figures which are both accurate and reliable. Two such rotary charges
gave enough fiber for one complete paper-making test.

[Footnote 2: For a description of this rotary digester, see Brand, C.
J., and Merrill, J. L., Zacaton as a paper-making material, U. S. Dept.
Agr. Bul. 309, p. 28, 1915.]


=OPERATIONS INVOLVED IN A TEST.=

A complete test on hurds comprises seven distinct operations, and the
method will be described, operation by operation, in the order in which
they were conducted.

_Sieving._--The hurds for the first test were not sieved to remove sand
and dirt, but the resulting paper was so dirty that sieving was
practiced in all subsequent tests. The hurds were raked along a
horizontal galvanized-iron screen, 15 feet long and 3 feet wide, with
11-1/2 meshes per linear inch, the screen being agitated by hand from
below. Various amounts of dirt and chaff could be removed, depending on
the degree of action, but it was found that if much more than 3 per cent
of the material was removed it consisted chiefly of fine pieces of wood
with practically no additional sand or dirt; in most of the tests,
therefore, the material was screened so as to remove approximately 3 per
cent. It became apparent that a finer screen would probably serve as
well and effect a saving of small but good hurds.

_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