THE ORIGIN OF GUM BOTH 107 



other lower forms of life are likewise producing change and under- 

 going change continually. 



According to Cameron^ the water within the soil is in reality 

 of two kinds, namely, ''film" water and "free" water. When a 

 relatively small quantity of water is added to an absolutely dry 

 soil or powdered solid there is some shrinkage in the apparent 

 volume of the solid; the water spreads over the surface in the form 

 of a film. With further addition of water the apparent volume of the 

 solid material increases until a maximum is reached. The optimum 

 water content which gives the maximum volume is a definite, 

 critical, physical, characteristic property for a given soil or solid. 

 A further addition of water will not increase the thickness of the 

 soil film but will produce free-water in the soil interstices. 



These two kinds of water play an important role. The film- 

 water is tenaciously held by the soil and subsoil particles. In 

 dry seasons it is practically a saturated solution of the dissolved 

 rock and soil materials. When the surface of the ground becomes 

 flooded, as in wet seasons or during heavy rains, the downward- 

 moving free-water extracts and carries away a part of the mineral 

 content of the film solution. Obviously the proportions of water- 

 soluble materials in soils containing moving free-water should be, 

 and are, less than those in soils containing only film-water. This 

 was conclusively shown by Hilgard.^ In the humid regions there 

 is a greater amount of rainfall, hence a greater amount of downward- 

 moving free-water, consequently a greater amount of leaching. 



Once the free-water is removed the saturation of the film-water 

 repeats. In this film-water are the dissolved rock materials, the 

 carbon dioxide, the oxygen, the nitrogen, the soluble humous 

 material, and the soil bacteria. Between these are evolved all of 

 the processes leading to the disintegration of the rock material 

 and the formation of the soil and its substrata. 



Chemical nature of glacial materials. — The rock materials 

 transported by the glaciers consisted chiefly of silicates, quartz, 

 some clays, and other previously weathered materials. The com- 

 plex silicates are salts of a very weak acid — silicic acid, with various 



' Cameron, Jour. Phys. Cheni., Vol. XIV (1910), p. 340. 

 ^ See Merrill's Rock Weathering and Soils, p. 368. 



