208 SOILS: PBOPERTIES AND MANAGEMENT 



vestigators ^ find that as the temperature is increased 

 the hygroscopicity becomes lowered, thus following the 

 general laws of adsorption. Hilgard, however, obtained 

 opposite results when the air was saturated, although his 

 data agreed with previous results when hygroscopicity 

 was studied in an atmosphere unsatisfied as to its capac- 

 ity for water vapor. King ^ explains this discrepancy 

 as being due to the very high vapor pressure generated 

 by a saturated atmosphere at high temperatures, causing 

 a more rapid taking-up of water by the soil than was 

 lost from its surface. The time necessarv for a soil to 

 assume its maximum thickness of adsorbed water is un- 

 certain. Hilgard^ used seven hours in his determina- 

 tions, while Mitscherlich ^ exposed his soil for several 

 days. A soil continues to increase in weight slowly as 

 its time of exposure to moist air is increased, so that a 

 sharp line of demarcation between capillary and hygro- 

 scopic water is difficult to establish. Capillary water 

 may even be present in the minute interstices before the 

 hygroscopic film is elsewhere satisfied.^ 



137. Determination of hygroscopicity. — The method 

 of the determination of the maximum hygroscopicity of a 

 soil, or, in other words, the hygroscopic coefficient, is 

 simple in outline. The soil, in a thin layer, is exposed 



1 Patten, H. E., and Gallagher, F. E. Adsorption of Vapors 

 and G*ases by Soils. XJ. S. D. A., Bur. Soils, BuL 51, p. 33. 

 1908. 



2 King, F. H. Physics of Agriculture, pp. 179-180. 

 Published by the author, Madison, Wisconsin, 1910. 



s Hilgard, E. W. Soils, pp. 196-201. New York. 1911. 



^ Mitseherlieh, E. A. Bodenkunde, pp. 56-58. Paul 

 Parey, Berlin. 1905. 



^Briggs, L. J. The Mechanics of Soil Moisture. U. S. 

 D. A., Bur. Soils, BuL 10, p. 12. 1897. 



