Various attempts have been made from time to time to remove from the waste liquors the soluble matter derived from the esparto by the addition of lime. This throws it down as a very voluminous precipitate, exceeding difficult of filtration. The filtrate contains only caustic soda. The difficulties of removal of the precipitate, however, are almost insurmountable. The usual method of disposing of the liquor is to evaporate it to dryness and ignite it, as will be subsequently described under the head of “Soda Recovery,” Chapter XII. _Straw._—Straw very closely resembles esparto in its chemical constitution; it is, however, more highly lignified, and for this reason (see p. 63) requires a more drastic treatment for its resolution. It is therefore usual to treat straw with rather larger quantities of caustic, and generally at high pressures. It is possible, however, to resolve it at 5–10 lbs. pressure, but the resulting pulp is more difficult to bleach, and can only be used for inferior kinds of paper. In treating straw for use in high-class paper, it is necessary to boil under such conditions that even the knots, which are much less readily attacked, shall be so far disintegrated that they may be bleached with a moderate quantity of bleaching powder. Such a treatment, on the other hand, involves a diminished yield of pulp, and at the same time a somewhat weakened fibre. The kinds of straw in general use are wheat, oat, rye, and barley; the two first constituting the bulk of the raw material, at least in this country. Some idea of the composition of straw may be gathered from the analyses given on p. 59. It will be seen that the amount of cellulose is quite as high as in esparto, but for the reasons above given and from the fact that a large proportion of the cellulose consists of cellular tissue (see Figs. 12 and 13), which is easily attacked by soda and readily passes through the meshes of the {99} drum-washers and the wire-cloth of the presse-pâte and paper machine, the yield obtained in practice falls considerably below that from esparto. In addition to the numbers already quoted on p. 59, the following results of analyses made from time to time by the authors, may be interesting. The percentages are calculated on the absolutely dry material. Cellulose per cent. Oat Straw 52·0 Oat Straw 53·5 Wheat Straw 49·6 Rye Straw (Foreign) 53·0 Oat Straw (Foreign) 46·5 Wheat Straw (Foreign) 50·2 The yield of pulp is greatly influenced by the conditions under which it is obtained; high pressures and temperatures exerting considerable influence on the result. This is probably the case with straw more than any other fibre, on account of the physical and chemical nature of the cellulose. It is doubtful whether much more than 35 per cent. is actually obtained in practice.