RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 113 
that the osmotic pressure varies inversely as the moisture content. 
The latter is probably true only within rather narrow limits. 
Further, since obtaining an equilibrium by vapor transfer is a 
slow process, it is desirable to be able to treat a large number of 
soil samples simultaneously. For this purpose a large-size bell jar, 
resting upon plate glass, may be employed. A pressure cooker, the 
cover joint being properly sealed, has also been found very useful. 
There may be one or more vessels of the solution, and as many ves- 
sels of soil as are desired, within the chamber. Using 2^-inch soil 
cans for soil containers and similar beakers for solutions, about 
60 soil samples may be treated at once under a 14 by 12 inch 
bell jar. However, a great deal of evidence shows the desirability 
of a relatively large vessel for the control solutions, and a decrease 
in the number of soil samples. 
It should be remembered that this treatment will only give the 
osmotic pressure for each soil at one particular moisture content, 
which will depend upon the total amount of water in all of the 
samples, as well as upon the concentration of the control solution, 
at the initiation of the test. To obtain a range of values for any 
soil, separate tests must be made under varying conditions. 
The fundamental provision in such a test is that the vapor cham- 
ber should have a constant temperature and be evenly heated on 
all sides. To accomplish this most simply a deep excavation in the 
ground is desirable. The very gradual seasonal change of tempera- 
ture in such a situation will not work any harm, since the vapor 
pressure within the chamber will adjust itself to such a change with- 
out necessitating any condensation. In the lack of this, a dark 
cellar may be chosen. 
In proof of the theory that a soil solution would continue to ab- 
sorb water indefinitely in the presence of saturated water vapor, 
and that, therefore, the hygroscopic coefficient as now known is a 
purely empiric quantity, a test has been conducted for slightly more 
than a year with clean sand and various modifications thereof which 
represent the different elements encountered in various types of 
soil. The sands and modified sands were all placed in the vapor 
chamber in an oven-dry state, together with a bottle of distilled 
water with linen wicks having about 10 square inches of evaporating 
surface. The successive weighings of samples and bottle indicate 
that some vapor is constantly escaping from the chamber, so that 
the vapor therein is never absolutely saturated. Early in the test 
it was found impossible to heat the chamber as evenly as desired 
in the cellar in which it was placed, so that it was removed for a 
time to an electrically heated incubator. Here the fluctuations, 
though small, were rapid and a slight overheating at one time caused 
a very severe loss from all samples, the chamber being unable to 
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