Chapter VIII. 



ABSORPTION BY SOILS. 



A property of soils, affecting profoundly the composition and 

 concentration of the soil solution, is absorption. 1 It is generally 

 recognized that soils are good absorbers for vapors, and this 

 fact finds practical expression in the common practice of bury- 

 ing things with a disagreeable odor, such as animal carcasses, 

 night-soil, etc. It is also well-known that dissolved as well as 

 suspended material can be more or less completely removed from 

 water by passing it through sand or soil, and this fact finds 

 important application in water supplies for cities and towns, 

 sewage disposal, etc. It was known as long ago as Aristotle's 

 time that ordinary salt is partly removed from water by passing 

 through sand or soil. In recent times the practical as well as 

 theoretical importance of this phenomenon has led to consider- 

 able study and experimental research, so that our knowledge of 

 absorption effects is now fairly extensive, though it can hardly 

 be claimed that it is satisfactory. The absorption of a dissolved 

 substance from solution by a soil may be one or more of at least 

 three kinds of phenomena. It may be a mechanical inclusion or 

 trapping, distinguished by the term imbibition, the most familiar 

 and striking case being the absorption of water itself by soil or 

 sponge or similar medium. It may be a partial taking up of 

 the dissolved substance to form a new compound or a solid 

 solution, 2 as probably is the absorption of phosphoric acid by 



1 For a detailed discussion and citations of the literature, see : Absorp- 

 tion of vapors and gases by soils, by H. E. Patten and F. E. Gallagher, 

 Bull. No. 51 ; and Absorption by soils, by H. E. Patten and W. H. Wagga- 

 man, Bull. No. 52, Bureau of Soils, U. S. Dept. Agriculture, 1908. 



2 That is, a homogeneous solid, which may be either crystalline or 

 amorphous. Probably the readiest criterion for distinguishing between 

 a definite compound and a solid solution, is that the former is stable in 

 contact with a liquid solution of its constituents over a measurable range 

 of concentrations, while the composition of the solid solution changes 

 with every change in the concentration of the liquid solution in contact 

 with it. 



