WATER SOILS. 



733 



is given of the following topics : Properties of water affecting its reten- 

 tion and movement in tlie soil — gravitation of water, surface tension. 

 viscosity, and hygroscopic state; properties of films — pressure of a film 

 and surface of no pressure; form of water surface between two soil 

 grains: establishment of equilibrium between two unequal masses of 

 capillary water; salts as affecting the movement of water in soils; 

 temperature as affecting the movement of water in soils; influence of 

 texture and structure of soils on the acquirement and retention of soil 

 moisture; and displacement of capillary water through gravitation. 



The water of the soil is considered to be of three kinds: Gravitation 

 water, capillary water, and hygroscopic water. These are defined and 

 the forces and principles involved in their movement and retention are 

 illustrated on the assumption that the soil is made up of spherical 

 particles arranged in different ways. 1 The two great factors in deter- 

 mining the movement and retention of soil water, gravitation and sur- 

 face tension, are given first attention, a clear distinction being drawn 

 between surface tension and the effective force of a film ; and then the 

 modifying influences of viscosity and hygroscopicity are considered. 



"With the exception of mercury, water possesses a higher surface tension than 

 any other substance which is liquid at ordinary temperatures. The surface tension 

 of water- expressed in dynes per centimeter is 75.6 at 0° C. and 72.1 at 25 . The 

 temperature coefficient is thus ahout — 0.14 dynea per degree Centigrade. The sur- 

 face tension of most aqueous solutions of salt is higher than that of water, and the 

 surface tension increases with the concentration of the solution, as is shown in the 

 following table: 



Surface tension of solution* of satis in water. 



a Approximate weight of the dissolved substance in 100 parts by weight of the solution. 



"It must he remembered, however, that the surface tension of solutions is very 

 greatly decreased by the addition of very small quantities of certain organic sub- 

 stances produced through the decomposition of vegetable matter. This action is 

 especially marked where there are present substances of an oily nature which do not 

 go into solution, but spread out over the surface in an extremely thin tilm. Owing 

 to such substances being continually produced by the decay of organic matter, the 



1 Soyka, Forsch. Agr. Phys. [Wollny], 18 (1895), p. 1; Whitney, Agr. Sci.,3 (1889), 

 p. 19! ». 



2 Smithsonian Physical Tables, 1896, p. 128. 



