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carbonic acid and dissolved limestone, deposits a fresh portion of the crystalline matter, and thus a new growth, or stalagmite, gradually rises to meet the stalactite which hangs from the roof. In this way a natural pillar of limestone is formed.

Calcic sulphate, CaSO,, occurs in nature, combined with 2H,O, as gypsum or alabaster. It is soluble in 400 parts of water, and is a very common impurity in spring water, giving rise to a hardness which cannot be removed by boiling. Gypsum, when moderately heated, loses its water, and is then called plaster of Paris; this, when moistened, takes up two atoms of water again and sets to a solid mass, and is therefore much used for making casts and moulds.

Calcic chloride (chloride of calcium), CaCl,, is formed when limestone or marble is dissolved in muriatic acid ; if the solution be then evaporated, colorless needle-shaped crystals are obtained. When these are dried, they form a porous mass which takes up moisture with great avidity, and is much used for drying gases.

114. STRONTIUM.— This element occurs in much smaller quantities than calcium, being found in only a few minerals. It likewise occurs in minute quantities in certain spring waters. The metal has a yellowish-white color, and is prepared by the action of a current of electricity on the fused chloride. It resembles calcium closely in its properties. When heated in the air, it burns, forming the oxide strontia.

The native salts of strontium, the carbonate and sulphate, are insoluble, and serve for the preparation of the remaining salts. The nitrate, Sr2NO3, and the chloride, SrCl2, are soluble in water. These are the only salts of this metal which are employed in the arts. They are used for the preparation of red fires, as the volatile

salts of strontium have the power of coloring the flame crimson.

115. BARIUM.

- Barium compounds occur somewhat more widely dispersed than those of strontium, the two most common barium minerals being the sulphate, or heavy spar, and the carbonate, or witherite. The metal barium has not yet been obtained in the coherent state ; but the metallic powder may be prepared in the same way as calcium and strontium, which it closely resembles in its properties.

The most important barium compounds are the chloride and the nitrate, used as tests for sulphuric acid, and for giving a green color to the flame in fireworks.

116. CHROMIUM. Chromium is a substance whose compounds do not occur very widely distributed, or in large quantities. They are, nevertheless, much used in the arts as pigments, many of them having a fine bright color. The chief ore of this metal is a compound of the oxides of chromium and iron, called chrome ironstone, found in America, Sweden, and the Shetlands. A compound of chromium, lead, and oxygen, called plumbic chromate (chromate of lead), is also found. Pure chromium appears to be the most infusible of all the metals, as it cannot be melted at a temperature sufficient to fuse and volatilize platinum.

Chromium forms three oxides : chromous oxide, Cro, chromic oxide, Cr,Og, and chromic peroxide, Cros, which, dissolved in water, has a strong acid reaction, and may then be called chromic acid. It neutralizes bases, and forms yellow or red salts, called chromates. The most common of these are known as the chromate and bichromate of potash, K,CrO, and K,Cr,Oy. The former is a yellow salt, and the latter a red salt, which

is made on a large scale, and used in the preparation of the various chrome pigments.

The chief of the insoluble chromates is plumbic chromate, PbCroq, or chrome yellow, obtained by precipitating a potassic chromate by a soluble lead salt, and largely used for dyeing and other purposes.

117. COBALT. — Cobalt is never found in a free state except in small quantities in meteoric iron. It is almost as infusible as iron. It is of a reddish-gray color, hard, and strongly magnetic. It is not used in a metallic state, but many of its compounds are remarkable for the beauty of their colors, and are used as pigments. It forms two oxides, CoO and Co,0z. The zaffre of merce is a very impure oxide of cobalt, mixed with two or three times its weight of sand. It is used for coloring glass. Thenard's blue, or artificial ultramarine, is a compound of cobaltic and aluminic oxides.

118. MANGANESE.-Manganese occurs in nature as an oxide, and it can be obtained, though with difficulty, in the metallic state, by heating the oxide very strongly with charcoal. The metal is of a reddish-white color; it is brittle, and hard enough to scratch glass. It decomposes water at the ordinary temperature with evolution of hydrogen ; it cannot be preserved in the air without undergoing oxidation, and must be kept under naptha, or in a sealed tube; it is slightly magnetic, and, like iron, combines with carbon and silicon. Metallic manganese is not used in the arts; but an alloy of this metal and iron is now made on a large scale, and used in the manufacture of steel. Some of its oxides are used for evolving chlorine from muriatic acid, and also for giving glass a purple color.

Manganese forms several well-marked oxides: (1) man

ganous oxide, MnO, a basic body, furnishing a series of salts; (2) manganic oxide, Mn,O2, which also forms salts, but of a much less stable character; (3) black oxide, MnO2, a neutral substance, occurring as an ore of manganese ; (4) and (5) two acid oxides, which give rise to salts known as manganates and permanganates.

Black oxide of manganese is found in many parts of Europe; but the most productive mines are in Thuringia and Moravia. More than 18,000 tons are used annually in England in the manufacture of bleaching powder.

THE RARER METALS.

The other metallic elements are very rare substances, and as yet are of very little practical importance.

119. Cadmium (Cd) is a white metal resembling tin, and is found in small quantities in the ores of zinc. It is remarkable for forming very fusible alloys. For example, an alloy of 15 parts of bismuth, 8 of lead, 4 of tin, and 3 of cadmium, melts at about 140° F. The sulphide is a valuable yellow pigment, and the iodide is used to some extent in photography.

120. Iridium (Ir) is the heaviest of the metals. An alloy of iridium and osmium is very hard, and is used for the points of gold pens. It forms three oxides, which readily pass one into another, giving rise to the variety of tints which characterize the solution of its salts. It is because of these changes of color that the metal is called iridium, from iris, the rainbow.

121. Tungsten, or wolfranium (W), forms with steel an alloy of remarkable hardness. WOg, or tungstic acid, combines with sodium to form sodic tungstate (tungstate of soda), Na,WO,, which is used in calico-printing, and in combination with starch to render linen and cotton fabrics incombustible.

122. Uranium (U) forms an oxide, UO, which gives a peculiar yellow color to glass, and also renders it fluorescent.

The other métals are of too little use or interest to be mentioned here.

SUMMARY.

The metals are seldom found in a free state, but usually combined with other elements as ores. Their separation from these ores is called reduction,

The most useful metals are iron, lead, copper, tin, and zinc.

The noble metals are gold, silver, mercury, and platinum.

The less useful metals are sodium, magnesium, aluminium, antimony, bismuth, and nickel.

The metals not used in a free state, but valuable for their salts, are potassium, calcium, strontium, barium, manganese, chromium, and cobalt.

Among the rarer metals, cadmium, iridium, tungsten, and uranium are of some interest.

Among the most important metallic compounds may be mentioned : salt, soda-ash, soda saltpetre, caustic soda, bicarbonate of soda, pearlash, saltpetre, caustic potash, lime, alum, glass, porcelain, green vitriol, blue vitriol, white vitriol, litharge, red lead, and white lead.

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