Pure cellulose is a white, hygroscopic substance, which is insoluble in water and in most other solvents. If heated with water under pressure to about 260° C., it dissolves completely without decomposition. If boiled with a strong solution of zinc chloride, or treated in the cold with zinc chloride and concentrated hydrochloric acid, or with an ammoniacal solution of copper hydroxide (Schweitzer's reagent), it dissolves to a clear solution from which it may be reprecipitated without chemical change by neutralizing or diluting the solution.
Cellulose has the formula (C6H12O5)n. When hydrolyzed under the influence of the enzyme cytase, it breaks down, first into cellobiose, an isomer of maltose, and then into glucose. It is, therefore, chemically like, but not identical with, starch; and structurally it is arranged in fibrous form instead of in granules. Under the action of fermentative enzymes, as when vegetable matter decays under stagnant water, in swamps, etc., cellulose breaks down into carbon dioxide and marsh gas, according to the equation
(C6H12O5)n + nH2O = 3nCO2+3nCH4.
Cellulose is acted upon by caustic alkalies in a variety of ways. When fused with a mixture of dry sodium and potassium hydroxides, it is decomposed into oxalic and acetic acids. When heated with a 10 to 15 per cent solution of caustic soda, cellulose fibers thicken and become translucent, thus resembling silk fibers. This process, known as "Mercerizing," is largely used for the production of commercial fabrics.
Acids also act on cellulose in a variety of ways. When heated with nitric acid (sp. gr. 1.25), it is converted into oxycellulose; while dilute sulfuric acid, under similar conditions, yields hydro-cellulose, a substance having the formula C12H22O11, which retains the fibrous structure of the original cellulose but which, when dry, may be rubbed up into a fine powder. Concentrated nitric acid, or better, a mixture of concentrated nitric and sulfuric acids, acts upon cellulose, converting it into various nitro-derivatives, several of which have great industrial value. The number of NO3 groups which unite with the cellulose molecule under these conditions depends upon the temperature, pressure, etc., employed during the nitration process; di-, tri-, tetra-, penta-, and hexanitrates are all known. Pyroxylin, or collodion, is a mixture of the tetra- and penta-nitrates, which is soluble in alcohol and is used in surgery, in photography, and in the manufacture of celluloid, which is a mixture of collodion and camphor. The hexanitrate, C12H14(NO3)6O4, is the violent explosive known as gun-cotton.
Gentler oxidizing agents, such as "bleaching powder," etc., have no effect upon cellulose, and hence are extensively used in the treatment of cotton and other vegetable fibers, in preparation for their use in the manufacture of textiles, paper, etc.
Cellulose is indigestible in the alimentary tract of animals, but the putrefactive bacteria which are generally present there ferment it, with the production of acids of the "fatty acid" series, carbon dioxide, methane, and hydrogen. Excessive fermentations of this kind are responsible for the distressing phenomenon known as "bloat."
The compound celluloses comprise the larger proportion of the material of the woody stems of plants. They consist of a base of true cellulose, which is either encrusted with or chemically combined with some non-cellulose constituent. Depending upon the nature of the non-cellulose component, the compound celluloses are divided into three main groups, known respectively as (1) ligno-celluloses, (2) pecto-celluloses, and (3) adipo-, or cuto-celluloses. As the names indicate, the non-cellulose component in the first group is lignin; in the second, pectic substances; and in the third, fats or waxes.
Ligno-celluloses.—In the young plant cell, the cell-walls consist of practically pure cellulose; but as the plant grows older, this becomes permeated with lignin, or woody fiber, until in the stem of a tree, for example, the proportion of cellulose in the tissue is only 50 to 60 per cent. In the preparation of wood pulp for the manufacture of paper, the lignin materials are dissolved off by means of various chemical reagents, leaving the cellulose fibers in nearly pure form for use as paper. The lignin material generally consists of two types of substances, one of which contains a closed-ring nucleus of unknown composition and the other is probably a pentosan. These materials are so extremely difficult to hydrolyze that their composition has not yet been definitely determined.