Cellulose, in those forms to which the application of the term has been hitherto restricted, is a comparatively inert substance, and its reactions are consequently few. One of these is available for the identification of cellulose, and is chiefly used in the microscopical examinations of tissues: this is its reaction with iodine. Cellulose is not coloured blue by a solution of iodine excepting under the simultaneous influence of hydriodic acid, potassium iodide, sulphuric acid, phosphoric acid, or zinc iodide or chloride. The solution is prepared in the following way: zinc is dissolved to saturation in hydrochloric acid, and the solution is evaporated to sp. gr. 2·0; to 90 parts of this solution are added 6 parts potassium iodide dissolved in 10 parts of water; and in this solution iodine is dissolved to saturation. By this solution cellulose is coloured instantly a deep-blue or violet. For the identification of cellulose in the gross, mere inspection is usually sufficient; confirmatory evidence is afforded by an observation of the action of the ammonio-copper reagent, and of the absence of reaction with chlorine water. (See p. 18.) Cellulose in its earlier stages of elaboration has no action upon light; but with age it acquires the property of double refraction, not, as has been shown by experiment, by virtue of its state of aggregation, but of its molecular constitution (Sachs). {7} _Animal Cellulose._—The mantles of many of the mollusca, e.g. the _Pyrosomidæ_, _Salpidæ_, and _Phallusia mammillaris_, contain a resistant substance which, after isolation by chemical treatment, has been found to be identical both in composition and properties with vegetable cellulose. Cellulose has also been stated to occur in degenerated human spleen and in certain parts of the brain. _Compounds of Cellulose._—The chemical inertness of cellulose is a matter of everyday experience in the laboratory, where it fulfils the important function of a filtering medium in the greater number of separations of solids from liquids. Its combinations with acids and with basic oxides are, as might be expected, few and of little stability. It has been shown by Mills that cellulose (cotton) in common with certain other organic fibrous substances, when immersed in dilute solutions of the acids or basic oxides, condenses these bodies within itself at the expense of the surrounding solution, which is proportionately weakened. This effect of concentration is sufficiently uniform and constant to lead us to assign it to a chemical cause, and the view is strengthened by a consideration of the relative effects upon the various acids and bases which have been investigated, and brought to the following numerical expression:—Weight of cotton 3 grm. (with 6·9 per cent. H_{2}O and 0·05 per cent. ash)—i.e. 2·893 anhydrous fibre. Strength of solution about 0·5 grm. of the reagent in 250 cc.