16 BULLETIN 620, U. S. DEPARTMENT OF AGRICULTURE. 
yield of pulp. One cook was made where the cooking liquor con- 
tained only 0.30 per cent of combined SO,, and the result was that 
the chips came out of the digester looking as if they had received 
only a steaming treatment. They were softened, but black, and 
gave a yield of 60.40 per cent, showing that the wood had not been 
thoroughly cooked. When the combined SO, is decreased too much, 
there is a darkening of the liquor without the corresponding cooking 
action, causing a browning of the pulp. Apparently a certain amount 
of lime must be present in the cooking liquor to get complete pulping, 
but its effect becomes less as it is increased beyond 1 per cent com- 
bined SOQ,. 
At the present time, the only method of giving a numerical value 
to the strength of the fibers of a pulp is to make. strength tests of 
paper produced from that pulp. In order to obtain any regularity of 
results in strength of paper in a series of tests it is necessary that the 
beating of the pulp, formation of the sheet, heat in the driers, humid- 
ity at which the strength tests are made, and probably other factors 
must remain constant. Most of these, then, depend on the personal 
equation of the machine tender, and where some of the runs may be 
made a week apart, conditions are likely to vary. This probably 
explains the irregularity of the strength results as given in Table 3. 
If they show anything at all, it is that above 0.80 per cent the amount 
of combined SO, has no influence on the strength of the paper pro- 
duced but below that there is a tendency to decrease in strength. 
Photomicrographs of the pulps secured in cooks 75, 74, 72, and 71 
are shown in Plates I, IJ, IJ, and IV. It will be noted that there is 
a gradual change in the appearance of the fibers. Cook 75 was made 
with liquor containing 0.62 per cent of combined SO,, and cook 71 
with 2.09 per cent. The former contains much lignified fiber, while 
the latter seems to be thoroughly cooked. 
