_Cooking._--Cooking is the technical term for the operation by which
fibrous raw materials are reduced to a residue of cellulose pulp by
means of chemical treatment. In these tests about 300 pounds of hurds
were charged into the rotary with the addition of a caustic-soda
solution, such as is regularly employed in pulp mills and which tested
an average of 109.5 grams of caustic soda per liter, or 0.916 pound per
gallon, and averaged 85 per cent causticity. Sufficient caustic solution
was added to furnish 25 or 30 per cent of actual caustic soda,
calculated on the bone-dry weight of hurds in the charge. After closing
the rotary head, it was started rotating at the rate of one-half
revolution per minute, and in about five minutes steam at 120 pounds per
square inch was admitted at such a rate that the charge was heated in
one hour to 170 deg. C., which is the theoretical equivalent of 100 pounds
of steam pressure per square inch. It was found, however, that when the
temperature reached 170 deg. C. the pressure was usually 115 or 120 pounds
instead of 100 pounds, due to air and gases inclosed in the rotary. At
this point the rotary was stopped and steam and air relieved until the
pressure dropped to 100 pounds, or a solid steam pressure. The
temperature was maintained at this point for the number of hours
required to reduce the hurds, which was found to be about five, after
which the rotary was stopped and steam relieved until the pressure was
reduced to zero, when the head was removed and the stock was emptied
into a tank underneath, measuring 5-1/2 by 6 by 2 feet deep, where it
was drained and washed. Samples of waste soda solution or "black
liquor," which were taken from some of the "cooks" for analysis, were
drawn while the stock was being thus emptied into the drainer.

_Determination of yield._--For determining the yield of cellulose fiber
the stock in the drain tank was washed with water until free from waste
soda solution, when, by means of a vacuum pump communicating with the
space between the bottom and the false perforated bottom, the water was
sucked from the stock, leaving the fiber with a very uniform moisture
content throughout its entire mass and in a condition suitable for
removing, sampling, and weighing for a yield determination. Tests have
shown that it is possible to sample and calculate the yield of bone-dry
fiber within 0.05 per cent of the actual amount.

It has been found that stocks from differe