164 
Journal of Agricultural Research 
Vol. XI, No. 4 
The data in Table XII show that in the case of soils of medium hygro- 
scopicity a 12-hour exposure gives results so close to those obtained by 
one of longer duration or by the use of metal trays that the differences 
may be considered within the range of experimental error. However, 
in the case of soils of very high hygroscopicity, such as I, it is not certain 
that the results are not slightly too low; in the case of these it is especially 
important to have the layer of soil very shallow. With L and I we used 
only from 2 to 5 gm., 8 to 16 gm. giving a decidedly lower result, 
while with soil A 10 gm. was not too much and with C as much as 80 
gm. could be used without lessening the value found. 
It is highly important that the method of Hilgard, carried out exactly 
as he described it gives reliable results, as this makes available as trust¬ 
worthy material for investigational purposes the great number of data 
on the hygroscopic coefficient which he has reported along with physical 
and chemical analyses of arid and humid soils. 
MODIFICATIONS OF METHOD WHEN SAMPLE CONTAINS GRAVEE OR 
PEBBEES 
On many soil types the sample in which moisture has been determined 
contains gravel or even small pebbles. These can not as such be in¬ 
cluded in the material exposed in the absorption boxes, as the quantity 
so used is so small that the coarse particles would constitute either a 
much smaller or much larger proportion of these than they did in the 
moisture sample. Accordingly they must either be separated and a 
correction introduced or be reduced to a state sufficiently fine to permit 
of a uniform sample being secured while still coarse enough to have 
practically no absorptive capacity. As a coarse sand has a hygroscopic 
coefficient of only about 0.5, the pebbles and gravel if crushed just 
sufficiently to pass a 2-mm. sieve will cause no difficulty on the trays and 
introduce no appreciable errors. 
In practice we have found it most convenient to crush the sample in 
a steel mortar, sifting out the fine material at frequent intervals with 
a 2-mm. sieve. 
CONCLUSIONS 
The amount of hygroscopic moisture absorbed increases with the rise 
of temperature. 
Drying of mineral soils at temperature of ioo° to no° C. does not 
appreciably decrease their hygroscopicity. 
Intractable samples may be reduced in a steel mortar to pass a i-mm. 
sieve without appreciably affecting their hygroscopicity. 
Twelve hours’ exposure in the absorption boxes is sufficient only when 
the soil layer is very shallow. In practice a longer interval is found more 
convenient, 20 to 24 hours proving very satisfactory. An exposure of 
