4 BULLETIN 1452, U. S. DEPARTMENT OF AGRICULTURE 
through the supercentrifuge. 4 These preparations probably differed 
somewhat from those of Bradfield, who must have discarded a part of 
the finest particles, since he recovered that portion of the colloid 
which was thrown out when greater centrifugal force was uti- 
lized (9). 
Ultra filters did not seem well adapted to the determination of the 
size of particles in the colloidal soil material, inasmuch as the material 
clogs the pores very quickly and renders the filter entirely imper- 
vious to colloidal particles. Among the methods proposed for 
determining particle size, that of Siedentopf and Zsigmondy (50) 
seemed most practical for determining differences in the average size 
of particles. This method consists in counting: with an ultramicro- 
scope the number of particles in a known volume containing a known 
weight of material of determined specific gravity, and then calculat- 
ing size by assuming a definite form of particles. In the present 
study the particles were assumed to be spherical and to have the 
specific gravities shown in Table 2. The accuracy of this method 
depends to a great extent upon the character of the material ex- 
amined. The error involved is primarily one of imperfect visibility 
which arises from two causes: (1) Some particles may be so small 
as to be invisible, and (2) the index of refraction of certain particles 
may be so near that of water as to render them practically invisible. 5 
The colloidal materials from seven soils were examined by the 
method of Siedentopf and Zsigmondy. The suspensions directly 
from the centrifuge were diluted with optically pure water in such 
proportions that in the final dilution an average of one to three 
particles were visible in the field at one time. The blank count of the 
water averaged 0.035 of a particle. In order to promote dispersion 
and to prevent as far as possible the aggregation of primary parti- 
cles, ammonia was added in each case to N/200. The particles were 
observed with an ultramicroscope of the Siedentopf-Zsigmondy type, 
which was provided by the microchemical division, Bureau of 
Chemistry. 
The average diameters of the particles in seven colloidal soil mate- 
rials are shown in Table 2. The surface of smooth spherical par- 
ticles of this size and the number of such particles that would be 
present in 1 gram of material are also given in Table 2. 
Table 2. — Average size of colloidal soil particles 
Kind of colloid 
Average 
diameter of 
colloidal 
particles 
Surface- 
per gram of 
colloid 
particles 
Number of 
particles 
per gram of 
colloid 
divided 
by 10 12 
• 
Susquehanna 
Millimi- 
crons 
141 
129 
128 
111 
106 
91 
102 
Square 
meters 
15.7 
17.1 
17.0 
20.4 
21.5 
24.2 
21.3 
263 
Norfolk 
322 
335 
505 
Marshall. .- 
613 
Sharkey 
960 
Fallon 
680 
4 Pasteur-Chamberland filters were used for concentrating the colloidal suspensions. 
The solutions passing through them were always clear and contained only 8 to 15 parts 
per million of dissolved material. 
3 Attempts were made to increase the visibility of particles by the addition of small 
quantities of methylene blue and other dyes. No significant advantage was obtained, 
however, by such treatment. 
