Although the pulp produced by these processes is of a very light-grey colour, so light indeed that they may be used for certain low classes of white paper without further treatment, yet to obtain a pure white colour almost as much bleaching powder is required as in the case of soda pulps, although the latter are much darker in colour. The chief points to be observed are (1) the complete removal of the sulphites from the pulp before the addition of bleaching liquor, which would otherwise be ineffectively consumed; (2) the avoidance, in washing, of waters containing iron, which would combine with certain constituents of the pulp to form dark-coloured bodies. The following Table, compiled from Dr. Hugo Müller’s ‘Pflanzenfaser,’ is of interest, as indicating the probable yield of pulp from different woods, compared with the amounts obtained in actual practice by the Watt and Burgess (soda) process:— ----------+--------+----------+--------+---------+------------+------------ | | Water | | Lignin, | | Pulp—Watt | Water. | Extract. | Resin. | &c. | Cellulose. |and Burgess | | | | | | Process. +--------+----------+--------+---------+------------+------------ Birch | 12·48 | 2·65 | 1·14 | 28·21 | 55·52 | 40·00 Pine | 12·87 | 4·05 | 1·63 | 28·18 | 53·27 | 34·70 Oak | 13·12 | 12·20 | 0·91 | 34·30 | 39·47 | 20·60 Chestnut | 12·03 | 5·41 | 1·10 | 28·82 | 52·64 | 25·10 ----------+--------+----------+--------+---------+------------+------------ The following Table, which gives a concise account of the various typical processes that have been employed for the preparation of wood pulp, will be readily understood in connection with the theoretical considerations previously discussed:— {78}