PAPER-MAKING MATERIALS 217 



then submitted to pressure to expel the water, and 

 is subsequently converted into sheets by passing it 

 between rollers. It is usually packed for export in 

 the moist state, as if it is kept dry for any considerable 

 time it loses its felting power. 



The process of preparing chemical wood pulp 

 consists essentially of the digestion of the chipped 

 wood with a chemical solution under pressure at a 

 high temperature. In the " sulphite " process of 

 manufacture, the liquor employed consists of an 

 aqueous solution of bisulphite of lime or magnesia, 

 prepared by passing sulphur dioxide (sulphurous acid 

 gas), produced by burning sulphur or pyrites, up 

 towers packed with lumps of limestone or dolomite 

 through which water is trickling. In the " soda " 

 process, the wood is digested with an aqueous solution 

 of caustic soda, and in the " sulphate " process, a 

 solution of sodium sulphate and caustic soda is 

 employed. 



Whilst mechanical wood pulp consists merely of 

 ground wood and contains all the encrusting sub- 

 stances of the fibres, chemical wood pulp is composed 

 of the fibres which have been freed from the resinous 

 and gummy matters by the action of chemicals. 

 Chemical wood pulp is therefore of much better 

 quality than that produced by the mechanical method 

 and realises considerably higher prices. It is mostly 

 exported in the dry state, as the loss of felting power 

 which takes place with mechanical pulp does not 

 occur in the case of this product. 



The total imports of wood pulp into the United 

 Kingdom during the years 1911-15 were as follows : 



Year. Quantity. Value. 



Tons. I 



1911 . . . 784,296 3,743,445 



1912 . . . 925,590 4,418,420 



1913 977,757 4,6i7,739. 



1914 . 990,272 4,888,170 



1915 . . . 954,050 5,314,245 



The quantity and value of the different kinds of 

 wood pulp imported into the United Kingdom in 1914 



