_Decay of Ligno-cellulose._—When a mass of ligno-cellulose (jute) is allowed to lie for a long period in a damp state, it undergoes resolution accompanied by structural disintegration. The soluble products of this change are acid bodies of the pectic class, and astringent substances closely resembling the tannins. In this decomposition also we have evidence of the plastic condition of the ligno-cellulose molecule, of the variety of its modes of resolution, according to the various directions in which it is attacked. _Decomposition by Heat._—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, CO_{2}, &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.