PROPERTIES OF THE COLLOIDAL SOIL MATERIAL U 
agglomeration of silica particles each surrounded by a film of water. 
Capillary spaces a few millimicrons in diameter are formed by the 
agglomeration of particles. The water-air interface of these capillaries 
is enormous when the material is nearly dry. When the material is 
immersed in water the capillaries are filled with adsorbed water and 
the water-air interface disappears; hence the evolution of heat. 
When the material is immersed in benzene, the adsorption of benzene 
in the capillaries replaces the water-air interface with a water-ben- 
zene interface. 
Patrick's concept of the mechanism of adsorption and heat of wet- 
ting emphasizes the idea of the attractive force between the mole- 
cules of the liquid as manifested in surface tension; the older con- 
cept emphasizes the attractive force between the surface molecules of 
the adsorbent and the molecules of the liquid. But according to 
either hypothesis, differences in the kind of surface presented by the 
adsorbent and differences in the extent of adsorbing surface should 
both produce differences in the heat of immersion in a given liquid. 
Soil colloids, like silica gel, contain considerable water not driven 
off at 110° C. 11 If it were assumed that the ultimate particles of all 
soil colloids possess water surfaces and are constituted similarly to 
silica gel. then the heats of wetting of the different colloids should be 
proportional to the capillary surfaces exposed. The total surfaces or 
the average diameter of the capillaries would depend on the average 
size of particles. On these assumptions, the heats of wetting of the 
different colloids should show a relation to the average size of par- 
ticles. 
If the different soil colloids, which vary widely in chemical com- 
position, possess different kinds of surfaces prior to immersion, then 
the heats of wetting may be expected to reflect the influence of both 
extent and nature of the surface. In this case the heats of wetting 
of various colloids should show some relation to the size of particles 
and some relation to other factors which are particularly dependent 
upon the nature of the surface. 
It is, of course, very probable that the area or size of the particles 
is intimately associated with, or dependent upon, the kind of sur- 
face; that is, upon the nature of the boundary layer of the colloid 
particle or micelle. If a surface with a great attraction for water 
were associated with small particle size, greater differences might be 
expected in the heats of wetting of the different colloids than in the 
average size of particles. If, on the other hand, the relation between 
kind of surface and size were the reverse, smaller differences might 
be expected in the heats of wetting than in the average size of 
particles. 
_ The heat of wetting in water was determined for colloidal mate- 
rials extracted from eight different soils. These values are given in 
Table 4. The determinations were made by immersing the samples 
in water after drying for 18 hours at 110° C. without outgassing. 1 ^ 
Details of the procedure are given in a previous publication" (2). 
11 It has been shown that soil colloids contain from 3 to 16 per cent of water not 
driven off at 110° C. (/,?). 
12 An attempt was made to obtain the heat of wetting on outgassed material, as it was 
thought that this might form a better basis for comparing the different colJoids. Experi- 
mental difficulties were encountered, however, in working with certain highly plastic col- 
i? ld i S- lt is P° ssiDle that the heats of wetting of evacuated colloids might have been 
higher in some cases, although no evidence to this effect was obtained. 
655?— 26 2 
