Oct. 22 ,1917 Direct Determination of the Hygroscopic Coefficient 159 
The above would indicate that in the case of a mineral subsoil a tem¬ 
perature as high as 175 0 C. may be permissible. If the subsoil should 
be one containing any considerable amount of organic matter, such a 
temperature should not be employed, but one of 105° to no° C. we 
found to have no distinct influence upon the values found for silt-loam 
surface soils containing 2 per cent of organic carbon. We made no 
experiments with peat soils or with mineral soils very rich in organic 
matter. 
EFFECT OF GRINDING 
As many fine-textured samples when first removed from the drying 
oven are so bricklike that it is difficult to reduce them with a rubber pes¬ 
tle so that they will pass a i-mm. sieve it is desirable to know whether 
the hygroscopicity is appreciably increased by freely grinding them in 
a steel mortar until they will pass through such a sieve. The samples 
we used were four control soils, three of which, unless they had first 
been moistened and then oven-dried, needed no grinding to pass a i-mm. 
sieve, while the fourth did not need it even then. A portion of each 
was ground in a steel mortar until it passed through a silk bolting cloth 
with openings of 0.25 mm. (Table IX). 
Table IX .—Influence of grinding upon the amount of moisture absorbed 
Soil. 
Hygroscopic coefficient. 
Unground 
(2-1 mm.). 
Finely ground 
(below 0.25 
mm.). 
c. 
0.8 
5-8 
10. 9 
22. 0 
1.4 
5-7 
k >-5 
21. 7 
A. 
B. 
I. 
Only in the case of the very coarse-textured soil, C, which in practice 
would not need grinding, did we find any increase in the hygroscopicity. 
If a ball mill were employed, the reduction in size of particles might 
easily be so great as to affect seriously the value found. 
It would appear that the dried samples, if very hard, may safely be 
reduced in a steel mortar until they pass a i-mm. sieve. 
INFLUENCE OF SIZE OF ABSORPTION BOXES AND THE NUMBER OF TABLES 
With two soils, H and S, larger absorption boxes were also tried, 
these being lined with blotting paper as with the smaller ones. We used 
one 23.25 inches long, 16 inches wide, and 6.25 inches high, and another 
12 inches square and 30 inches high, these being the inside dimensions. 
In the first an exposure of about 72 hours was necessary to bring the 
