98 Bt'LLETIX 1059. T\ S. DEPARTMENT OE AGBICULTUEE. 
The discovery that within certain limits the moisture of the soil 
follow- the laws of osmosis, or more precisely speaking, the laws of 
dilute solutions with respect to its freezing point, has naturally led 
to the idea that the soil solution might also be considered as having 
a definite vapor pressure at a definite osmotic concentration. If this 
were true, then a soil placed in a moist atmosphere should give off 
or absorb vapor, according to whether its original solution repre- 
sented a lower or higher osmotic pressure than that represented by 
the atmosphere of vapor in which it was placed. Furthermore, if 
this vapor pressure manifested itself properly and in accordance 
with the laws of solutions, then, through vapor transfers, one soil or 
a hundred soils simultaneously might be brought into vapor-pressure 
equilibrium, and thereby into osmotic equilibrium, with a solution 
whose osmotic pressure is readily determined: and the moisture con- 
tents corresponding to such osmotic pressure might then be readily 
measured for one or all of the soils. This plan was conceived as a 
possible means of avoiding some of the difficulties of the freezing- 
point method of osmotic determinations, which are especially bother- 
some in treating coarse soils. That the theory is correct may hardly 
be questioned now. and full discussion of the available data will be 
given later. This subject has been mentioned here because of its 
possible bearing on the hygroscopic coefficient determinations. It is 
rather readily seen that, if the laws of solutions prevailed under all 
conditions of soil moisture, a soil exposed to completely saturated 
water vapor should go on absorbing moisture indefinitely, because 
the dilute solution of the soil would always stand for some osmotic 
pressure, while saturated water vapor would stand for none at all. 
Whether this does not occur in the hygroscopicity tests because of 
the failure to create a completely saturated atmosphere, or because 
there is a sharp line between the behavior of water vapor in the soil 
and liquid water, is for the future to decide. That it probably has 
no practical bearing on the hygroscopic coefficient under the empiric 
condition- set for that test, is perhaps enough in itself. It will 
help to clarify the matter if it is remembered, first, that Bouyoucos 
(109) has shown that at about the moisture content at which wilting 
occurs, the water of the soil ceases to behave as a liquid and refuses 
to freeze: and secondly, that Briggs and Shantz (114) have shown 
that the hygroscopic coefficient falls considerably below the wilting 
coefficient, the former being usually about 0.7 of the magnitude of the 
latter. 
Since the determination of the hygroscopic coefficient begins with 
air-dry soil, it does not deal with liquid water in the soil, but more 
probably with water molecules more or less separated, like individual 
vapor molecules. 
