The compound celluloses may be divided, as we have seen, into three classes—pecto-celluloses, ligno-celluloses, and adipo-celluloses. The last-named being present in only a few paper-making materials, and then only in very small proportion, is without any practical interest. We are concerned therefore with the two former only. (1) _Pecto-celluloses._—From what we have seen of the nature of the pecto-celluloses, it follows that for their resolution a tolerably simple treatment with soda will be sufficient. The chief members of this group in which the paper-maker is interested, are esparto and straw. The former being but very slightly lignified, its resolution can be effected at a very low pressure; in fact it is even possible by using tolerably strong solutions to do it by boiling in open vessels. Straw, on the other hand, though it possesses certain features in common with esparto, is more lignified, and in {64} consequence, necessitates a much more energetic action to complete its resolution. It is therefore usual to boil it at somewhat high pressures (40–70 lb.), and with larger quantities of soda than are demanded in the case of esparto. It is, moreover, essential, so to regulate the treatment, that even the most resistent portions of the straw, such, for example, as the knots, shall be completely resolved, otherwise unbleached portions are liable to find their way into the finished paper. By increasing the quantity of soda, it is possible to boil under reduced pressure, and _vice versâ_; this may be taken as a general principle applicable in all other cases. We now come to the second class of celluloses, that is to say, the ligno-celluloses. Of these, perhaps the most important are jute, manilla, and wood. For their _complete_ resolution, it is imperative to employ very strong solutions at very high temperatures and pressures; thus in the case of wood it is necessary to boil at temperatures corresponding to a pressure of 100–120 lb. per square inch. The use of strong caustic soda solutions at high temperatures is attended with very serious objections, apart from the question of cost. Among these may be mentioned the destructive action of the soda upon the cellulose itself, involving a considerable loss of pulp; the de-hydration, condensation, and oxidation of the non-cellulose portion of the fibre (which, as we have seen (p. 25) is of the nature of an aldehyde, and therefore peculiarly susceptible of such changes), and the formation of dark coloured products which remain intimately combined with the residual cellulose; the inconvenience of working at very high temperatures and pressures.