242 TIMBERS AND THEIR USES 



brought about by removing tbe bark entirely 

 and taking out all knots. The wood is first 

 chipped, then disintegrated between crushing 

 rollers, all unsound parts are taken away, and 

 the rest transferred to the digesters. 



The digesters are lined with some substance 

 which is not afiected by the free acid, usually 

 some kind of cement, i.e. a mixture of Portland 

 cement and silicate of soda. The period of 

 digestion varies from 16 to 20 hours under a 

 pressure of 75 lbs. per square inch to 70 to 80 

 hours under a pressure of 45 lbs. per square 

 inch. 



" The products are the pulp, or fibre, and the 

 waste liquor containing the sulphonated lignone 

 bisulphite compounds. Careful mechanical puri- 

 fication of the digested material, washing, separa- 

 tion of the fibres from imperfectly digested chips, 

 small knots and dirt by a system of traps and 

 strainers, are necessary before the pulp is run 

 oE into webs or sheets on a presse-pate or simple 

 form of paper machine." 



There are two main groups of wood cellulose ; 

 those obtained from deciduous trees such as 

 the Poplar, and those derived from conifers, the 

 latter being the more frequently used in paper 

 making. " The chief commercial varieties are 

 Aspen and American Poplar, prepared mostly 

 by the soda process, but also to some extent by 



