THE FORMS OF SOIL WATER 209 



to an atmosphere of definite humidity under conditions 

 of constant temperature and pressure. Complications 

 arise from the necessity of using a very thin layer of soil, 

 from the difficulty of controlling humidity, and from the 

 tendency of capillary water to form in the soil interstices 

 before the hygroscopic film is satisfied. The question of 

 how long the exposure should take place is a very serious 

 factor, as has already been pointed out. In the drying 

 of the soil after exposure a vexing condition also is en- 

 countered, in that as the temperature is raised, the giving- 

 off of water vapor continues. It is evident, therefore, 

 that not only must any method be more or less arbitrary, 

 but that its value can be only comparative. The method 

 of Mitscherlich, as already described, 1 is probably the 

 most nearly accurate. He exposes the dry soil under 

 partial vacuum over 10 per cent sulfuric acid and water. 

 The partial vacuum is to hasten adsorption, and the acid 

 to prevent a fully saturated air, thereby cutting down 

 chances of dew deposition. 



138. Heat of condensation. The amount of energy 

 necessary to expel the hygroscopic film from around a 

 soil particle is very great, since its only movement is 

 thermal. As a matter of fact, it is really impossible to 

 divest the soil grain entirely without causing the loss of 

 moisture other than that simply adsorbed. As so much 

 energy is expended in removing this film, it is reasonable 

 to expect that a certain amount of heat of condensation 

 when the film is resumed would become apparent. Pat- 

 ten 2 offers the following quantitative data concerning 

 this point : 



1 Mitscherlich, A. E. This text, paragraph 111. 



2 Patten, H. E. Heat Transference in Soils. U. S. D. A., 

 Bur. Soils, Bui. 59, p. 34. 1909. 



