HEMP HURDS AS PAPEK-MAKING MATERIAL. 13 
It is well known that the method of conducting tests necessarily 
varies with the size of the test. In the matter of yield determination, 
for example, laboratory tests may be on such a small scale that the 
weighing and sampling of the resulting cellulose fibers may be con- 
ducted by means of chemical laboratory apparatus and analytical 
balance, while in tests involving a matter of 5 to 10 pounds of material 
larger and different types of equipment are necessary. When 
the tests are so increased in size as to employ 300 or 400 pounds, 
still other types of equipment are necessary for the treatment of 
the material and for a determination of the yield of fiber. In 
tests involving tons of material the equipment involves the use of 
machines. Accuracy in degree of control and in results will vary 
materially with the size of the test. As the size of the test increases, 
certain factors will vary in a beneficial manner, while others will vary 
in a detrimental manner, so it is a question for each investigator to 
decide, after taking all factors into consideration, as to the size of 
test which will give the most satisfactory results. In work of this 
nature it is found, on the whole, that better results are obtained in 
large tests, although the control of the factors and the determination 
of the yield of fiber are more difficult than in smaller tests. 
In the tests described in this bulletin, the Department of Agricul- 
ture employed a rotary digester of its own design, 1 comprising a shell 
5 feet 5 inches in length by 4 feet in diameter, capable of holding 
about 300 pounds of air-dry hurds. It is believed that a test of this 
size is large enough to give satisfactory results and that the results 
are susceptible of commercial interpretation, while at the same time 
they are sufficiently small for complete control and to afford fiber- 
yield figures which are both accurate and reliable. Two such rotary 
charges gave enough fiber for one complete paper-making test. 
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 J meshes per linear inch, the screen being agitated by hand from 
below. Various amounts of dirt and chaff could be removed, depend- 
ing 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 
1 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. Bui. 309, p. 28, 1915. 
