Preparation of the Wood.—Although the acid sulphite processes can, by suitable modifications, be made available for the preparation of cellulose from almost all kinds of wood, yet it is necessary, in order to insure uniformity of result, not only to take care that the wood is carefully selected, but that it is freed from bark and roots, which, if allowed to remain, would be imperfectly pulped, and would be exceedingly difficult to bleach. The most suitable kind of wood is _Pinus sylvestris_. After the bark has been carefully removed the logs are sawn into boards of ordinary sizes. This treatment {74} exposes the knots, which are then removed either by boring or cutting out with an axe. The pieces are then passed through a machine, which cuts them or rather breaks them into fragments about an inch long. In order that the acid sulphite may as quickly as possible enter into the wood, these fragments must be crushed and opened out by passing through heavy rollers: moreover, the wood is cut _across_ the grain, by which means it is more or less split up into laminæ. A machine for this purpose has been patented by Fry, Ransome, and Wilkie (No. 11,389, 1884). The Boiling Process.—Ekman uses a lead-lined, jacketed cylindrical boiler, suspended on trunnions, so that it can be turned upside down for discharging. The pressure in the outer jacket is 70 lb. per square inch, while inside the boiler it is 90 lb., the difference being due to the tension of the sulphurous acid gas. The duration of boiling is from ten to twelve hours. Partington, on the other hand, boils with live steam, and therefore is enabled to use a boiler of simpler construction. His boilers are spherical, lead-lined and rotary. The pressure employed does not exceed 60 lb. per square inch, the time of boiling being about sixteen hours. Various other sulphite processes have been from time to time devised, which differ but little either in principle or in working detail from those described, and therefore a brief mention of them will suffice. Mitscherlich employs a bisulphite of lime in somewhat weaker solution than that employed by Francke or Partington, and in order to produce the same result he is obliged to extend the time of boiling to from 40 to 60 hours. Graham, after treating the wood with a mono-sulphite, injects into the boiler either a solution of a bisulphite or gaseous or liquid sulphurous acid. Flodgvist uses a solution prepared by dissolving bones in sulphurous acid solution.