102 BULLETIX 1059. U. S. DEPARTMENT OF AGRICtJLTDBE. 
between the osmotic pressure within the plant and the antiosmotie 
pressure exhibited by the soil moisture, with an allowance for the 
distance through which this force must operate. This same basis 
was used in the preceding section as a rough means of showing 
changes in the soil condition, but without any allowance for changes 
in the absorbing power of the plant which occur with its loss or 
gain of water, or without considering the factor of height and dis- 
tance as it may affect tall trees. 
In attempting thus to express the availability of water to the 
plant, in precise terms or osmotic pressures, currently for any con- 
dition that may be encountered in the soil or plant, it is necessary 
to determine the osmotic pressure of the soil or plant quickly and 
accurately. 
The osmotic pressure of an aqueous solution is determined by the 
increase in its boiling point over that of pure water : by the depression 
of its freezing point : by the decrease u\ the vapor pressure over the 
solution; and. possibly, by the increase in the latent heat of vapori- 
zation. It is only recently that investigations of the last have been 
made, so that there is no known formula which would make this 
process available. 
Within the limit- of so-called dilute solutions a rise of l c C. in the 
boiling point represents an osmotic pressure of about 57 atmos- 
pheres: a depression of IV C. in the freezing point indicates P= 
12.05 atmospheres, and a depression of 1 per cent in the saturated 
vapor pressure over the solution, the temperature being the same, 
indicates about 12 atmospheres pressure. These approximate figures 
permit us to judge of the practical utility and accuracy of different 
methods. 
It may also be useful at this point to refer to the fact that in pure 
solutions, such a- may be used in the vapor-transfer method or in 
plasmolytic tests on tissues, the osmotic pres-ure is very readily de- 
termined by the concentration of the solution, in terms of the molec- 
ular weight of the solute, provided the solute is chemically pure and 
anhydrous. According to Xernst ( 131 1 the " molecular lowering 
of the freezing point" for water is 18.4° C., 12 or 1.^4 : C. when 1 
gram molecule of the substance is dissolved in a liter of water. A 1- 
molecule solution, therefore, stands for 22.12 atmospheres osmotic 
pressure. 
From the-e data it would seem that the boiling-point method 
would insure the greatest precision in osmotic pressure determina- 
"-- More recent investigations reported by Jones 128 <tu>w that the molecular lower- 
ing may be twice this amount in the case of salts which are dissociated by water into 
int detprminations should quickly decide this, in case of doubt. 
