PEODTJCDSTG SODA PULP FROM ASPEN. 31 
The milder treatments are apparent in the increasing number of 
ray cells and vessels, the latter being well preserved and showing 
their markings quite clearly. The fibers are twisted or broken to only 
a small extent, and yet are so well separated that there are no fiber 
bundles. 
Undercooked pulps. — Plates VI and VII illustrate the character- 
istics of undercooked pulps, and show plainly the mildness of the 
digestion treatments employed in their production. Well-preserved 
vessels with, sharply defined markings are clearly visible, ray cells 
are numerous, and the walls of the fibers are less dissolved away 
than in the more thoroughly cooked pulps. Coincident with these 
characteristics there are also present many fiber bundles or shives? 
noticeable even when the microscopic slides are examined with the 
naked eye. Undercooked fibers develop a deep color from the particu- 
lar stain used in mounting, and on this account appear very distinct. 
Of the several groups of tests, the one varying the amounts of 
caustic soda per pound of wood resulted in the greatest range of 
severity of cooking as determined by the microscopic appearance 
of the pulp fibers. A small amount of chemical resulted in an under- 
cooked pulp. With increases in the amount the strength of cell 
walls gradually decreased, the wood vessels suffered gradual destruc- 
tion, and their markings were dimmed. The ray cells and fiber 
bundles disappeared soon after the point was reached where the 
maximum yield was attained. The higher amounts of caustic gave 
the overcooked effects. 
For varying durations of cooking the effect was practically the 
same, and undercooked pulps were obtained at the shortest duration 
used. However, the highest durations employed did not give as 
severely cooked pulps as were obtained with large amounts of chem- 
ical. While all of the tests varying the cooking pressures resulted in 
fairly well cooked pulps, there was a tendency toward undercooking 
at the lowest pressure used. The tests varying the initial concentra- 
tions also resulted in well-cooked pulps, except for the highest con- 
centration, where a slight overcooking effect was observed. 
INFLUENCE OF COOKING CONDITIONS ON COST. 
While it is not feasible from the data at hand to discuss all cost 
factors affecting the commercial production of pulps, the more direct 
effects of the cooking conditions employed can be shown. The actual 
effects on the cost of production, of course, depend upon various 
other operating conditions at any particular mill, but the general 
trend of the effects is the same, irrespective of local conditions. 
TIME. 
Shorter durations of cooking result in more efficient use of the 
digesting apparatus; more cooks can be made per day or per week, 
