THE WATER OF SOILS. 197 



It will be noted that the greater fineness of grain in the 

 Washington dust soil induces a higher absorption of moisture 

 than occurs in the sandy soil from Mississippi, although the 

 latter contains more clay. Comparison of the figure for the 

 Mississippi pipeclay and clay soil with the ferruginous soils, 

 from the same state and from Oahu, indicate plainly the in- 

 fluence of the ferric hydrate in increasing absorption ; although 

 in the latter case the clay determination was not made, because 

 of the excess of ferric hydrate. The influence of humus is 

 plainly shown in the case of the marsh muck and soil, neither 

 of which contain any appreciable amount of either clay, or fer- 

 ric hydrate in the finely diffused condition. The relatively 

 slight difference in the absorptions of muck and soil is due to 

 the only partial humification of the organic matter in the 

 former, while in the soil the humification is sensibly complete, 

 and the sand forming the body of the material serves to render 

 it more loose. 



These data, referring to natural materials, while not as com- 

 plete as could be desired, are sufficient to prove the facts, and 

 seem preferable to any artificially devised imitation of their 

 kind. 



Influence of Temperature, and Degree of Air-Saturation. 

 The amount of moisture absorbed varies materially both with 

 the temperature, and with the degree of saturation of the air 

 to which the soil is exposed. Schiibler, Knop and other earlier 

 observers, operating with earth exposed to air only partly 

 saturated, and with soil layers of considerable thickness (in 

 watch glasses), found that the absorption decreased as the 

 temperature increased, according to a law formulated by Knop. 

 The writer found that under the conditions established in the 

 experiments of Knop and others, the air was not nearly satur- 

 ated, 1 so that these determinations are marred by ineliminable 



1 It should he understood that it is by no means easy to insure full saturation 

 in any considerable volume of air. 



It has generally been considered sufficient to cover with water the bottom of 

 the space in which absorption was to occur. The writer found that in order to 

 insure uniform results, it was necessary to cover the entire inner surface of the 

 vessel with wet blotting paper, and even then to exclude carefully all circulation 

 of air by padding the joints with such paper. When only the bottom of the box 

 was covered, samples placed at different levels above the water surface gave dis- 

 cordant results. It was also observed that whenever the thickness of the soil 



