of the alkaline silicates possess the property of cooling, after fusion, to a perfectly amorphous mass, passing over gradually from the liquid, through an intermediate pasty state, to solidification. This property is retained by them even when they are mixed with other silicates which crystallize; nay more, they exert an influence over these, preventing their crystallization, and thus rendering them manageable in the hands of the glassblower.1

The silicates of the alkaline earths fuse only at a very high temperature. The silicates of lime and magnesia which correspond about to the formula MO.SiO, are the most fusible, requiring, however, the highest heat of a blast-furnace. The silicate of lime of that composition assumes a crystalline structure as it cools. The silicates of alumina are still more infusible; that corresponding to the formula AlО,.3SiO,, which appears the most fusible, cannot be liquefied in a blastfurnace. The silicates of iron and manganese are far more easily fusible, but become crystalline on cooling; the silicates of lead are the more readily fusible, in proportion to the amount of oxide of lead they contain.

With regard to the silicates obtained in the manufacture of glass, it is difficult to say whether they are really in chemical combination; thus much is certain, that the properties of different simple silicates undergo considerable modifications when mixtures are made; thus, we have already stated that the presence of alkaline silicates will prevent the silicates of iron and manganese from assuming a crystalline structure; again, it is found that the temperature at which a complex silicate fuses is always below the mean of the fusing point of its component silicates; indeed, sometimes even below that of the most fusible of the silicates present.

The choice of bases, as also the proportions, employed in the manufacture of glass, must of course be regulated by the application which the product is to receive. The effect produced upon the nature of glass by the different metallic oxides already mentioned may be briefly stated to be as follows:

Potassa and soda render the glass easily fusible; the soda adding to its brilliancy, but imparting to it a greenish tint; potassa does not tint the glass, but yields a somewhat less brilliant product.

Lime does not affect the color of the glass, but adds to its lustre, and also increases its hardness, while it decreases its fusibility. Alumina diminishes the fusibility of glass more than any other metallic oxide, while, on the other hand, oxide of lead renders it easily fusible, and also imparts to it a great degree of softness and brilliancy. Lead glass is also the most colorless, and possesses the

It may be mentioned under this head, that the so-called soluble glass is an alkaline silicate, and is prepared by fusing together, in an earthen crucible, 15 parts of sand, 10 parts of pearl-ashes (crude carbonate of potassa), and 1 part of charcoal. The charcoal aids the production of the silicate, by the conversion of the carbonic acid into carbonic oxide, which escapes more readily, and also by the reduction of sulphuric acid, which is present in pearl-ashes. Cold water does not dissolve the resulting mass, it only removes any foreign alkaline salts that may be present. Upon boiling the silicate thus purified with 5 parts of water, it gradually dissolves completely; the solution may be concentrated to a syrupy, sticky liquid which gelatinizes on cooling, and on exposure to air becomes a transparent, colorless, brittle, but not very hard glass, possessing an alkaline reaction; it is itself unalterable by exposure to air, but becomes coated with a film of alkaline salt (carbonate), which may be removed by cold water.

This substance has been applied for diminishing the combustibility of wood, stuffs, paper, &c., by coating such substances with it, whereby they become protected from air. It may also be employed as an excellent cement for glass and porcelain, and finally for silicifying objects made of chalk and gypsum, by impregnating them with a solution of the glass, and afterwards exposing them to the air. A considerable degree of hardness may thus be imparted to such objects; they are even thereby rendered susceptible of a high polish.

2 The only means by which these refractory silicates may be perfectly fused, is by the flame of the oxy-hydrogen blowpipe.

highest refractive power. The action of baryta is similar to that of lead. The fusibility of glass is also increased by iron and manganese, but iron is liable to color the glass, particularly if it is present as protoxide, the green color thereby produced being more intense than the brown color afforded by an equal quantity of sesquioxide. Hence, glass which has a green tint, from the presence of protoxide of iron, may be almost decolorized by oxidation. A small quantity of binoxide of manganese, when added to a glass containing protoxide of iron, destroys the green color imparted by the latter, whilst the protoxide of manganese produced does not impart any color to the glass; if more binoxide of manganese be used than is necessary to oxidize the protoxide of iron, an amethyst color is imparted to the glass.

Various other metallic oxides are frequently used in the manufacture of glass, not so much to alter its nature as to impart to it various tints; we shall presently return to a brief consideration of these. It is found, in accordance with the above observations, that those kinds of glass which possess a high specific gravity (from 2.8 to 3.6, in consequence of the great density of the silicates they contain) are the softest, and also possess the greatest brilliancy and refractive power, while those varieties that are far less fusible, possess less brilliancy and a lower specific gravity (2.37 to 2.6).

It appears that the composition of a glass is not only dependent upon the proportion of the ingredients mixed together, but upon the temperature employed; the tendency possessed by silicic acid to unite in large proportions with bases being somewhat counteracted by the difficult fusibility of high silicates. A very high temperature tends also to volatilize portions of the alkaline bases from glass; thus complex silicates are found to decrease gradually in fusibility by continued exposure to a high temperature; this is not only owing to the volatilization of the alkalies, but also to the extraction of alumina from the glass-pots by the silica liberated. Excess of base in glass is equally prone to act upon the pots in the opposite manner, extracting the silica.

We have already stated that glass (in the extended sense of the word) is amorphous; when in its most liquid state (at the highest heat of the furnace) it is of the consistence of thin syrup, and may, when in that state, be cast into moulds or sheets. As the temperature cools down to a bright red heat, the glass becomes very tough and tenacious, and may, when in that state, be blown into any form, or drawn out into threads.

If a mass of glass be allowed to cool in the air, the outer portion solidifies first, and forms a coating which resists any change of position that might otherwise be assumed by the interior particles in the act of cooling; hence these latter exert a certain degree of tension upon the external layer of glass, and a slight concussion of the latter, insufficient of itself to destroy its continuity, will determine the rupture of the mass. In order to avoid this inconvenience, glass vessels are generally annealed by slow cooling in ovens, the temperature of which is very gradually diminished; here the whole mass of glass solidifies almost simultaneously, and a state of permanent equilibrium is established among the particles.

The devitrification of glass, which is observed in masses which are exposed for a length of time to a temperature approaching to fusion, depends mainly upon the volatilization of alkali, but partly, also, upon a molecular change in the structure of the glass, resulting in a crystalline arrangement of the particles. Réaumur's porcelain, as perfectly devitrified glass has been termed, possesses a remarkable opacity and hardness, striking fire with steel.

If a drop of melted glass is allowed to fall into cold water, it assumes an elongated form, terminating in a point, as is well known in the case of the Rupert's drop. If the thin extremity be broken, the tension existing between the particles will be immediately exerted, and the whole drop will fall to powder.

Having briefly noticed the general nature and properties of glass, we shall confine ourselves to an account of the materials employed by the glass-maker, and of the composition of the principal varieties of glass, referring the reader to special works on the subject for the practical details of glass-making.

Sand is the most general source of silica, since it requires less preparation than other varieties of silicious minerals. Ordinary sands generally contain iron, lime, and clay, and sometimes organic matter. The clay is removed by levigation, the lime is harmless, but the iron is objectionable in most cases; though easily removed by hydrochloric acid, the expense is, in many instances, too great; hence, sand free from iron, such as is found at Alum Bay (in the Isle of Wight), in Norfolk, Lancashire, Sydney, and New Holland, is always preferred. The sand is generally heated in reverberatory furnaces before use, to expel organic matter, and to reduce it to a fine state of division.

Rock-crystal, massive quartz, and flint, are used for some kinds of glass; the fragments are heated to redness, and then thrown into water, when they may be very easily reduced to powder.

The potassa used is either common ashes or crude potashes, according to the quality of the glass. For soda, barilla or soda-ash is generally employed; soap-boilers' waste is also used for common kinds of glass; the sulphates of soda and potassa may likewise be advantageously employed, the sulphuric acid being reduced by means of a little carbon, and thus more readily expelled by the silica. The lime may be furnished by limestone of any description, if not too poor, or too rich in alumina and magnesia. An excess of lime in the manufacture of glass is avoided, since it extracts the silica from the pots, thus speedily destroying them. Lead-glass is made from litharge or red-lead, the latter being preferred, since it is in a finer state of division, and furnishes available oxygen for the oxidation of impurities. As these oxides, in the commercial state, both contain iron and copper, red-lead is generally purified for glass-making. Too much lead is also injurious to glass, rendering it too soft, and coloring it yellow. When lead is employed, potassa is used exclusively as alkali, since soda imparts a bluish tint to lead-glass.

When baryta is required (as in bottle-glass, to increase its fusibility), heavy spar (sulphate of baryta) is added.

Some silicious minerals, which are more or less easily fusible, are used at times in the manufacture of glass. The principal are basalt, clinkstone, loam, marl, pumice-stone, &c. Some may be used alone as glass, such as basalt; others require the addition of lime or alkali to increase their fusibility.

The presence of impurities in some of the materials employed in glass-making, and which frequently impart a color to the product, renders the use of decoloriz ing agents necessary at times. The principal substances used are, binoxide of manganese, arsenious acid, and saltpetre, all of which effect the oxidation of the impurities (such as carbon or iron).

Finally, broken glass (cullet) is always ground up and mixed with the materials for glass of a similar kind, before their introduction into the pots, the fusion of the mass being much assisted by its presence.

The production and fusion of glass is effected in reverberatory furnaces of various kinds, by hot flame-fires (the fuel employed is coal, or baked wood). The glass, or metal, as it is termed, is fused in large crucibles, or pots, of very refractory fire-clay, which have been very carefully annealed before use.

The different varieties of glass may be divided into two classes: first, those which consist mainly of silica, lime, and alkali, of which the principal are crownglass, window-glass, the various kinds of bottle-glass, and plate glass;1 the second

1 Bohemian glass, which belongs to this class, and is indispensable to the chemist on account of its difficult fusibility and power of resisting sudden changes of temperature, owes its properties especially to the circumstance of its containing only silica, potassa, and lime.

class comprises those glasses of which lead is an important ingredient, of these the principal are flint-glass, of which a kind is especially manufactured for optical purposes; strass or glass-paste, used for artificial gems, and glass-enamel.

The following tables exhibit the percentage composition of the principal kinds of glass, as determined from the analyses of various specimens::

We have already referred to the property possessed by various metallic oxides, of imparting different colors to glass; the extensive application it has received from the earliest times in the production of colored or stained glass, painted glass, and glass-paste, renders an enumeration of the various coloring substances

necessary.

Sesquioxide of iron, suboxide of copper, and gold (either in the state of purple of Cassius, terchloride of gold, or fulminating gold), are employed for obtaining a red color; the first of these three yields a brownish-red glass; the coloring power of the suboxide of copper is very intense, the smallest quantity rendering glass opaque; hence, in coloring glass with this oxide, the red glass is first prepared, and colorless glass coated with a thin layer of it (by what is termed flashing). The suboxide will readily seize oxygen during the fusion of the glass, producing the protoxide, which yields a green glass; hence reducing substances are generally added in fusions of this kind of glass.

Various shades of red, from carmine to rose, are obtained by the use of gold. Yellow glass is obtained by means of glass of antimony, consisting of a mixture of tersulphide of antimony and antimonious acid, produced by incomplete roasting of the former. Antimoniate of potassa is also used. Charcoal is sometimes used for imparting a brownish-yellow tint (in some kinds of bottle-glass), the color being produced by the distribution of carbon in a fine state of division through

The composition of Bohemian hard glass tubing (combustion-tubing), has been found by Rowney to be as follows:

Silicic acid 73.13, lime 10.43, alumina 0.30, sesquioxide of iron 0.13, magnesia 0.26, protoxide of manganese 0.46, soda 3.07, potassa 11.49.

the mass. Chloride of silver yields a brilliant yellow with glass containing alumina. Sesquioxide of uranium is now extensively employed for coloring glass yellow; it yields a brilliant color, exhibiting a greenish tinge.

A green color is obtained, as already stated, by (prot-) oxide of iron, but the color obtained by means of oxide of copper is far more brilliant and beautiful, particularly if the glass to be colored contains lead. If the glass is dull or translucent, the color produced by oxide of copper is not green, but blue.

Sesquioxide of chromium yields the finest green color. Oxide of cobalt is the only substance used to impart a blue color to glass. The ores of cobalt contain a number of other substances (arsenic, sulphur, copper, nickel, iron, &c.), which it is necessary to remove as far as possible, since they exert great influence over the blue color yielded by the cobalt. The ores are levigated, roasted, and afterwards fused with proper proportions of finely-divided quartz and potashes. A deep blue glass is thus obtained, which, when ground and washed, receives the name of cobalt smalts, and is employed for coloring glass.

Other colors (called mixed colors) are imparted to glass by using mixtures of the substances just enumerated. Thus binoxide of manganese and smalt yield a brown garnet color; orange is obtained by the use of silver and iron; flesh-color by iron and alumina, &c.

These various colors are either introduced into the fused glass, or a colored glass is first prepared, with which the colorless glass is coated, or lastly, colored lead-glasses are prepared and finely ground as pigments, with which paintings are made on the glass, and afterwards burnt in.

In the manufacture of artificial gems, a very brilliant colorless flint-glass, containing a great quantity of lead, is prepared and carefully fused with the various colors employed. Topaz is obtained by the use of sesquioxide of iron, purple of Cassius, and glass of antimony; a ruby color is produced by purple of Cassius; beryl, by oxide of cobalt and glass of antimony; garnet, by purple of Cassius, antimony, and binoxide of manganese; emerald, by the oxides of copper and chromium, &c.

Enamel-glasses, employed extensively for coating vessels of various kinds, either for ornament or protection, consist of easily fusible lead-glass, which is either transparent or colored as above, or rendered opaque or milky, by the uniform dissemination, throughout its mass, of fine particles of some substance which cannot be fused at the temperature at which the glass is made. The substances generally employed for producing opaque, or opalescent enamels, are bone-earth, binoxide of tin, or teroxide of antimony.

Enamels may also be applied, like the colored glasses, as pigments, for which purpose they are reduced to a fine state of division, and are burnt on to the vessels, which are exposed to the necessary heat in muffles.

For the analysis of glass, see Quantitative Analysis, Special Methods.

§ 139. SILICON AND HYDROGEN.-In the description of the preparation of silicon, mention was made of the retention of hydrogen by that substance, obtained by the action of potassium on the fluoride, when it is washed with water. A chemical compound, the silicide of hydrogen, appears to be formed, which, when freed from silicic acid, by treatment with hydrofluoric acid, burns with brilliancy in oxygen or air, water being always produced. It parts with its hydrogen, yielding silicon, when very strongly heated in a covered crucible.

SILICON AND CHLORINE-When heated in chlorine, silicon takes fire, forming terchloride of silicon, SiCl,, as a vapor, which condenses, upon cooling, to a mobile liquid.

This compound may also be prepared by passing chlorine over an intimate mixture of charcoal and flocculent silica, strongly heated in a porcelain tube, or earthenware retort. It is purified from excess of chlorine by agitation with mercury, and distillation. It is then obtained as a transparent, colorless liquid, of