52 



chemical. These two methods give different results as regards paper 

 manufacture, the product of the mechanical method, termed ''paper pulp'' 

 being more gi'anular, whilst that of the chemical method, termed "cellu- 

 lose," is more fibrous, and hence stronger. In making the mechanical 

 pulp, the wood is cut into suitable lengths for grinding, the bark removed, 

 and the blocks held by hydraulic pressure against the edge of a rapidly 

 revolving sandstone and by attrition reduced to a mushy consistency. 

 The fiber as thus ground is passed through filterers of various fineness. 

 The fibrous mass is now brought to another machine, where the water is 

 pressed out, and it is cut into slabs, baled, and shipped to regular paper 

 mills without drying. The pulp so made is the basis of all lower grades 

 of paper. As already noted, the pulp industry has become an integral part 

 of the paper business, over half of the ground wood produced being made 

 into paper on the spot. 



By the chemical process, which is more recent and more costly, but 

 which produces a much longer fiber, the finely ground wood fragments 

 are placed in large boiling tanks or digesters, lined inside with lead or 

 other acid-resisting material. Chemical pulp or "cellulose" is of two 

 Ivinds, depending upon the use of caustic soda (alkali) or calcium sulphite 

 (acid) to macerate the wood. It should be remembered that all chemical 

 processes of wood pulp manufacture are based upon the underlying 

 principle that the middle lamella and infiltrated material which sur- 

 rounds and holds together the individual fibers of wood is soluble and 

 produces a chemical reaction with certain aqueous solutions, notably that 

 of the bisulphite of lime. The problem is to apply the macerating liquid 

 under conditions Avhich will completely and quickly eliminate tlie infil- 

 trated substances without imnecessarily weakening the fiber, and for this 

 purpose the solution must be applied at a high but carefully governed 

 temperature and under a mechanical pressure that will force the chemical 

 solution into every pore of the woody structure, thus permitting it to 

 attack the non-cellulose matter in which the fiber is embedded and by 

 which it is permeated. The matrix thus loosened and dissolved is re- 

 moved by washing with water. 



Where sulphite of lime is used, the wood fragments are boiled in sul- 

 phurous acid from 24 to GO hours under a pressure of about three atmos- 

 pheres. The soft, crumbling, reddish-yelloAv pieces are then pounded, 

 washed, filtered, and pressed into sheets about the thickness of paste- 

 board. 



