Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/533

Rh CARBON.] powerless to produce organic substances from their elements as they were formed in the animal or plant under the influence of life, it being supposed that, therefore, the interposition of a special force, termed the vital force, was requisite. The first step towards the disproval of this hypothesis was made by Wohler, who in 1828 succeeded in artificially producing urea, the characteristic crystalline constituent of the urine ; but its final overthrow was not accomplished until 1845, when Kolbe showed that it was possible to produce acetic acid from carbon by a compara tively very simple series of reactions. Berthelot s discovery in 1862 of the formation of acetylene, C 2 H 2, from its elements, however, affords the simplest solution of the problem of the artificial production of organic substances from their elements, as acetylene may be without difficulty converted into ordinary alcohol, and from this body all the carbon compounds which have been artificially pre pared may be more or less directly derived. Chemists have already succeeded in preparing a large number of substances which occur as products of animal or vegetable life, for example, oxalic, tartaric, and salicylic acids ; coumarin, the crystalline substance from which the Tonka bean derives its pleasant odour ; and alizarin, the colour ing matter derived from madder root. From the insight we have recently gained into their constitution, there is little doubt that eventually the synthesis of even the most complex organic bodies, such as albumen, will be possible. Although carbon combines with hydrogen in a great variety of proportions, it furnishes only a very limited number of compounds with other elements ; thus, only one sulphide of carbon, CS.,, and one nitride of carbon, C 2 N , are known, and no well characterized compounds of carbon with the metals have been obtained. The compounds of carbon with the halogens are more numerous. The only compound that calls for consideration here is the sulphide ; the others will be noticed in the section on Organic Chemistry. Carbon disulphide, CS 2, the analogue of carbon dioxide, is obtained by passing sulphur vapour over charcoal heated to redness. It is a colourless, very mobile, highly re fracting liquid of faint unpleasant odour, insoluble in water ; it boils at 46 C. It is extremely inflammable, and its vapour when mixed with air is highly explosive ; the products of its combustion are carbon dioxide and sulphur dioxide. The formation of carbon disulphide from its elements is attended with the absorption of no less than 22,000 units of heat, which explains how it is that it so readily inflames, and also the fact that it is necessary to apply heat continuously in preparing it ; whereas the combustion of carbon in oxygen, when once it commences, proceeds spontaneously, owing to the large amount of heat developed in the process ; we may suppose that the ex penditure of energy is necessary because less heat is developed by the combination of the atoms of carbon and sulphur than is requisite to convert the carbon and sulphur from the state in which they ordinarily exist into that in which they exist in carbon disulphide, or, in other words, than is requisite to effect the separation from each other of the atoms of carbon and of sulphur in the carbon and sulphur molecules. Carbon disulphide belongs to the class of acid sulphides or sulphur-anhydrides. It readily dissolves in solutions of alkaline hydroxides and of metallic sulphides, forming salts analogous to the metallic carbonates, and which may be regarded as derived from the carbonates by the partial or entire displacement of the oxygen in the latter by sulphur. The stability of these salts and of the corre sponding acids increases with the amount of sulphur ; thus, sulphocarbonic acid, H 2 CS 3, which is formed from the salt produced by combining carbon disulphide with metallic 521 sulphides, may be obtained as a yellow oily liquid, whereas carbonic acid, H 2 CO 3, can only exist in extremely dilute solution. Carbon disulphide readily dissolves sulphur and phos phorus, and also oils and fatty matters ; on this account it meets with many practical applications. Ey gently heating a mixture of carbon disulphide and sulphuricanhydride, carbon oxysulphide is produced, one-half the sulphur in the former being displaced by oxygen : CS 2 4- SO 3 = COS + SO 2 + S ; this compound may also be obtained by combining carbon monoxide with sulphur, by passing a mixture of the gas with sulphur vapour through a red hot tube. It is a colourless gas, possessing an odour like that of carbon disulphide ; in properties, as in composition, it is intermediate between carbon dioxide and disulphide. SILICON. Symbol, Si; At. wt, 28 ; Valency, v. This element always occurs in combination either with oxygen alone as silicon dioxide or silica, or with oxygen and metals as silicates, constituting, in fact, in these forms of combination, the greater part of the earth s crust. Silicon may be obtained from its chloride or fluoride by theaction of metals such as potassium, sodium, or aluminium; like carbon it exists in three distinct modifications. Amor phous silicon produced by heating potassium silicofluoride, K SiF, with potassium, or the corresponding sodium salt with sodium, is a dull brown powder, heavier than water ; it is not affected by nitric or sulphuric acid, but is readily dissolved by hydrofluoric acid, and by a warm aqueous solution of potassium hydroxide. It fuses at a temperature below that at which steel melts ; and when heated in air or oxygen it burns brilliantly and is converted into silicon dioxide. When strongly heated in a platinum crucible it becomes much denser and darker in colour, and much less oxidizable, being converted into graphitoidal silicon. On heating a mixture of aluminium with potassium silicofluoride to the melting point of silver, a metallic button is obtained, which, when treated successively with hydrochloric and hydrofluoric acids, yields graphitoidal silicon partly in isolated hexagonal tables. This modification has the specific gravity 2 49, and may be heated to whiteness in oxygen without burning ; it is not attacked by any acid excepting a mixture of nitric and sulphuric acids, and is only slowly dissolved by a solution of potassium hydroxide. When the vapour of silicon tetrachloride mixed with hydrogen is passed over fused aluminium, the chloride is reduced, and the silicon dissolves in the aluminium ; after a time a point is reached at which the silicon separate? from the fused metal in large beautiful needles, having a dark iron-grey colour. These crystals constitute the adamantine variety of silicon. Silicon appears to be capable of combining with hydrogeii in the nascent state, as when a plate cr wire of aluminium containing silicon is connected with the positive pole of a galvanic battery, and made to decompose a solution of sodium chloride, gas is evolved which spontaneously inflames. Silicon hydride mixed with much hydrogen is also obtained on decomposing with concentrated hydro chloric acid the magnesium silicide produced by heating a mixture of magnesium chloride, sodium silicofluoride, and sodium chloride with sodium. The pure gas is pro duced by the decomposition of triethylsiliconorthofoimate- in contact with sodium 4SiH(OC 2 H 5 ) 3 = SiH 4 + 3Si(OC 2 H,) 4. Triethylsiliconorthoforniate. Silicon hydride. Tetrethylortnot&tate. It is a colourless gas, not spontaneously inflammable under the ordinary temperature or pressure, but only when gently heated under reduced pressure, or when mixed with V. 66