PROPERTIES OF THE COLLOIDAL SOIL MATERIAL 
15 
the increases in volume recorded were probably only indicative 
of differences in degrees of swelling. It was assumed in the pro- 
cedure that the various colloids did not swell in benzene. Although 
this may not be strictly true, it was at least approximately correct 
for the soil colloids, since the lumps did not break down in benzene 
as they did in water and no change in volume was apparent under 
the microscope. In the case of gelatin or bentonite the difference 
between the volume in water and in benzene was so great that- 
some swelling in benzene would not have affected the comparisons 
made. 
,The swelling capacities of the different colloids indicated by this 
method are shown in Table 6. The magnitude of swelling, given 
in the last column, is the difference between the volume of the 
material in water and in benzene, calculated as per cent of the 
benzene volume. 
Table 6.- 
—Swelling 
(t 
olunie increase) 
or 
colloids 
in water 
Kind of colloid 
Volume 
per gram 
in benzene 
Volume 
per gram 
in water 
Swelling 
Gelatin 
Cubic 
centimeters 
1.8 
1. 1 
1.1 
1.0 
1.0 
1.1 
1.1 
1. 1 
1.1 
1.3 
1.3 
1.3 
Cubic 
centimeters 
18.0 
1.14 
9.0 
2.5 
1.9 
2.0 
1.8 
1.8 
1.8 
1.9 
1.9 
1.8 
Per cent 
900 
Silica gel 
4 
Bentonite 
718 
Fallon.. 
150 
Sharkey 
90 
Susquehanna 
82 
M arshall 
64 
Miami. 
64 
Sassafras 
64 
Huntington - 
46 
Cecil... 
46 
Norfolk 
38 
All the materials except silica gel occupied an appreciably larger 
volume in water than in benzene. The soil colloids swelled more 
than silica gel but much less than gelatin or bentonite. Thus, with 
respect to volume changes in water, the soil colloids stand between a 
typical nonelastic and a typical elastic gel. Probably not much 
significance should be attached to the smaller differences among the 
various soil colloids, considering the lack of strict accuracy of the 
method, but it is safe to conclude that the greatest volume change 
took place in the Fallon colloid and possibly the least in the Norfolk. 
Further light on the volume changes of the different materials was 
obtained by observing the behavior of separate particles under a 
low-power microscope. The ignited silica gel remained unchanged 
in water, and a particle of gelatin slowly increased in size without 
losing its apparent homogeneity. This, of course, is in accord with 
the conception of a rigid structure for ignited silica gel and an 
elastic structure for gelatin. A particle of bentonite increased 
quickty its volume in water and broke down progressively into a 
nearly uniform, finely granular mass which apparently had only a 
slight coherence, since some small aggregates separated from the 
edges. Bentonite might- therefore be regarded as a somewhat elastic 
gel of a less homogenous and coherent structure than gelatin. 
