Chapter VI. 



THE FILM WATER. 



When a relatively small quantity of water is added to an 

 absolutely dry soil or other powdered solid, there is some 

 shrinkage in the apparent volume of the soil or powder. The 

 water spreads over the surfaces of the solid particles in a film, 

 and a rise in temperature shows that a noticeable energy change 

 accompanies the formation of the film. 1 With further incre- 

 ments of water the apparent volume of the soil increases until a 

 maximum is reached. The water content at which this 

 maximum volume of soil can be attained is a definite physical 

 characteristic for any given soil. What is popularly known as 

 the "optimum water content" corresponds to this critical content. 2 

 It is the point at which further additions of water will not 

 increase the thickness of the moisture film on the soil grains, but 

 will give free water in the soil interstices. Just as the apparent 

 volume of a given mass of soil varies with the water content, and 

 reaches a maximum at a critical moisture content, so do all the 

 physical properties vary and have either a maximum or minimum 

 value at this same critical moisture content. Thus the apparent 



1 See, in this connection, Energy changes accompanying absorption, by 

 Harrison E. Patten, Trans. Am. Electrochem. Soc., 11, 387-407 (1907) ; 

 see also the recent valuable research, Les degagements de chaleur qui so 

 produisent au contact de la terre seche et de 1'eau, par A. Muntz et H. 

 Gaudechon, Ann. sci. agron. (3), 4, II, 393-443 (1909), where it is shown 

 that probably a part of the heat is due to chemical combination between 

 the water and the other soil components. To quote, "Ces diverses obser- 

 vations nous conduisent a penser, sans nous en donner toutefois la preuve 

 absolute, que la fixation, de 1'eau sur les elements terreux tres fins et sur 

 les materiaux organises, est tout au moins, en partie, attribuable a une 

 combinaison chimique qui se manifeste non seulement par un fort degage- 

 ment de chaleur, mais aussi par la soustraction de 1'eau a des substances 

 aux-quelles elle semble chimiquement liee." 



*The moisture content and physical condition of soils, by Frank K. 

 Cameron and Francis E. Gallagher, Bull. No. 50, Bureau of Soils, U. S. 

 Dept. of Agriculture, 1908. See also Uber physikalische Bodenunter- 

 suchung, von H. Rodewald, Schriften Naturwiss. Vereins Schleswig- 

 Holstein, 14, 397-399 (1909)- 



