The apparatus employed is shown in Figs. 28, 29, and 30. Fig. 28 illustrates an arrangement of grinding apparatus fixed direct on to a turbine. The stone is fastened on to the shaft S worked by the turbine T. The wood in the form of small blocks is kept in contact with the stone by a number of hydraulic presses P. {108} A somewhat similar arrangement, but placed horizontally, is shown in front and side elevation in Figs. 29 and 30. The same letters correspond. [Illustration: FIG. 31.] The ground and sorted pulp is made into thick sheets by means of the board machine shown in Fig. 31. The pulp mixed with water passes down the shoot D into the vat B in which the cylinder K revolves. This cylinder is covered with wire-cloth, and as it revolves it takes with it a certain quantity of pulp in the form of a continuous sheet. This sheet is taken on to the endless travelling felt F by the small couch roll E. When it reaches the rolls C it is wound round the upper one, from which it is removed when a sufficient thickness is obtained. Obtained in this form the pulp is readily transported. The woods commonly employed are white pine and aspen. The latter yields a pulp of a better colour, but of inferior strength than the former. Paper containing mechanical wood pulp is very liable to become discoloured by the action of air and light, the ligno-celluloses being much more readily acted upon than the celluloses isolated from them. Such fibre is, moreover, devoid of much felting power; it has, in fact, little to recommend it but its comparative cheapness. It is nevertheless used in large quantities, some cheap papers being made entirely from it. {109} TABLE OF STRENGTH OF CAUSTIC SODA SOLUTIONS (15° C. = 59° F.) (TÜNNERMAN).