RESEARCH METHODS IX STUDY OE EOREST ENVIRONMENT. Ill 



With partial vapor pressures over acid solutions, practical equi- 

 librium was reached in all soils at the same ultimate moisture con- 

 tent, whether the soil was started in a moist or dry condition. Thus 

 sea-island cotton soil, which was the finest used, dried out at a vapor 

 pressure of 17.90 millimeters (76 per cent of saturation) in 97 days 

 from 55 per cent to 6.08 per cent moisture, and the same soil ab- 

 sorbed in the same period 5.6 per cent, starting from a dry condition. 

 In the presence of a vessel of water the drying out was always very 

 slow, and the fact that any drying whatever occurred is believed to 

 be sufficient evidence that the atmospheres in the desiccator were not 

 saturated, owing to the presence of the dry soils in the same at- 

 mospheres. 



Finally, the energy effects of absorption must not be overlooked. 

 Patten and Gallegher cite a number of investigations which show 

 that the heat released when vapor is absorbed by a soil is in excess 

 of the latent heat which is released when vapor condenses. This 

 fact indicates that water held in the soil, like water held in a solu- 

 tion, is brought to a greater density than that in which only water 

 molecules are attracting each other. This density can only be ob- 

 tained through the release of additional energy. 



Examined kinetically, then, the whole situation is fairly simple. 

 Molecules of a gas or vapor repel one another, and this repulsion 

 increases with the temperature and energy of each molecule. When 

 a certain density is obtained in a volume of vapor, the so-called 

 saturation density, the molecules may either return to the liquid 

 from which they emanated or be compelled to unite with other 

 molecules, starting condensation in the molecular sense. In the case 

 of atmospheric vapor, solid particles, such as dust particles, may 

 start condensation through their attraction for vapor molecules, 

 which latter would otherwise repel one another too strongly to be 

 brought together. 



The same phenomena occur in the soil. A soil particle of given 

 size, mass, and gravitational power can attract to itself a certain 

 number of vapor molecules, this number depending upon the space 

 available and the distance at which the vapor-molecules begin to repel 

 one another, or the temperature and energy of these molecules. A 

 vapor molecule which has been trapped, and has given up some of 

 its energy in this process of " individual condensation,' 1 is relatively 

 inert, but not so insert as the soil particle, and is still capable of re- 

 pelling other molecules to some extent. The ultimate number of 

 molecules that can be held in a given soil, therefore, must depend 

 (1) primarily on the energy of the free molecules as governed by 

 temperature; (2) on the area or surface of soil particles exposed, 



