many of their properties.

§ 2. Silicon and its Compounds.

SILICON.-Symbol, Si. Atomic Weight, 28; Quantivalence, IV.; Molec. ular Weight 56 (?); Molecular Volume, 2; Specific Gravity, 2.49.

477. Silicon. This element is never found native, but may be prepared by decomposing silicic fluoride or chloride with sodium or aluminium. It has three allotropic states: first, amorphous silicon-a brown powder; second, a crystalline hexagonal variety resembling graphite; and a third, octahedral form which is exceedingly hard. It is of no importance except to the scientific chemist.

478. Silicic Dioxide, SiO,, Silica.-This compound is one of the most important and widely-distributed of substances; forming the bulk of the minerals, quartz, flint, agate, chalcedony, opal, etc., and of most sandstones, and sandy soils. It is also an essential and sometimes predominating ingredient in granite, and many other rocks. It exists in two modifications: one crystalline, the other amorphous. In both of these conditions it is almost infusible. By the intense heat of the oxy-hydrogen blow-pipe it is reduced to a transparent glass, and may be spun out into threads.

Both modifications of silica are insoluble in water, and in all acids, except the fluohydric, but it is dissolved by solu