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kinds of glass are easily fusible, while the second or potash glass is much less fusible; the addition of plumbic oxide increases the specific gravity and lustre of the glass as well as its fusibility. The common glass articles of household use are generally made of flint glass, while for chemical apparatus a soda-lime glass is to be preferred. The potash-lime glass is much employed where an infusible or hard glass is needed. The fourth kind of glass is an impure mixture of various silicates, employed for purposes

in which the colors and fineness of the glass are not of

consequence. In the preparation of all the fine qualities of glass, great care is requisite in the selection of pure materials, as well as in the processes of manufacture. Generally the materials are melted together with a quarter to a half their weight of cullet, or broken glass of the same kind.

After the glass articles have been blown or cast, they must be annealed, that is, made less brittle by being very slowly heated and as slowly cooled in an oven arranged for the purpose.

Certain metallic oxides have the power of coloring glass when they are added in small quantity. Thus ferrous oxide produces a deep green color (bottle-glass), while the oxides of manganese impart a purple tint to glass. Advantage is taken of this in the preparation of colorless glass, for as it is difficult to obtain materials perfectly free from iron, which imparts a green color, a small quantity of black oxide of manganese is added to the mixture, and the violet color thus produced neutralizes the green, and a nearly colorless glass is the result. The addition of arsenious oxide effects the same end by oxidizing the ferrous to ferriç oxide.

The colors of precious stones are imitated by adding certain oxides to a brilliant lead glass called paste; thus the blue of the sapphire is given by a small quantity of cobaltic oxide,

while cuprous oxide imparts a ruby-red color, and ferric oxide a yellow color resembling topaz.

The various forms of porcelain and earthen-ware consist of aluminic silicate or clay in a more or less pure state, covered with some substance which fuses at a high temperature, and forms a glaze, giving a smooth surface and binding the material closely together, and thus filling up the pores of the baked clay. For the manufacture of porcelain the finest kaolin or China clay is used, while for common earthen-ware an inferior clay may be employed. The glaze used for porcelain is generally finely powdered felspar, the biscuit, or porous ware, being first dipped into a vessel containing this substance suspended in water and then strongly heated. The articles thus coated can be used for chemical purposes, as this glaze withstands the action of acids. For earthen-ware the so-called salt glaze is used; obtained by throwing some common salt into the furnaces containing the strongly heated ware. The salt is volatilized and undergoes decomposition on the heated surface, depositing a fusible silicate upon it, and rendering the ware impervious to moisture.

109. SALTS OF ANTIMONY.- Antimony has two oxides, Sb,Оg, and Sb,Og, and two corresponding sulphides and chlorides. The most important of its salts is tartar-emetic, or tartrate of antimony and potash, used in medicine.

110. SALTS OF BISMUTH. Bismuth forms two oxides, analogous to those of antimony. The most important of the salts is the nitrate, Bi3NO,, which is easily procured by dissolving the metal in nitric acid.

III. SALTS OF NICKEL.

There are

two

oxides of nickel, Nio, and Ni,Og. The former of these gives rise to salts which have a peculiar apple-green color. The latter is a black powder, prepared by adding a solution of bleaching-powder to a soluble nickel salt. The most important soluble salt is the nitrate, Ni2NO3.

METALS NOT USED IN A FREE STATE, BUT

VALUABLE FOR THEIR SALTS.

There are certain metals which are not used in a free state, but which form salts of considerable importance. These metals are potassium, calcium, strontium, barium, manganese, chromium, and cobalt.

112. POTASSIUM. This metal was discovered in the year 1807, by Sir Humphrey Davy, who decomposed potash by means of a powerful galvanic current. Before this time the alkalies and alkaline earths were supposed to be elementary bodies. The metal is now prepared by heating together potash and carbon to a high temperature in an iron retort.

Potassium is a bright, silver-white metal, which can be easily cut with a knife at the ordinary temperature; it is brittle at 32°, and melts at 144o. It rapidly absorbs oxygen when exposed to the air, and becomes converted into a white oxide.

The original source of potassic compounds is in the felspar of the granitic rocks, as these contain from .02 to .03 of this metal. Up to the present time, no cheap and easy mode has been found of separating the potash from the silicic acid with which it is combined in felspar. Plants, however, are able slowly to separate out and assimilate the potash from these rocks and soils; so that, by burning the plant and leaching the ashes with water, soluble potash-salt is obtained.

This is the crude potassic carbonate, called, when purified by crystallization, pearlash. It is the substance from which most of the potassic compounds are obtained.

The most important compounds of potassium are caustic potash, HKO, potassic carbonate (carbonate of potash), K,CO2, and potassic nitrate, or saltpetre, KNO3. This last compound readily parts with its oxygen, and is therefore extensively used in making gunpowder and fireworks.

Gunpowder consists of an intimate mixture of saltpetre, charcoal, and sulphur. When the gunpowder is ignited, the sulphur and charcoal burn at the expense of the oxygen in the saltpetre, while the nitrogen is set free. It is to the expansive force of the large volume of gases suddenly set free in a confined space, that the destructive power of gunpowder is due.

Potassic chlorate, KCIO,, gives up its oxygen even more readily than the nitrate, and is used, as we have seen, in the preparation of oxygen (10). It is also used in making fireworks and percussion caps.

113. CALCIUM.

Calcium forms a considerable portion of the rocks of which the earth is composed, and occurs in very large quantities, forming whole mountain chains of chalk, gypsum, and limestone. The metal is obtained by the decomposition of the chloride by the electric current, or by heating the iodide with sodium. It is a light, yellow metal, which easily oxidizes in the air. When heated in air, it burns with a bright light, forming lime, CaO, the only oxide of calcium.

Lime is prepared on a large scale for building and other purposes, by heating limestone (the carbonate) in kilns by means of coal mixed with the stone; the carbonic acid escapes, and quicklime or caustic lime re

Pure lime is a white, infusible substance, which

combines with water very readily, giving off great heat, and falling to a white powder called calcic hydrate, or slaked lime. The hydrate is slightly soluble in water, I part of it dissolving in 730 parts of cold, but only in 1,300 parts of boiling water, and forming lime-water, which, like the hydrate, has a great power of absorbing carbonic acid from the air. It is indeed partly owing to this property that the hardening or setting of mortars and cements made from lime is due. Mortar consists of a mixture of slaked lime and sand. A gradual combination of the lime with the silica occurs, and this helps to harden the mixture. Hydraulic cement, which hardens under water, is prepared by carefully heating an impure lime containing clay and silica; a compound silicate of lime and alumina appears to be formed on moistening the powder, which then solidifies, and is not acted upon by water. Lime is largely used in agriculture, its action being, (1) to destroy the excess of vegetable matter contained in the soil; and (2) to liberate the potash for the use of the plants from heavy clay soils by decomposing the silicate.

Calcic carbonate (carbonate of lime) CaCO, occurs most widely diffused, as chalk, limestone, coral, and marble; many of those enormous deposits being made up of the microscopic remains of minute sea-animals. The carbonate is almost insoluble in pure water ; but readily dissolves when the water contains carbonic acid, and is deposited again, in crystals, as the gas escapes. In this way enormous masses of crystalline limestone are formed. Water charged with carbonic acid and calcic carbonate makes its way through the roof of limestone caverns, and, as the carbonic acid gradually escapes, the calcic carbonate is deposited in dependent masses, like icicles, termed stalactites, while the water, falling on the floor of the cavern before it has parted with all its

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