igi6] SHULL— SOILS 3 



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 (io) 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 (b) physiological. 



PHYSICAL MEASUREMENTS 



A. Heat of wetting method. — The principle of heat of wetting 

 was discovered by Pouillet (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 Joule 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. 



