MOST INTERESTING FOREST PRODUCTS 139 
by hand was very durable but quite expensive. With 
the rapid increase in the number of newspapers in 
order to fill the demand for an elastic free-running 
paper, the process of manufacturing paper from wood 
was invented. 
Spruce on account of its long strong fiber is especially 
desirable as a source of wood pulp and at present sup- 
plies more than half the stock used for paper. How- 
ever, more than a score of species furnish material for 
wrapping paper and fiberboard. 
Manufacturers divide the kind of pulp made from 
wood into two classes—mechanical and chemical. In 
the manufacture of pulp by the mechanical process the 
wood is ground up into bits by pressing each piece of 
the tree trunk—they are called “bolts’—against a 
rapidly revolving grindstone. These wood fragments 
make a very coarse pulp from which fiberboard and 
ordinary pasteboard boxes are made. This ground 
wood may be mixed with chemical pulp to make news- 
paper stock, about three-fourths of the paper being com- 
posed of ground wood. The ease with which such paper 
is torn is largely due to the presence of the untreated 
pulp, but the discoloration of a newspaper after a short 
time is due to the presence of sulphur in the chemical 
pulp. 
In the manufacture of paper by use of chemicals the 
wood is first chipped instead of being ground, as the 
fiber must be kept whole. The chips are then treated 
with soda or sulphurous acid which dissolves away all 
the impurities and leaves behind nothing but the wood 
fibers. This part of the manufacture—the “cooking’”— 
is done in large upright evlinders called digestors. 
After the cooking is finished'the pulp is washed, beaten 
and then diluted with water until it is very thin, in 
