PROPERTIES OF THE COLLOIDAL SOIL MATERIAL O 
The accuracy of the determination of particle size was undoubtedly 
much greater in the case of the Susquehanna, Xorfolk, and Sassafras 
colloids than in the case of the Fallon. In the former, most of the 
particles gave distinct, brilliant spots of light, whereas the latter 
contained many particles near the extreme limit of visibility. It is 
probable that in the case of the Fallon colloid a comparatively large 
number of particles were actually invisible either because of ex- 
tremely small size or low index of refraction. Such an error in 
counting would ascribe to the Fallon colloid a particle size larger 
than was actually present. It is therefore probable that the particles 
of the Fallon colloid were actually the smallest of the group studied. 
Since the turbidity of a suspension is more or less proportional to 
the size of the particles or aggregates, some evidence concerning com- 
parative particle size in colloid suspensions was obtained by observ- 
ing the comparative turbidities. Suspensions containing 0.1 gram per 
liter of each of the seven colloidal materials were made up and ar- 
ranged in the order of relative turbidity. The Susquehanna colloid 
showed greatest turbidity, the Xorfolk less, and so on in the same 
order as the particle size determined by the count method, with the 
exception of the Fallon, which was least turbid. This evidence 
serves further to substantiate the conclusion stated above that the 
particles of the Fallon colloid were in reality the smallest of the 
group studied. 6 
The size of particles shown in Table 2 is in general accord with the 
results obtained by Ehrenberg and Given {11). They found an 
average diameter of 140 millimicrons for a colloidal clay sample 
prepared by long subsidence of an ammoniacal suspension. The re- 
sults obtained by Svedberg and Xicliols (-57) on a colloidal clay show 
also a similar magnitude. They used a centrifugal method for 
measuring the minimum s.ize and the ultramicroscopic count method 
for measuring average size of particles. The smallest particles were 
80 millimicrons in diameter and the average only slightly larger. 
Ultramicroscopic determinations doubtless give a fairly accurate 
estimate of particle size in sols of colloidal soil material, provided 
all the assumptions involved in the determination are true; namely, 
that all the particles are primary ones, and all visible, and that 
the specific gravity of the particles as determined in water represents 
the true volume relation to mass. It has just been shown thar. in 
some cases at least, there is strong probability that some of the 
material is invisible. There is no direct evidence indicating whether 
the particles counted are all primary ones, all aggregates, or a mix- 
ture of both. The latter case seems most probable. Furthermore, 
since the specific gravity of the material was determined on the basis 
of its weight when dried at 110° C, it is probable that in the swollen 
and perhaps hydraced state of the colloid in the suspension, particle 
size may have been considerably different from that calculated when 
dry weight was used. 
SPECIFIC GRAVITY 
Specific gravity is considered a definite constant of most liquids 
and solids. But in the case of materials which appear to be sab- 
6 The case with which water is removed from the different colloidal suspensions is 
probably also indicative of comparative particle size. The Fallon, Sharkey, and Marshall 
colloids very quickly clog Pasteur-Chamberland filters, thus making the filtration pr 
very slow. On the other hand, the Susquehanna and Xorfolk colloids filter verv rapidlv. 
