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 C_{12}H_{22}O_{11}, 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 NO_{3} 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, C_{12}H_{14}(NO_{3})_{6}O_{4}, 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.