Читать книгу A Text-book of Paper-making - C. F. Cross - Страница 33

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—The celluloses burn in the air with {24} a quiet luminous flame. When heated out of contact with the air, they are completely resolved into gaseous and volatile products on the one hand, and a residual black mass, which, although containing a high percentage of carbon, and approximating in its properties to the element itself, nevertheless contains hydrogen and oxygen in essential chemical union with the carbon, in consequence of which it is expedient to apply the term pseudo-carbon to this as to all similar substances. The volatile products of the destructive distillation of pure cellulose have not been studied, but those of certain of the ligno-celluloses (woods) have been exhaustively investigated, and have moreover, a very considerable commercial importance. They divide themselves into two groups (a) soluble in water, and containing, besides water, acetic acid, methyl alcohol, methylic acetate and acetone; (b) insoluble in water, or wood tar, containing tolyene, xylene, cymene, naphthalene, chrysene, anthracene, retene, pittacal, cedriret solid paraffins, &c. The quantity of the pseudo-carbon (charcoal) obtained is, in the case of the hard woods, about 25 per cent. of their weight. The liquid portion of the products amount to some 50 per cent., and contains about one-fifth of the carbon of the original wood; the remainder of the carbon is eliminated in the gaseous form (CO, CO2, &c.). The proportion of acetic acid is 3–4 per cent., the tar amounting to 7–8 per cent.

The coals themselves may be regarded as pseudo-carbon derivatives of celluloses, formed by a process of molecular condensation, the true nature of which remains a matter of speculation. In this view, the whole of our vast series of aromatic or benzene compounds, derived as they are from the products of the destructive distillation of coals, may be traced back to a cellulose origin.

Pseudo-carbons are obtained as products of the action of various reagents upon the celluloses, and other of the so-called carbo-hydrates. These reagents, such as sulphuric acid, act in virtue of their dehydrating power; and the recognition of this fact, together with the supposed “carbonaceous” {25} character of the product, led to the erroneous conclusion that the carbohydrates are in such decompositions simply resolved into carbon and water; a conclusion which seems to derive additional warrant from the peculiar numerical relationship which exists between the C, H, O atoms of all the members of the group. Their relation is expressed in the general formula C_n H_2(n−m) O_(n−m), and in the somewhat misleading term carbohydrate, which is applied to the whole group. We now know that the removal of water from these bodies by the action of dehydrating agents—including heat—follows the ascertained laws of chemical dehydration, involving molecular con­den­sa­tions and rear­range­ment, and that the pseudo-carbons are the extreme terms of a series of such con­den­sa­tions or cum­u­la­tive res­o­lu­tions. The matter, however, is not as yet sufficiently investigated to enable us to state with any preciseness the mechanism of these changes. Still this general statement will enable us to avoid many of the erroneous views which have existed on the subject, and in a measure to anticipate the results of future investigation.6

6 It is worthy of mention that by the action of chlorine in presence of water, and by the action of concentrated nitric acid upon the cannel coals, substitution derivatives are formed resembling those obtained by the action of these reagents respectively upon the ligno-celluloses.