206 SOILS: PROPERTIES AND MANAGEMENT 



or chemical means or both, been thrown into a colloidal 

 condition. Especially do the humous colloids, as has 

 already been shown, function in this regard, so that the 

 organic matter must be of very great importance in deter- 

 mining the hygroscopic capacity of any soil. The finer 

 the soil, the greater is the amount of hygroscopic water 

 merely because of the large area of surface exposed. 

 Also, any practice that will increase the colloidal material 

 — the humous colloids being very susceptible to increase 

 by proper soil management — the higher will be the per- 

 centage of this hygroscopic moisture. Texture and humus, 

 then, govern the hygroscopicity of most soils. 



136. Nature of the film. — The nature of this thin film 

 which is designated as hygroscopic water has not as yet 

 been determined. Held so strongly by a molecular force 

 averaging probably 10,000 atmospheres, generated by 

 adhesion and cohesion, it is not definitely known whether 

 the film exists as a liquid or a vapor. Consequently it 

 cannot be expected to conform to the laws that are gen- 

 erally found to apply to capillary films. In many cases 

 the film may not be continuous, and being so very, very 

 thin^ it may even possess a negative surface tension. The 

 radius of influence of a particle in water has been shown 

 by Chamberlain^ to be about 1.5X10""^ centimeters. 

 Within this zone the molecules of water are much restricted 

 in their motions. The thickness of the hygroscopic film on 

 quartz particles as calculated by Briggs ^ is 2.66 X 10~^ 

 centimeters, showing that the outer edge of the hygroscopic 



^ Chamberlain, C. W. The Radius of Molecular Attrac- 

 tion. Physical Review, Vol. 31, pp. 170-182. 1910. 



2 Briggs, L- J. On the Adsorption of Water Vapor and of 

 Certain Salts in Aqueous Solution by Quartz. Jour. Phys. 

 Chem., Vol. 9, pp. 617-641. 1905. 



