_Pecto-Cellulose._—The compound cellulose described under the names adipo-cellulose and ligno-cellulose have this feature in common, that they are richer in carbon than cellulose. On the other hand there is a large number of cellulosic tissues which contain in the aggregate less carbon, i.e., in which the cellulose is combined or mixed with substances of lower carbon percentage than itself. Such tissues, or rather the substances which compose them, are resolved by simple treatment with dilute alkaline solutions into insoluble cellulose, and soluble bodies of the pectic class. Thus the purified bast of the Russian flax, i.e., the raw flax, is found to contain— 43·7 per cent. C. 5·9 „ H. On boiling with an alkali it loses in weight about 20 per cent., the substance dissolved being identical in properties with Frémy’s pectic acid, a substance containing 42 per cent. C and 4.8 per cent. H = C_{16}H_{22}O_{15}. Many other bast fibres, as well as cellulose tissues, exhibit similar properties, and although this branch of the subject has been but little investigated, there is sufficient evidence for constituting a special class of compound celluloses under this term, pecto-cellulose, having properties indicated in the type selected above. In addition to pectic acid there exists in, or can readily be obtained from certain plants, a series of bodies of a similar nature, such as pectose, pectin, parapectic acid and metapectic acid. The most important of these are pectic acid and metapectic acid. Pectic acid, though it rarely exists ready-formed in the plant, can readily be obtained from it by the action of weak alkalis. It is best procured by boiling the pulp of turnips with a 10 per cent. solution of sodium carbonate for about half an hour. If an acid be added to the filtered liquid, pectic acid is precipitated as a transparent colourless jelly, which {29} dries up to a transparent horny mass. By the prolonged action of alkalis or acids, pectic acid is converted into metapectic acid (C_{8}H_{14}O_{9}), which may be obtained as a syrupy, strongly acid liquid. From what we know of the properties of the members of the pectic group, and therefore of the pecto-celluloses, it will be readily seen that the latter are very liable under the action of alkalis to undergo conversion into soluble derivatives. Conversely, a study of the action of alkalis upon plant substances, enables us to determine to what extent the latter partake of the nature of the pecto-cellulose, the importance of which to the practical paper maker we need hardly point out. For a further development of this idea (see p. 43). {30} CHAPTER II. PHYSICAL STRUCTURE OF FIBRES.