The above results are calculated on the ash-free wood. The ash varies from about 0·3 to 0·7 per cent. {62} CHAPTER V. PROCESSES FOR ISOLATING CELLULOSE FROM PLANT SUBSTANCES. We are now in a position to discuss generally the various methods by which the paper-maker obtains cellulose from the different raw materials. The special treatments necessary for each of the more important of these will be given more fully subsequently. These methods, though they vary considerably with the different materials to be treated, may be roughly divided into two groups—the alkaline and the acid processes. The former, which we will call _a_, comprises all processes in which caustic soda, carbonate of soda and lime, or mixtures of these are employed; the latter, _b_, the various processes which have been introduced of late years, involving the use of sulphurous acid, either alone or in combination with a base, such as lime or magnesia. The processes in group _a_ may be applied to the treatment of every known fibrous vegetable material; those in group _b_ are at the present time applied exclusively to the preparation of pulp from wood. Group _a_ may be conveniently subdivided according to the nature of the materials operated upon. It is obvious that those materials, such as bleached cotton and linen threads and rags, which have already undergone treatment by the textile manufacturer, and are therefore more or less already in the state of pure cellulose, require but little chemical treatment at the hands of the paper-maker, whose attention is therefore chiefly directed to the removal of such adventitious matters as size, grease, &c. This can generally be effected by the employment of weak solutions of caustic soda, or even of lime at a low pressure. {63} We now have to consider the treatment of those materials which consist of the compound celluloses. All methods for the isolation of cellulose from the compound celluloses depend upon a hydrolytic resolution of their constituents, i.e. a splitting up by combination with water into cellulose on the one hand, and a series of soluble derivatives on the other. This, as will be shown, may to a certain extent be accomplished by the action of water itself at a high temperature. As, however, the products of such an action are acid bodies, which, if allowed to remain in contact with the cellulose, would injuriously affect it, and would induce the production of complicated bodies, the removal of which from the cellulose would become increasingly difficult, it is necessary to have present a body such as caustic soda, which by combining with these acid bodies, removes them as such from the sphere of action. In certain raw materials, such as straw and esparto, we have, in addition to the compound celluloses which form, so to speak, the groundwork, a certain proportion of fatty and resinous bodies, whose removal is brought about by the action of the caustic soda converting them into soaps.