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the same time, much steam and heat. This is called the slaking of lime. In this process, the water unites to the lime, and becomes solid; for slaked lime is quite dry. It is, therefore, called a hydrate of lime.

Lime is soluble in water: the solution has an acrid taste, and is called lime-water. When limewater is exposed to the air, a stony film forms upon the surface, owing to the lime attracting carbonic acid, and returning to the state of carbonate, which is insoluble in water. This film breaks, falls down, and is succeeded by others in succession. Fresh quicklime has a strong tendency to attract moisture from the air, and also carbonic acid, so that it must be kept in closely-stopped vessels.

Quicklime is used for making mortar for building, by mixing it with sand. This, by solidifying the water and attracting carbonic acid, becomes a very hard substance like stone. . The lime should be newly burnt, and the sand silicious and free from impurities. It is also extremely valuable as a manure when put upon the land.

Carbonate of lime is not caustic, nor soluble in water. It is decomposed by the stronger acids. Put chalk or marble into a vessel, and pour upon it diluted sulphuric or muriatic acid ; an effervescence will ensue, which is owing to the escape of the carbonic acid. Hence these acids are employed to distinguish lime-stones.

Lime combines with phosphorus, forming phosphuret of lime, to be afterwards described. With sulphur it forms sulphuret of lime.

It also combines with all the acids, forming a great number of neutral salts.

Sulphate of lime, called also gypsum, exists largely in a natural state. When burnt, it forms the sub

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stance called plaster of Paris, so much employed in making casts of statues, and in plastering

rooms.

Fluate of lime, or lime united to the fluoric acid, is the substance so well known by the name of Derbyshire spar, and which is much used for vases and other ornaments.

Nitrate of lime is a very soluble salt; its taste is acrid and bitter. It is often found efflorescing on old plaster walls.

MAGNESIA.

This earth, when pure, is white, nearly destitute of taste and has no smell. It is insoluble in water, but changes vegetable blues to green, and unites to the acids. It is conjectured to be composed of a metallic base, magnesium, united to oxygen; but the metal has not yet been distinctly obtained. .

Native magnesia is a rare substance, but it enters as a constituent in many rocks, as serpentine, steatite, &c.

Carbonate of magnesia is extensively employed as a medicine. When a red heat is applied to it it loses its carbonic acid, and becomes calcined magnesia.

Sulphate of magnesia is known by the name of Epsom salt, because formerly procured from the springs of Epsom, in Surrey.

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BARYTES.

This earth was formerly called terra ponderosa, from its great specific gravity. It has strong alkaline properties, a caustic taste, and changes vegetable blues to green.

It slakes in the air like lime, is soluble in water, and also in alkohol, the flame of which it causes to assume a yellow colour. It is a deadly poison. It is also found to consist of oxygen and a metallic base called barium.

Carbonate of barytes is found as a mineral, but it is not abundant.

Sulphate of barytes is found native more frequently. When calcined, it forms the Bolognian phosphorus.

Barytes is used as a white paint under the name of permanent white, not being liable to change its colour.

STRONTIA.

The name of this earth is derived from Strontian, in Argyllshire, in Scotland, where it was first discovered by Dr. Hope.

It is soluble in water, and changes vegetable blues to green.

It is also considered to be the oxide of a metal called strontium.

Strontia is not very abundant, and is always in nature found combined with the carbonic or sulphuric acids.

The other salts of strontia are but little known. All the salts of Strontia have the property of tingeing the flame of alkohol red.

SILICA.

This earth, which forms a large portion of the surface of the earth, exists nearly pure in flint and rock-crystal: hence it has been called the earth of Aints.

It may be obtained pure as follows: calcine gun-flints till they become brittle, then pulverize them. Mix this powder with three or four times its weight of carbonate of potash, and fuse the mixture in a crucible, by a strong red heat. We shall thus obtain a compound of alkali and siliceous earth: dissolve it in water, and add to it diluted muriatic or sulphuric acid; a precipitation will take place, which, when well washed, is pure silex.

Siliceous earth, when pure, is white and tasteless. . It is infusible by itself, and insoluble in water. It has a harsh feel, and does not form a cohesive mass with water.

No acid can act upon it, except the hydrofluoric, which dissolves it. When mixed with an equal weight of carbonate of potash, and fused in a strong furnace, it forms glass. With a larger proportion of alkali it forms a substance soluble in water, which has been called silicated alkali. The solution of this was called liquor of flints. The silex is precipitated from it in the state of a gelatinous hydrate by acids.

It is supposed that silica consists of oxygen united to a certain base, which has been assumed to be a metallic substance, and which has been called Silicium : but its real nature has not been ascertained. It is imagined, however, that silicium forms an alloy with iron, and that the properties of some sorts of iron are owing to the addition of this substance.

ALUMINA.

This earth forms a part of all clays, and hence has been called argillaceous earth. It exists also in numerous rocks, particularly slate, and even constitụtes some of the hardest gems and stones, as the sapphire, ruby, and corundum.

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It rarely occurs in a pure unmixed state. But it has been found native, in small masses, at Newhaven, in Sussex, and also in Hall, in Saxony.

Clay consists of this earth, joined to silex. Porcelain clay proceeds from the decomposition of felspar; it consists of silica, alumina, and sometimes a little lime and potash. Pipe-clay, and potters'-clay are pure clays, but of variable composition.

Alumina has no smell nor taste; is insoluble in water, but forms with it a ductile paste, and shrinks much when exposed to beat. It is dissolved by the liquid fixed alkalies, and unites chemically with barytes, strontia, lime, and magnesia. It is dissolved by most of the acids.

The salt called alum, which gives its name to this earth, is a sulphate of alumina and potash. Sulphate of alumina alone will not crystallize; but when sulphate of potash is added, octahedral crystals of alum are produced. When alum is exposed to heat, it loses part of the acid and water of crystallization, becomes light and spongy, and is called burnt alum. Alum is extensively employed in the arts of dyeing and calico printing, in consequence of the attraction which alumina has for colouring matter. Alumina also forms the basis upon which are precipitated certain colours used as pigments.

YTTRIA.

This rare earth, so called from Ytterby, in Sweden, where it was discovered, is found only in a stone called gadolinite, so named from Professor Gadolin. It is insipid, and insoluble in water, but dissolves in carbonate of ammonia. It forms salts

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