26 THE SOII, SOLUTION 



ol the soil can be readily diminished to the critical water content ; 

 but to diminish it further by mechanical means is not easy. The 

 tenacity with which film water is held by the soil grains has been 

 shown in several ways. In one of these, for instance, a semi- 

 permeable membrane was precipitated in the walls of a porous 

 clay cell, which was then filled with sugar solution having an 

 osmotic pressure of about 35 atmospheres. When this cell was 

 buried in a soil having a moisture content above the optimum, 

 water flowed into the cell. On the contrary, when the cell was 

 buried in another sample of the same soil having a moisture con- 

 tent well below the optimum, there was a marked flow of water 

 from the cell. It would appear, therefore, that the attraction 

 between the soil grains and the film-forming water was certainly 

 greated than the solution pressure of the sugar. 1 Again, by 

 whirling wetted soils in a rapidly revolving centrifuge, 2 fitted 

 with a filtering device in the periphery, and developing a force 

 equivalent on the average to 3,000 times the attraction of gravita- 

 tion, the soils could not be reduced below the critical water con- 

 tent. From the results of Lagergren, 3 Young, 4 and Lord 

 Rayleigh, 5 it appears that the force holding a very thin moisture 

 film on the soil grains would be of an order of magnitude from 

 6,000 to 25,000 atmospheres. This force, however, must greatly 

 decrease with thickening of the film, as is shown by the fact 

 that at the critical moisture content a small further addition of 

 water produces no marked heat manifestation, though making 

 a noticeable difference in the physical properties of the soil. 



*The chemistry of the soil as related to crop production, by Milton 

 Whitney and Frank K. Cameron, Bull No. 22, Bureau of Soils, U. S. 

 Dept Agriculture, 1903, p. 54. 



*The moisture equivalent of soils, by Lyman J. Briggs and John W. 

 McLane, Bull. No. 45, Bureau of Soils, U. S. Dept Agriculture, 1907. 



*Uber die beim Benetzen fein verteilter Korper auftretende Warme- 

 tonung, von Lagergren, Bihang till K. sv. Vet-Akad., Handl., 24, Afd. II, 

 No. 5 (1898). 



* Hydrostatics and elementary hydrokinetics, by George M. Minchin, 

 p. 311, 1892. 



'On the theory of surface forces, by Lord Rayleigh, PhiL Mag. (5), 

 30, 285-298, 456-475 (1890). 



