The illuminating gas obtained from coal consists essentially of free hydrogen, marsh-gas, olefiant gas, and carbonic oxide, with small quantities of acetylene, benzole vapour, and some other substances.

A fair general idea of its composition is given by the following table :

The only constituents which contribute directly to the illuminating value of the gas are the marsh-gas, olefiant gas, oil-gas (acetylene, and benzole vapour).

The most objectionable constituent is the sulphur present as sulphuretted hydrogen and bisulphide of carbon, for this is converted by combustion into sulphuric acid, which seriously injures pictures, furniture, &c. The object of the manufacturer of coal-gas is to remove, as far as possible, everything from it, except the constituents mentioned as essential, and at the same time to obtain as large a volume of gas from a given weight of coal as is consistent with a good illuminating value.

The mode of purifying the gas, and the general arrangements for its manufacture, will be described in a later part of the work.

The destructive distillation of coal may be exhibited with the arrangement represented in fig. 109. The solid and liquid products (tar, ammoniacal liquor, &c.) are condensed in the globular receiver (A). The first bent tube contains, in one limb (B), a piece of red litmus paper to detect ammonia; and in the other (C)

102

QUARTZ SAND-FLINT.

a piece of paper impregnated with acetate of lead, which will be blackened by

A

Fig. 110.

the sulphuretted hydrogen. The second bent tube (D) contains enough lime-water to fill the bend, which will be rendered milky by the carbonic acid. The gas is collected over water, in the jar E, which is furnished with a jet from which the gas may be burnt when forced out by depressing the jar in water.

The presence of acetylene in coal-gas may be shown by passing the gas from the supply-pipe (A, fig. 110), first through a bottle (B) containing a little ammonia, then through a bent tube (C), with enough water to fill the bend, and a piece of bright sheet copper immersed in the water in each limb. After

a short time the bright red flakes of the acetylide of copper will be seen in the

water.

SILICON.

78. In many of its chemical relations to other bodies this element will be found to bear a great resemblance to carbon; but whilst carbon is remarkable for the great variety of compound forms in which it is met with in nature, silicon is always found in combination with oxygen, as silicic acid, or silica (SiO2), either alone or united with various metallic oxides, with which it forms silicates.

Silica. The purest natural variety of silica is the transparent and colourless variety of quartz known as rock crystal, the most widely diffused ornament of the mineral world, often seen crystallised in beautiful sixsided prisms, terminated by six-sided pyramids (fig. 111), which are always

Fig. 111.-Crystal of quartz.

easily distinguished by their great hardness, scratching glass almost as readily as the diamond. Coloured of a delicate purple, probably by a little organic matter, these crystals are known as amethyst; and when of a

brown colour, as Cairngorm stones or Scotch pebbles. Losing its transparency and crystalline structure, we meet with silica in the form of chalcedony and of carnelian, usually coloured, in the latter, with oxide of iron.

Hardly any substance has so great a share in the lapidary's art as silica, for in addition to the above instances of its value for ornamental purposes, we find it constituting agate, cat's eye, onyx, so much prized for cameos, opal, and some other precious stones. In opal the silica is combined with water. Sand, of which the whiter varieties are nearly pure silica, appears to have been formed by the disintegration of siliceous rocks, and has generally a yellow or brown colour, due to the presence of oxide of iron.

The resistance offered by silica to all impressions has become proverbial in the case of flint, which consists essentially of that substance coloured with some impurity. Flints are generally found in compact masses, distributed in regular beds throughout the chalk formation; their hardness, which even exceeds that of quartz, formerly rendered them useful for striking sparks with steel, by detaching small particles of the metal, which are so heated by the percussion as to continue to burn (see p. 10) in the air, and to inflame tinder or gunpowder upon which they are allowed to fall.

SILICA RENDERED SOLUBLE.

103

The part taken by silica in natural operations appears to be chiefly a mechanical one, for which its stability under ordinary influences peculiarly fits it, for it is found to constitute the great bulk of the soil which serves as a support and food-reservoir of land-plants, and enters largely into the composition of the greater number of rocks.

But that this substance is not altogether excluded from any share in life is shown by its presence in the shining outer sheath of the stems of the grasses and cereals, particularly in the hard external coating of the Dutch rush used for polishing; and this alone would lead to the inference that silica could not be absolutely insoluble, since the capillary vessels of plants are known to be capable of absorbing only such substances as are in a state of solution. Many natural waters also present us with silica in a dissolved state, and often in considerable quantity, as, for example, in the Geysers of Iceland, which deposit a coating of silica upon the earth around their borders.

Pure water, however, has no solvent action upon the natural varieties of silica. The action of an alkali is required to bring it into a soluble form.

To effect this upon the small scale, a few crystals of common washingsoda (carbonate of soda) may be powdered and dried; a little of the dried powder is placed upon a piece of platinum foil slightly bent up (fig. 112),

and is fused by directing the flame of a blowpipe upon the under side of the foil. As soon as the carbonate of soda is perfectly liquefied, a small quantity of very finely powdered white sand is thrown into it, when brisk effervescence will be observed, and the particles of sand will dissolve; fresh portions of sand may now be added as long as they produce effervescence, which is due to the escape of the carbonic acid, and since, in general, one acid can only be displaced by another, it is but reasonable to infer that the sand really possesses acid properties, and hence the fitness of its chemical name, silicic acid.

The piece of platinum foil with the melted mass upon it may now be placed in a little warm water, and allowed to soak for some time, when it will gradually dissolve, forming a solution of silicate of soda. This solution will be found decidedly alkaline to test-papers; for silicic acid, like carbonic, is too feeble an acid to neutralise entirely the alkaline properties of the soda.

If a portion of the solution of silicate of soda in water be poured into a test-tube, and two or three drops of hydrochloric acid added to it with occasional agitation, effervescence will be produced by the expulsion of any carbonic acid still remaining, and the solution will be converted into

a gelatinous mass by the separation of hydrated silicic acid. another portion of the solution of silicate of soda be poured into an excess of dilute hydrochloric acid (ie., into enough to render the solution distinctly acid), the silicic acid will remain dissolved in the water, together with the chlorde of sodium formed by the action of the hydrochloric acid upon the soda.

In order to separate the chloride of sodium from the silicic acid, the process of dialysis* must be resorted to.

Dialysis is the separation of dissolved substances from each other by taking advantage of the different rates at which they pass through moist diaphragms or septa.

If the mixed solution of chloride of sodium and silicic acid were poured upon an ordinary paper filter, it would pass through without alteration; but if parchment paper be employed, which is not pervious to water, although readily moistened by it, none of the liquid will pass through. If the cone of parchment paper be supported upon a vessel filled with distilled water (fig. 113), so that the water may be in contact with the outer surface of the cone, the hydrochloric acid and the chloride of sodium will pass through the substance of the parchment paper, and the water charged with them may be seen descending in dense streams from the outside of the cone. After a few hours, especially if the water be changed occasionally, the whole of the hydrochloric acid and chloride of sodium will have passed through, and a pure solution of silicic acid in water will remain in the cone.

Fig. 113.

This solution of silicic acid is very feebly acid to blue litmus paper, and not perceptibly sour to the taste. It has a great tendency to set into a jelly in consequence of the sudden separation of hydrated silicic acid. If it be slowly evaporated in a dish, it soon solidifies; but, by conducting the evaporation in a flask, so as to prevent any drying of the silicic acid at the edges of the liquid, it may be concentrated until it contains 14 per cent. of silicic acid. When this solution is kept, even in a stoppered or corked bottle, it sets into a transparent gelatinous mass, which gradually shrinks and separates from the water. When evaporated, in vacuo, over sulphuric acid, it gives a transparent lustrous glass which is composed of 22 per cent. of water and 78 per cent of silicic acid (HO.SiO2).

This hydrate of silica cannot be redissolved in water, and is only soluble to a slight extent in hydrochloric acid. If it be heated to expel the water, the anhydrous silicic acid which remains is insoluble both in water and in hydrochloric acid, but is dissolved when boiled with solution of potash or soda, or their carbonates.

Silicic acid in the naturally crystallised form, as rock crystal and quartz, is insoluble in boiling solutions of the alkalies, and in all acids except hydrofluoric; but amorphous silica (such as that found at Farnham) is readily dissolved by boiling alkalies. These represent, in fact, two distinct modifications of silica. A transparent piece of rock crystal may be heated to bright redness without change, but if it be powdered previously to being heated, its specific gravity is diminished from 2.6 to 2.4, and it becomes soluble in boiling alkalies, having been converted into the amorphous modification.

* From diaλúw, to part asunder.

ACID CHARACTER OF SILICA.

105

When

Crystals of quartz have been obtained artificially by the prolonged action of water upon glass at a high temperature under pressure. fused with the oxyhydrogen blowpipe, silica does not crystallise, being thus converted into the amorphous variety of sp. gr. 2·3.

To prepare the amorphous modification of silica artificially, white sand in very fine powder may be fused, in a platinum crucible, with six times its weight of a mixture of equal weights of carbonate of potash and carbonate of soda, the mixture being more easily fusible than either of the carbonates separately. The crucible may be heated over a gas-burner supplied with a mixture of gas and air, or may be placed in a little calcined magnesia contained in a fire-clay crucible, which may be covered up and introduced into a good fire. The platinum crucible is never heated in direct contact with fuel, since the metal would become brittle by combining with carbon, silicon, and sulphur derived from the fuel. The magnesia is used to protect the platinum from contact with the clay crucible. When the action of the silicic acid upon the alkaline carbonates is completed, which will be indicated by the cessation of the effervescence, the platinum crucible is allowed to cool, placed in an evaporating dish, and soaked for a night in water, when the mass should be entirely dissolved. Hydrochloric acid is then added to the solution, with occasional stirring, until it is distinctly acid to litmus paper. On evaporating the solution, it will, at a certain point, solidify to a gelatinous mass of hydrated silicic acid, which would be spirted out of the dish if evaporation over the flame were continued. To prevent this, the dish is placed over an empty iron saucepan (fig. 114), so that the heat from the flame may be equally distributed over the bottom of the dish. When the mass is quite dry the dish is allowed to cool, and some water is poured into it, which dissolves the chlorides of potassium and sodium (formed by the action of the hydrochloric acid upon the silicates of potash and soda), and leaves the silicic acid in white flakes. These may be collected upon a filter (fig. 115), and washed several times with distilled water. The filter is then carefully spread out upon a hot iron plate, or upon a hot brick, and allowed to dry, when the silicic acid is left as a dazzling white powder, which must be strongly heated in a porcelain or platinum crucible to expel the last traces of water. It is remarkable for its extreme lightness, especially when heated, the slightest current of air easily blowing it away.

79. For effecting such fusions as that just described, an air-gas blow-pipe (A, fig. 116) supplied with air from a double action bellows (B), worked by a treadle (C), will be found most convenient. Where gas is not at hand, the fusion may be effected in a small furnace (fig. 117) surmounted with a conical chimney, and fed with charcoal.

Fig. 114.

80. Silicates. The acid properties of silicic acid are so feeble that it is a matter of great difficulty to determine the proportion of any base which is required to unite with it in order to form a chemically neutral salt. Like carbonic acid, it does not destroy the action of the alkalies upon test-papers, and we are, therefore, deprived of this method of ascertaining the proportion of alkali which neutralises it in a chemical sense. In

Fig. 115.-Washing a precipitate.