20 
colloidal material can not be directly compared, but an indirect 
comparison of the two kinds of colloid is possible. If the ex tractable 
colloidal material is completely separated from the soil as in the work 
just described, the unextracted colloidal material will all be present 
in the residual fractions of fine and coarse material, mixed, of course, 
with the mineral fragments above colloidal size. 
A comparison, then, of the adsorptive capacity of the fine or 
coarse fraction for dye, water, and ammonia with the adsorptive 
capacity of the extracted colloidal material for these three sub- 
stances will show whether the unextracted colloid differs from the 
extracted colloid more in its adsorption of dye than in its adsorption 
of water or ammonia. This comparison would show only the rela- 
tion between the dye, water, and ammonia adsorptive capacities of 
the extracted and unextracted colloids. It would not show, for 
example, whether the unextracted colloid was 80 per cent or 40 per 
cent as adsorptive for all three substances as the extracted colloid. 
However, the adsorption ratios of the fine and coarse fractions, 
adsorption per gram of fraction 
adsorption per gram of extracted colloid 
will, other things being equal, show the absolute quantities of colloid 
in the fractions in proportion as the adsorptive capacities of the 
extracted and unextracted colloids are alike. Now it is possible to 
determine the quantities of colloid in the fine and coarse fractions 
by microscopical observation with considerable accuracy (Fry, 13). 
Hence a comparison of the quantities of colloid found by microscopical 
observation with the quantities indicated by the adsorption ratios 
will show approximately the relative adsorptive capacities of the 
extracted and unextracted colloidal materials. If the adsorption 
ratio shows a smaller percentage of colloidal material than the 
microscopical determination, it indicates that the unextracted col- 
loidal material has a lower adsorptive capacity than the sample of 
extracted colloid and vice versa. 
The soil fractions prepared in order to compare the successive 
fractions of extractable colloidal material were not well suited for 
a comparison of the extractable and unextractable colloid since 
considerable portions of the fine and coarse fractions were lost, 
except in the case of the Vega Baja soil. Accordingly new frac- 
tionations of soils were made, taking care to avoid losses of colloidal, 
fine, or coarse material. 
The methods of extracting and concentrating the colloidal material 
and of separating the fine and coarse residues were essentially the 
same as those described on pages 16 to 17. The important difference 
was that in this work a bottle centrifuge was used for separating the 
colloidal material from the noncolloidal particles instead of the 
supercentrifuge, the speed and time of centrifuging being so regulated 
as to leave in suspension all particles less than 1 micron in diameter. 
The centrifuge used had a diameter of 22 inches. When running at 
a speed of 850 revolutions per minute about 45 minutes were usually 
required for throwing out particles larger than 1 micron in diameter. 
This was checked by microscopic observation of the colloidal, fine, 
and coarse fractions. The colloidal material passing through the 
supercentrifuge in the extractions described on pages 16 and 17 
