XXII. SILICIUM.

This substance, although the basis of the most abundant product of the mineral kingdom, has been but recently obtained in the separate state. It may be procured simply by the action of potassium on silica, by which the latter is deprived of its oxygen; or better, by means of the double fluoride of silicium and potassium. This salt being heated with a quantity of potassium, the latter unites with fluorine, and the silicium is detached. The silicium is then subjected to a powerful heat, in order that it may be freed from a portion of hydrogen which remains attached to it with great force; and then, in order that it may be freed of some silica which remains mixed with it, an acid is employed, the hydrofluoric, which dissolves the silica, but does not act upon the silicium.

Silicium, as produced by the common processes, is in the form of a powder, of a dark nut-brown colour, which blackens what it comes in contact with, in the manner of powdered charcoal. When heated sufficiently in air or oxygen gas, it inflames, and is partially converted into silica. But after being exposed to a strong heat, it becomes more dense, and has its characters so altered, that it now remains without change under the most intense action of the blowpipe.

From the resemblance of this substance to carbon and boron, it has been proposed to term it silicon, in place of silicum or silicium, the names originally given to it, and still retained by the continental chemists. The propriety of the change of name adopted in England is not very apparent. The term silicon, applied to this substance, is intended to distinguish it from the metals; but the term metal indicates a difference of form, and not of substance; and

there is no substantial distinction between metals and other kinds of matter, and therefore the adoption of a term to distinguish silicium from the metals so called, being based on an erroneous hypothesis, should be avoided. Besides, we do not know that the powdery form which this substance assumes, in the common methods of preparing it, is that which, under other conditions, it would assume. If heat could be applied sufficient to fuse the grains, there is reason to believe that they would assume the metallic aspect, just as various metals do. I have more than once found, in preparing this substance, that, for an instant or two, it assumed a colour as bright as silver, which it immediately lost.

Silicium combines with hydrogen, carbon, oxygen, sulphur, chlorine, and other bodies. Its combining weight was estimated by Berzelius at 22.221, on the supposition that silica contains 3 equivalents of oxygen to 1 of base. But silica may contain 2 equivalents, or 1 equivalent of oxygen, to 1 of base, and then the equivalent of silicium would be or of the number assigned to it; and there are distinguished chemists who maintain the latter opinion. M. Dumas, in particular, contends that silica must be held to contain only 1 equivalent of oxygen; and Dr Thomson favours the same opinion. Dr Clark, on the other hand, conceives that silica contains 2 equivalents of oxygen; which would assimilate the constitution of silica to that of carbonic acid. We may adopt the opinion of Dr Clark as being intermediate between the other views; but it is a more probable supposition, founding on the feeble acid powers of silica, that it does not contain more than 1 equivalent of oxygen.

Silica, whether a monoxide or binoxide, is the most abundant product of the mineral kingdom, forming the greater part of the solid crust of the globe, and entering

into the composition of numerous mineral species. As it is obtained by the common processes, it is a white granular powder, gritty to the touch, and destitute of taste. Its specific gravity is 2.65. It is infusible in the strongest wind-furnace; but in the flame of the oxyhydrogen blowpipe, it melts into a limpid glass. When obtained in a certain manner, or after being exposed to heat, it is insoluble, or nearly insoluble, in water; but as it is obtained by other means, it is soluble, or at least combines with water, and forms slightly soluble hydrates. These solutions, when concentrated, produce a gelatinous mass. Silica is found largely in the mineral kingdom in the crystalline form, as in quartz and rock-crystal, leading us to the conclusion, that the earth was once covered with a fluid, holding silica as well as other substances in solution.

Silica exhibits feeble acid characters. It does not even redden the blue colour of plants, and yet it stands in the relation of an acid to various bodies. Thus, combining with the alkalies, it forms glass, and with alumina, the substances termed clays.

The relations of this substance with alumina are very marked. Alumina is obtained, like silica, as a white powder, destitute of taste, but differing from silica in this, that it is soft to the touch, and not gritty. It is insoluble, or nearly insoluble, in water; yet, like silica, it combines with that fluid, forming hydrates; and these hydrates, when moist, are gelatinous, like the hydrates of silica. Alumina is infusible in the strongest heat of a wind-furnace; but by the flame of the oxyhydrogen blowpipe, it melts into a semi-transparent glass or enamel. It is a feeble base, and stands sometimes in the relation to other bodies of an acid; but its acid powers are slight, and insufficient to redden the blue colour of plants. These characters, common to silica

and alumina, indicate an intimate relation between them; and further, they are found in close association throughout the mineral kingdom, forming together, not only numerous mineral species, but the great mass of what, in common language, is termed earth. Further relations between silica and alumina conduct us, by an irresistible analogy, to the conclusion, that their bases, silicium and aluminum, are closely connected as natural bodies.

Silicium combines with chlorine, forming a colourless liquid, limpid, and exceedingly volatile. When exposed to the air, this substance is evaporated almost instantaneously, in the form of a white vapour. It has a suffocating odour, resembling that of cyanogen. When brought into contact with water, mutual decomposition takes place.

Silicium, on the suppositions made, is represented by HC; but if we suppose silica to be formed of 1 equivalent of silicium and 3 of oxygen, silicium is resolvable into H10 C2.

Whether by any natural processes this substance is converted into carbon by parting with its hydrogen, or whether, on the other hand, carbon, by combination with the same. element, is converted into silicium, is not known, although probable in a high degree. If carbon be the primary element, then the silicium, which forms the basis of the solid earth, may not unreasonably be supposed to have been formed by combination of carbon, or the molecules of carbon with hydrogen, or the molecules of hydrogen, in that pristine state of the globe in which natural changes took place, under conditions of pressure and temperature, and by the influence of great masses of matter upon one another, of which we cannot predicate the results from any action now known to us. If, in any former state of the globe, the aqueous portion predominated, as there is reason to believe it did, over the solid, we may perhaps believe that one of the means

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of diminishing the volume of water was the combination of its hydrogen with the molecules of carbon to form silicium, and of its oxygen with silicium to form silica.

The well known phenomenon of the petrifaction of organic substances is exceedingly like a change of their carbonaceous matter into silica. The change into silica of the numerous plants that have been found imbedded in the older sedimentary deposites is so complete, that generally not a vestige of carbon can be detected in them. But if the common solution were correct, that petrifaction is produced by an infiltration of siliceous particles, the whole carbonaceous matter should remain. Besides, the petrified bodies present the appearance of nothing in any degree like infiltration, but every appearance that indicates an alteration of the particles of the carbon itself as complete as any chemical change. What shall we say to the flinty petrifactions in chalk? Why should not carbonate of lime infiltrate the matters of the animal rather than silica? The other result may be explained, by supposing that the carbon of the animal has combined with hydrogen, supplied by the water under which the animal matter had been buried during countless ages.

Another natural phenomenon renders it not improbable that there are cases in which carbon is extracted from silica. Great forests grow on sands almost entirely siliceous, in which there seems to be no more carbon than is supplied by the fallen leaves on the surface, and where the trees have extended their roots below the surface into matter in which not a trace of carbon can be detected by analysis. Whence do these trees derive their carbon? It will be answered, from the carbonic acid of the air, which is decomposed in the leaves, the oxygen being set free. It is not denied that carbonic acid is abstracted from the air, and decomposed in the leaves during the day, but it must be re