membered that, during the night, carbon is given off, and that the difference is so inconsiderable between the carbon abstracted and that given off, that many physiologists are in doubt whether the carbon given off does not equal that abstracted; and if this were so, it might lead to the conclusion that this decomposition of carbonic acid has other ends than the supply of matter of nutrition to the plant. It is, indeed, an opinion continually expressed, that a function of growing plants is to supply that oxygen to the atmosphere of which it is deprived by the breathing of animals. But this hypothesis, so universally received, is very far from being satisfactorily established. It is known that the clearing away of forests is in every case a mean of rendering the air more salubrious, and that in the highest mountains, and in the circumpolar regions, enveloped in perpetual snow, there is as much, or rather it may be believed there is more, oxygen than directly over the plains covered with the vegetation from which this prodigious stream of oxygen is supposed to be continually evolved. But be all this as it may, there are many reasons for believing that by far the greater part of the carbon which plants consume in growing is derived from the earth; and it is a supposition not devoid of probability, that silica, acted upon by the fibrils of the roots, furnishes carbon. That the roots of plants are capable of acting upon the hardest rocks appears from this, that a column or a statue formed of siliceous rock will remain for ages as perfect as if it had been chiselled the day before. This may be supposed to be because its smooth surface presents no points of attachment for the seeds of plants; but let the column or the statue be overthrown, and broken, or be so placed that plants may grow upon it, and we shall find that it becomes subject to disintegration and change; plants take root upon it, first the fungi and lower tribes, and then the more developed

species, until at length it becomes a part of the soil. From these and other considerations upon which I will not here dilate, plants seem to possess the power of appropriating the matter of the harder minerals to their uses, and so perhaps of deriving even carbon from siliceous matter.

Whether in the numerous processes of the laboratory, changes of carbon into silicium take place, is not easily determined. Chemists are sometimes startled by results which do not accord with known chemical actions, and this being so, the results are ascribed to impurity of the substances employed, or to errors of experiment; but more frequently analyses themselves are partial and imperfect, and the whole results are not observed. It is far, therefore, from being an improbable supposition, that, amongst thousands of experiments, there are many in which the change in question has taken place.

XXIII. BORON.

Boron is derived from boracic acid which exists in several minerals, and especially in the impure alkaline salt, borax, a substance derived from certain elevated lakes in Thibet, which dry up in summer, leaving the borax as an incrustation. It was long familiar in Europe, for the fusing of metals, and for forming glass employed to imitate gems. But boracic acid is now chiefly obtained from certain hot pools in a district of Tuscany, the water of which is charged with the free acid, apparently derived from volcanic vapours. The boron may be obtained by heating the acid along with potassium, by which means it is deprived of its oxygen; but boron may be procured by better means from the double fluoride of boron and potassium. It is obtained as a dark olive-coloured powder, destitute of taste, smell, and metallic lustre. Like silicium, it is a non-conductor of electricity. When heated in vacuo, or close vessels, it bears the most intense heat that can be applied, without fusing, or undergoing any change, beyond an increase of hardness and density; but if heated in the open air to 600°, it takes fire suddenly, and burns with brightness, forming boracic acid, which fuses as it is formed. It is insoluble in water, in ether, alcohol, and oils.

The combining weight of this substance has been very variously estimated, according to the views taken of the constitution of its compounds. M. Berzelius estimates it at 10.91, on the supposition that boracic acid contains 3 equivalents of oxygen. M. Dumas draws a different conclusion, and Dr Thomson estimates its combining weight at 8, on the supposition that boracic acid consists of equal proportions of oxygen and boron.

Boracic acid has been long known in medicine and the arts, in the alkaline salt, borax. It is found native in certain waters, apparently in contact with volcanic vapours, and it is a constituent of several minerals. As it is obtained from borax by the action of sulphuric acid, it is in the state of a hydrate, which may be rendered anhydrous by the application of heat. It has a bitter taste, and is destitute of smell. Its acid properties are feeble. It reddens litmus paper slightly, but, like the alkalies, renders turmeric paper brown. The weakness of its acid properties is further manifested in its saline compounds, all the borates in solution being decomposed by the stronger acids. The anhydrous acid fuses readily, forming a hard, colourless, transparent glass, and this property of ready fusibility it communicates to its compounds, on which account borax is employed as a flux.

Boron combines with chlorine, but the product is not liquid like the chloride of silicium, but in the state of gas at common temperatures. It is rapidly absorbed by water, and then decomposed. When it is brought into contact with moist air, a dense cloud of vapour is produced.

Although in these combinations, and still more in those with hydrogen and fluorine before referred to, a great analogy is observed between silicium and boron, there are certain differences which prevent us from assuming, that they are precisely the same substance. But they are both connected in an intimate manner with carbon, and it cannot be doubted are derived from it as an immediate root. But boron and silicium differ in their relations with the great mass of mineral products in this, that while the one is diffused throughout the whole earth, the other is found only in a few localities, and has manifestly been produced under peculiar conditions. So far as observation yet shews, boron is formed in connexion with volcanic agents. Now, what

is the conclusion which we ought to draw from this? Not surely that boron and silicium are distinct products of nature, each formed at the beginning of matter proper to itself, but that the one is either derived from the other, or that both are formed from a common root. Silica constituting the greater part of the solid matter of the earth, and extending to the sources of volcanic action, or at any rate acted upon by the heated matter of volcanoes, it is easy to conceive how its constitution may be so altered by so powerful an agent as to place its elements in a different chemical relation to one another. If both consist of carbon and hydrogen, or any other supposable form of matter, nothing is so likely as that silica, in the vast natural crucible in which its products are contained, may give rise to another body similar to itself. Such a supposition is in accordance with the known laws of chemical changes. The supposition that Nature has found it necessary, if we may use such an expression, to form two distinct kinds of matter, the one in the smallest quantity, and in no sensible degree, so far as we know, influencing the constitution of the great mass of mineral substances, seems to be opposed to a just analogy, and inconsistent with the simplicity of natural laws.