110 BULLETIN 1059, IT. S. DEPARTMENT OF AGRICULTURE. 



vapor from escaping, their results are not indicative of those to be 

 expected when vapor pressures are maintained in true equilibrium 

 with solutions of reasonable osmotic pressure. As has been stated, 

 a solution showing 12 atmospheres pressure will be in equilibrium with 

 vapor 99 per cent saturated, and even this degree of saturation, it is 

 believed, would be extremely difficult to maintain except in a fully 

 sealed vessel. 



On the other hand, Patten and Gallagher (136) have carried out 

 experiments, both on absorption of vapor and evaporation from soils, 

 in "desiccators" which are assumed to be similar to the bell jars 

 mentioned hereafter, and which, while usually not strictly vapor 

 proof, approach much nearer to the ideal condition. Patten and 

 Gallagher, both in their review of earlier work and in their own 

 experiments, have established a number of salient points which assist 

 in the proper conception of the relation between vapor (or, to a cer- 

 tain extent, gas) molecules and solid particles, such as those of the 

 soil. Schiibler 17 and Davy 1T are quoted as having shown that, in 

 general, the finer the texture of the soil and the greater its content 

 of humus, the higher is the absorption capacity of soil for water 

 vapor. These results, while actually referring to the initial rate of 

 absorption, are fairly indicative of the forces with which various 

 soils attract water vapor. Von Dobeneck 17 obtained similar results, 

 though concluding that large grains absorbed more vapor per unit 

 of surface than small ones. Each soil particle reacts upon vapor 

 molecules independently, and each has a specific relation to different 

 kinds of gases. Several investigators have decided that the absorp- 

 tion of vapor decreases with an increase in temperature, even though 

 the absolute vapor pressure increases proportionately. Patten and 

 Gallagher have carefully proven this. Hilgard's contrary finding 

 may be explained on the basis that he was dealing almost wholly 

 with rate of absorption, and higher absolute vapor pressure should 

 more quickly bring about equilibrium. Mason and Richards 17 found 

 that cotton fiber containing water resembles a solution in exhibiting 

 a definite partial vapor pressure. 



Patten and Galleghers most important results have to do with the 

 rates of absorption of vapor, and with evaporation, in the presence 

 of various vapor pressures controlled by sulphuric acid solutions 

 and vessels of water, within desiccators. The rate of absorption by 

 drjr soils increases, and the rate of evaporation from" wet soils de- 

 creases quite regularly as the partial vapor pressure in the desiccator 

 is increased. It is, however, evident in all of the results that as the 

 vapor pressure approaches saturation the amount of absorption in- 

 creases in greater proportion than does the vapor pressure. A num- 

 ber of the graphs are strongly suggestive of the idea that, if com- 



17 For complete citations see Bureau of Soils, Department of Agriculture, Bulletin 51. 



