containing it, without any flowing out. On a further desiccation the silicic acid parts with its combined water, and becomes aggregated and insoluble.

The properties of the silicates of earthy bases and of oxides of metals proper, which also enter into the composition of glass, and therefore require our examination, are quite different, in some respects, from those of the alkaline silicates.

When an intimate mixture of silicic acid and carbonate of lime (chalk or marble) is strongly calcined, combination always ensues, with the formation of silicate of lime; and if the carbonate constituted more than 36 per cent. of the weight of the mixture, the resulting mass is entirely soluble in muriatic acid. But the mass does not enter into a state of fusion unless the proportions of lime and silicic acid are confined within limits not very distant, and even in the proportions most favourable for fusion it requires the highest temperature attainable in a wind furnace for the mass to become quite liquid. The silicates of lime which contain 53, 62, and 75.5 per cent. of silicic acid are the most fusible (Berthier). The formulæ of these silicates are Ca O+ Si O,; 2 Ca O +3 Si O, and Ca O+ 2 Si 02. These compounds

are not vitreous, like the alkaline silicates, but stony and slightly translucent like porcelain, if they have been brought to a state of perfect fusion. Silicate of lime enters into the composition of plate-glass, window-glass, bottle-glass, and crown-glass.

It is difficult to form silicate of alumina by the calcination of a mixture of alumina and silicic acid, in consequence of the very intense heat requisite to

determine the combination. But the presence of some other bases, which form fusible silicates, greatly assists the union of the silicic acid with alumina. Silicates of alumina are found in great abundance in nature, most frequently in combination with other silicates. Clays are composed almost entirely of silicate of alumina; felspar and leucite are silicates of alumina and potash; analcime and albite are silicates of alumina and soda; prehnite, stilbite, mica, mesolite, tourmaline, and several other minerals are silicates of alumina and lime. All the proper silicates of alumina are quite infusible at the highest temperature of a wind furnace.

The variety of glass of which silicate of alumina always forms a constituent is bottle-glass.

If

Silicates of lead may be readily prepared by calcining in a common furnace mixtures in atomic proportions of silicic acid and oxide of lead. the mixture contains any quantity less than four equivalents of silicic acid to one of oxide of lead, it fuses readily; oxide of lead imparts fusibility to all silicates infusible by themselves, if applied in sufficient quantity.

Silicate of lead forms an essential constituent of flint-glass, crystal, and the kind of glass called strass. An excess of oxide of lead gives the glass a yellowish tint.

Protoxide of iron forms several definite compounds with silicic acid, some of which are met with in nature. Mixtures of silicic acid with the protoxide in proportions corresponding to the following formulæ, 4 Fe 0+ Si 02, 2 Fe O+ Si O2, Fe O + Si O2, and 2 Fe 0 + 3 Si O2, may be fused in a wind furnace ;

black or dark olive-grey compounds are thereby formed, all of which possess a crystalline or lamellar structure, with the exception of 2 Fe 0 + 3 Si O2, the fracture of which is conchoidal (Berthier). The dark colour of common bottle-glass is chiefly owing to the presence of one of the silicates of the protoxide of iron; and the first three of the above silicates are the basis of the slags of iron blast furnaces, in which they are often found in the state of regular crystals.

M. Berthier could not succeed in forming a silicate of peroxide of iron by igniting a mixture of

the peroxide with silicic acid. After exposure to a very intense heat, the mixture still remained in a pulverulent state, without any combination having taken place. In the presence of an easily fused silicate, peroxide of iron forms a greenish yellow compound with silicic acid.

The silicates of protoxide of manganese formed artificially by heating carbonate of manganese with silicic acid, are fusible, crystalline, and coloured either slightly red, green, or olive.

From the preceding brief description of the properties of all the silicates of which the common varieties of glass are composed, it appears, that neither of them separately is fit to make glass intended to bear exposure to the weather. The essential qualities required in common glass are transparency, absence of colour, moderate fusibility, and insolubility in water, which qualities are never found combined in a single silicate.

Among this class of bodies, permanent transparency and solubility in water are possessed only by

VOL. II.

silicates of the fixed alkalies, potash, soda, and lithia: all the remaining silicates, not excepting silicate of lead when it does not contain an excess of oxide of lead, are either perfectly opaque or translucent like porcelain, and quite insoluble in water, and most of them have a crystalline structure. Here we have the principles of a distribution of all simple silicates into two classes, which will be found of great convenience when considering the practical application of these bodies more in detail.

But although no simple silicate can be used as glass, for the reasons just mentioned, yet mixtures of different silicates may be obtained fulfilling all the required conditions. When a mixture of two silicates belonging to the opaque or crystalline class is strongly calcined, combination sometimes ensues, with the formation of a true double salt, the properties of which are not always the mean of those of its constituents. Such double silicates are generally more fusible and more translucent than their constituents separately. But no combination consisting of two opaque or translucent silicates only can be obtained artificially in a state of transparency; nor can a perfectly insoluble glass be prepared from mixtures in any proportions of the fixed alkaline silicates only.

From this class of bodies a combination of the qualities of transparency and insolubility in water, with absence of crystalline structure, can only be obtained (with a single exception),* by combining together silicates belonging to the different classes above referred to; and as such a combination, every ordi

The exception referred to is the silicate of oxide of lead, containing an excess of base. This substance forms a transparent but yellow glass.

nary variety of transparent glass may be considered. The earthy or metallic silicate thereby loses its opacity, and in a great measure its disposition to crystallize; while the alkaline silicate, on the other hand, becomes rendered insoluble. The temperature at which the combination of the two silicates softens or fuses is also much more convenient for working than that necessary for the fusion of the earthy or metallic silicate alone.

Potash or soda, or a mixture of both of these alkalies, forms an essential constituent, therefore, of all the common kinds of glass: the differences in the properties of the various kinds are principally produced by varying the quality and the proportion of the base of the earthy or metallic silicate. Considering glasses as true chemical, and therefore definite, combinations of different silicates, the various kinds may be distributed under the following heads:

1. Silicate of potash and oxide of lead;

2. Silicate of soda and lime, or silicate of potash, soda, and lime; 3. Silicate of potash and lime;

4. Silicate of soda, lime, alumina, and oxide of iron.

To the first class belong flint-glass, crystal, and strass; flint-glass contains more oxide of lead than crystal, and strass more than flint-glass.

Common window-glass, English crown-glass, and plate-glass constitute the second class in the above arrangement.

Foreign crown-glass and the refractory Bohemian glass are the double silicate of potash and lime.

The fourth class comprises the coarse green glass of which wine-bottles are made.