THE FILM WATER 27 



Therefore, while recognizing that our knowledge of this force 

 still lacks a desirable precision, it is nevertheless clear that the 

 force is very great 



The function of the film water in maintaining the soil structure 

 is undoubtedly important A soil in good tilth, or good condition 

 for crop growth, shows a peculiar structural arrangement of 

 the individual soil grains or soil particles, which it is very difficult 

 to describe in precise terms, but which is readily recognized in 

 practice. This condition is usually described as a "crumb struc- 

 ture," either because of its appearance or because of the peculiar 

 crumbly feeling which a soil in this condition gives when rubbed 

 between the fingers. The individual grains of soil are gathered 

 into groups or floccules. While other causes may be more or 

 less operative in particular cases, it seems very probable that 

 the film water is primarily the agency holding together the grains 

 in these floccules. The obvious explanation is that the film is 

 exerting a holding power because of its surface tension. It 

 follows, therefore, that anything which affects the surface tension 

 of water should affect the structure of the soil; that is, the 

 flocculation or granulation of the particles. But certain agents 

 which produce respectively flocculation or deflocculation, never- 

 theless modify the surface tension of the solution in the same 

 direction, and in not widely varying degree. Similar difficulties 

 arise in attempting to correlate "crumbing" phenomena with the 

 viscosity of the film water, 1 and it must be admitted frankly 

 that present views on this subject are very unsatisfactory, and 

 that more careful investigation is urgently needed on this funda- 

 mental and important problem. Not only is the absence of a 

 satisfactory theory embarrassing in considering the problems of 

 soil structure and a rational control, but the difficulties are no 

 less in the equally important problems of the movement of film 

 moisture, and the distribution of moisture in a soil. 



1 Equally unsuccessful is the attempt to correlate flocculating agents 

 with changes in the density of water. See, The condensation of water by 

 electrolytes, by F. K. Cameron and W. O. Robinson, Jour. Phys. Chem., 

 14, i-n (1910). 



