138 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
possibilities as an avenue of approach. Thus, by the freezing-point 
method the osmotic pressure of the soil solution corresponding to 
any given moisture content may be determined: and by several 
trials, the point at which the soil water ceases to behave as a liquid, 
that is. ceases to show a definite freezing point, may be determined 
quite closely. This water content is probably the term sought as a 
standard. The freezing-point method has one serious objection: 
namely, that it forbids keeping the soil during test in a natural state 
of compactness, or with a natural arrangement of the soil particles. 
The vapor transfer method has also many possibilities. In the 
ordinary determination of the hygroscopic coefficient the watei 
vapor is probably not entirely saturated: and. in consequence, under 
certain empiric conditions of the test, there may be obtained in a 
limited time a limited absorption of vapor by the soil which was 
air-dry at the outset. The quantity absorbed, so far as the tests go 
bears a fairly constant relation to the wilting coefficient, the ratio of 
the two quantities being approximately 2:3. The objection to this 
method is that the conditions are purely empiric, and the quantity 
of soil treated is very small. 
By exposing soils to water vapor in a vapor-tight chamber, such 
as a bell jar, and in the presence of a solution of an active salt which 
represents a given osmotic pressure and vapor pressure, considerable 
masses of soil may. after a long period, be brought to vapor pres- 
sure equilibrium with the solution and with one another. The mois- 
ture content of each soil is then the " osmotic equivalent " of the con- 
trol solution, whose osmotic pressure is readily calculated. The 
osmotic pressure to exist at the end of the test may be in part con- 
trolled by calculations at the outset, and later by changing the con- 
trol solution. A solution, at the end of the test, representing 50 
atmospheres osmotic pressure, for example, might be taken as a 
standard for establishing osmotic equivalents in lieu of wilting coeffi- 
cients. The objections to this method are the long time required to 
complete a test and the absolute need for a constant temperature, or 
at least for the elimination of rapid changes. The former objection 
is partly counterbalanced by the number of soils which may be 
treated simultaneously. 
In the moisture-equivalent determination, as so far conducted, the 
moisture of any soil is submitted to a definite centrifugal force which 
tends to separate it from the soil. Within the limits of agricultural 
types of soil, at least, the force of 1,000 gravity employed by Briggs 
and Shantz (114) appears to leave in the soils amounts of water 
which bear a nearly constant relation to the respective wilting co- 
efficients. Experiments with a force of 100 gravity have shown 
wide variations in results with different types of forest soils, indicat- 
ing that, as humus and clay proportions vary, both strong and weak 
