208 SOILS: PROPERTIES 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 1 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 2 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 necessary for a soil to 

 assume its maximum thickness of adsorbed water is un- 

 certain. Hilgard 3 used seven hours in his determina- 

 tions, while Mitselierlich * 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. 5 



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 Gases by Soils. U. S. D. A., Bur. Soils, Bui. 51, p. 33. 

 1908. 



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

 Published by the author, Madison, Wisconsin, 1910. 



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



4 Mitscherlich, E. A. Bodenkunde, pp. 56-58. Paul 

 Parey, Berlin. 1905. 



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

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



