owing to their possession of large forest areas. They also export large quantities of wood-pulp to other countries. In Europe the Scotch fir (Pinus sylvestris), the spruce (Picea excelsa),
Wood.the poplar {Populus alba) and the aspen (Populus tremula), are the timbers principally employed; and in America the black spruce (Picea nigra), the hemlock (Tsuga canadensis), the poplar (Populus grandidentata) and the aspen (Populus tremuloides). Two kinds of wood-pulp are used for paper manufacture, one prepared mechanically and the other chemically. The former is obtained by disintegrating the wood entirely by machinery without the use of chemicals, and is, as may readily be understood, a very inferior pulp. In the manufacture of chemical wood-pulp, very great advances have been made since 1880, and wood-pulp has grown to be one of the most important fibres for paper-making purposes.
Fig. 3.—Sinclair Esparto Boiler.
Two methods are in use, known respectively as the soda or alkaline process, and the sulphite or acid process, according as soda or sulphur (or rather sulphurous acid) forms the base of the reagent employed. Trees of medium age are usually selected, varying from seventy to eighty years' growth and running from 8 to 12 in. in diameter. They are felled in winter and reach the mill in logs about 4 ft. long. After being freed from bark and the knots taken out by machinery, the logs are cut into small cubical chips about 5 to 34 in. in size by a revolving cutter. The chips are then bruised by being passed between two heavy iron rolls to allow the boiling solution thoroughly to penetrate them, and are conveyed to the boilers over a screen of coarse wire-cloth, which separates out the fine sawdust as well as any dirt or sand. In the soda process the wood is boiled in large revolving or upright stationary boilers for about seven or eight hours, in a similar manner to esparto and straw, though it requires much severer treatment. The steam pressure varies from 90 ℔ to as much as 150 ℔ per sq. in., and the amount of soda required is about 16% of Na2O, estimated on the barked and cleaned wood. The essential feature of the sulphite process is the employment of a solution of sulphurous acid combined with a certain amount of base, either magnesia or lime. As the acid reaction of the bisulphite solution would attack any exposed ironwork with which it comes in contact, the boilers in all cases should be lined with lead. The type of boiler employed varies according to the process adopted. The principal patents connected with the sulphite process are those of Tilghman, Ekman, Francke, Ritter-Kellner, Mitscherlich, and Partington. The subsequent operations, in both the acid and alkaline processes of washing, bleaching and straining the pulp, are all very similar to those described for esparto. Wood-pulp produced by the sulphite process differs in a marked degree from that made by the soda process; the fibre in the former case is harsher and stronger, and papers made from it are characterized by their hardness and transparency, whereas those made from soda pulp are softer and more mellow, corresponding in some way to the difference between linen and cotton fibres. Each class of pulp is largely used, both alone and mixed with other materials.
Fig 4.—"Presse-Pâte," or Half-stuff Machine.
Within recent years important modifications and improvements have been adopted in the preparation of esparto and wood half stuff with a view to reduce the cost of manufacture and save waste of material. From the boiler to the beater the process becomes a continuous one, so that the prepared pulp requires practically no
