142 FIRST BOOK OF FORESTRY 



merely be pressed into a soft, pasteboard-like paper, baled, 

 and shipped to regular paper mills without drying, or else 

 it is made at once into ordinary paper, such as is used by 

 newspapers. 



In making " chemical pulp," or " cellulose," as it is 

 called, the blocks are chipped into short, small pieces, the 

 fragments of knots are sifted out, and the rest of the 

 wood is placed in large boilers called digesters, where it 

 is boiled in a solution of sulphite of lime, more rarely of 

 caustic soda. During the boiling the digester is kept 

 firmly closed, so that a steam pressure of about a hundred 

 pounds per square inch is developed. The boiling is con- 

 tinued for about six hours if soda, and for twenty -four 

 hours, and even longer, if sulphite of lime is used. In 

 this boiling the wood softens and becomes mushy. It is 

 then ground, washed, and finally treated in the same way 

 as ground pulp. 



The cellulose, or chemical pulp, is much finer than 

 ground pulp, and can be used for ordinary papers without 

 any bleaching. 



One cord of spruce gives about six to eight hundred 

 pounds of chemical pulp or twelve hundred pounds of 

 ground pulp. So far, nearly all pulp is used in the manu- 

 facture of paper. 



Acid Wood. In some districts of Pennsylvania, New 

 York, and other states "acid factories" require large 

 quantities of wood, preferably beech, maple, and birch. 



