8 BULLETIN 1452, U. S. DEPARTMENT OF AGRICULTURE 
nor part of a regular structure; they may be irregular interstices 
formed by the irregular agglomeration of submicroscopic particles. 
The magnitude of the difference in specific gravity in the two 
liquids varies considerably with the different soil colloids. In the 
case of most colloids, the difference is about as great as Harkins 
and Ewing (23) found for the difference between the specific gravi- 
ties of activated charcoals in water and benzene. 
If the compressibility hypothesis is used to explain the different 
specific gravities of the soil colloids in water and toluene, it would 
be necessary to assume that water is held by the colloids with a 
far greater adhesional force than toluene, since the specific gravity 
of the material is higher in water than in toluene and yet toluene 
is more compressible than water. The compressibility explanation 
would, therefore, involve a further explanation of differences in 
adhesional forces, or " affinities." 
The results might also be explained on the grounds of unequal 
penetration of the two liquids, water, which gives the higher specific 
gravity, penetrating more completely than toluene. It should be 
noted in this connection that, although some precautions were taken 
in the specific gravity determinations to promote the penetration 
of the liquid and draw out adsorbed or occluded air, it was im- 
possible to evacuate the colloid under high temperature without 
altering the nature of the material. This explanation also reduces 
itself to the somewhat vague concept of varying affinities of the 
material for water and for toluene, since it would be necessary to 
explain why water replaces adsorbed air better than toluene. 
HEAT OF V/ETTING 
It has been shown that the heats of wetting of colloidal materials 
from different soils vary widely (£, 8)^ hence the heat of wetting 
determination serves to characterize the colloidal material and to a 
certain extent distinguishes one kind from another. 
The quantity of heat evolved by the colloid on wetting is in- 
dicative of the nature of the material. However, specific conclu- 
sions regarding the ultimate structure or constitution of the colloids 
can not be drawn from measurements of the heats of wetting, since 
the nature of the reactions involved when a substance is wetted 
is not well understood. The heat evolved by a substance on being 
wetted is of course a manifestation of the net energy change accom- 
panying adsorption of a liquid. Two different hypotheses have 
been suggested to explain what takes place in the process of adsorp- 
tion and to account for this energy change. 
The older hypothesis, which is supported by recent work of Lamb 
and Coolidge (31) and of Harkins and Ewing (23) on activated 
charcoals, holds that the adsorbed liquid is compressed at the surface 
of the adsorbent with a force equivalent to 20,000 to 37,000 atmos- 
pheres in the case of certain charcoals. The compressive force is 
considered as being due to an attraction between the molecules of the 
adsorbent and the molecules of the liquid, whereas the heat evolved 
is largely the heat of compression of the adsorbed liquid. 
Patrick and his coworkers (44-) have advanced a different hy- 
pothesis to account for the heat of wetting of silica gel. They regard 
the heat of wetting as arising from the formation of new interfaces 
having a smaller total surface energy. Silica gel is conceived of as an 
