1916] SHULL—SOILS 4g 
Laboratory of the University of Chicago, where all needed facilities 
have been generously provided. 
II. Historical 
~The general status of our knowledge of the forces operative in 
soils was briefly discussed by CAMERON (10) several years ago. 
It is obvious from this account that up to the present time we have 
known very little about soil forces within the range of unavailable 
moisture, that is, between the wilting coefficient and air-dry 
condition of the soil. 
The attempts thus far made at measurement of the surface 
forces which are known to exist in finely divided matter of all 
kinds have been made from various angles, but they can be classed 
under two main heads: (a) physical, and (0) physiological. 
PHYSICAL MEASUREMENTS 
A. HEAT OF WETTING METHOD.—The principle of heat of wetting 
was discovered by PovurILLeT (26) a good many years ago. He 
found that all kinds of dry powders, from inorganic substances 
and porous organic matter, yielded heat on being wet with fluids 
like water, oil, alcohol, etc. The organic substances yielded the 
greater amount of heat because, he stated, the organic matter was 
composed of particles incomparably thinner than the finest inorganic 
powders. 
The literature dealing with the application of this principle to 
measurements of surface force has been reviewed so recently by 
PATTEN (25) that it will not be necessary to go into the details of 
it here. It will be sufficient to point out that through the work of 
Rose (28) and JuNGK (19) we gained the conception that water is 
condensed on the surface of the powdered inorganic or finely divided 
organic substances, and that the release of heat is due to this com- 
pression. The quantitative studies’of NAGELI (23) made it possible 
for SacHs (29) to calculate the surface forces in starch grains. 
Since Joure had shown that 34.3 atmospheres of pressure raises 
the temperature of water 0.03° C., the amount of heat produced 
by starch on being wet would indicate much more than 10,000 
atmospheres of surface force compression. SACHS assumed, of 
course, NAGELI’s theory of the structure of organic matter. 
