160 NATUBE AND PROPERTIES OF SOILS 



of the water film. The distinctive characteristics of these two 

 portions of the capillary water are due to their controls — 

 colloidal in one case, surface tensional in the other. 1 



While the outer portion of the capillary water is undoubt- 

 edly in the form of a more or less continuous film from par- 

 ticle to particle, the bulk of such moisture probably exists 

 normally in the interstices between the soil grains. Such a 

 condition arises because of the pressure developed by the 

 force of surface tension. The pressure due to surface tension, 

 however it may be expressed, varies with the curvature of 

 the film and is proportional to twice the surface tension di- 

 vided by the radius. The less the radius the greater the cur- 

 vature and, therefore, the greater the stress developed by sur- 

 face tension. 2 



The situation so far as the soil is concerned may be ex- 

 plained in an empirical way as follows : Suppose that two par- 

 ticles, each carrying a capillary water film, be brought into 

 such contact that the films coalesce. There are now two 

 distinct surfaces, that at A, A' (see Fig. 28), with the curva- 



1 Bouyoucos classifies these two types of capillary water as free (the 

 more active) and capillary-absorbed (the inner group). The distinction 

 is made on the basis of his dilatometer results, the portion which freezes 

 at about 0°C being considered as the more active or free. 



Bouyoucos, G. J., A New Classification of the Soil Moisture; Soil 

 Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 



"Surface tension is the tension of a liquid surface by virtue of which 

 it acts like an elastic enveloping membrane, tending always to contract 

 to the minimum area. While molecules in the interior portion of the 

 liquid are attracted in all directions and are thus at equilibrium, those 

 on the surface are attracted by an overbalancing force toward the 

 interior. In measurement, surface tension is considered as the force with 

 which the surface on one side of a line, one centimeter long, pulls against 

 that on the other side of the line. It is generally expressed in dynes. 

 The pressure due to surface tension varies with the curvature of the film. 

 It is usually expressed as: 



p = 2T 

 r 

 where P is the pressure; T, surface tension; and r, the radius of the 

 drop. As the radius becomes less, the curvature increases and the pres- 

 sure due to surface tension increases. An increase of T will increase 

 the pressure, P. v 



