May 31, 1924 
The Heat of Wetting of Soil Colloids 
931 
tive in adsorptive capacity of all the colloidal material in the soil; and the possi¬ 
bility of the colloidal material having a different adsorptive capacity after isola¬ 
tion from what it had in the unextracted soil. 
While it is a relatively simple process to isolate a small portion of the colloidal 
material from a soil, it is a long and tedious operation in most cases to extract 
as much as 50 per cent of the total colloid present. The data given in Tables I 
and II were obtained using samples which comprised not more than 5 per cent 
of the total colloidal material in the soil. Previous work indicated that such 
small samples differed by only about 10 per cent in adsorptive capacity from 
the bulk of the colloid that it was possible to extract. This small variation was 
shown by extracting successive portions of the colloidal material from several 
soils and testing the adsorptive capacities of the different fractions. A few of 
these samples which were not exhausted in the previous investigation were 
tested for heat of wetting. The results are shown in Table III, together with 
the adsorption data previously determined ( 4 , p. 18). ' 
Table III. Heats of wetting of colloid samples successively extracted from the 
same soil 
Heat of 
wetting 
per gram 
of sample 
Mala¬ 
chite 
green 
adsorbed 
per gram 
of sample 
Water 
adsorbed 
per gram 
of sample 
Ammonia 
adsorbed 
per gram 
of sample 
Vega Baja clay loam soil: 
First fraction of colloid 
Calories 
4.8 
4.4 
, 12.2 
9.8 
Gram 
Gram 
n ofOK 
Gram 
0.0206 
.0165 
.0611 
.553 
Second fraction of colloid. 
V. UOo*i 
U. olZQ 
0*7 AC\ 
Marshall silt loam soil: 
First fraction of colloid. 
• UIOo 
• z/4y 
QAOrt 
Fourth fraction of colloid... 
• oioy 
1Q1A 
• ouyu 
OQOQ 
. ZoZo 
The variation in the heats of wetting of successively extracted portions of col¬ 
loid was very similar to the variations in adsorptive capacities of the same 
samples. It is, therefore, to be assumed that the conclusion reached regarding 
adsorptive capacities on the basis of more extensive data applies in a general 
way to heat of wetting. That is to say, a small sample of colloid has, as a rule, 
only a slightly different heat of wetting from that of a sample which is a larger 
part of all the colloid. 
Although the extractable part of the colloidal material of a soil may be fairly 
uniform in character, it does not follow that the part which is exceedingly diffi¬ 
cult to isolate necessarily has the same properties. In determining the repre¬ 
sentativeness of a colloid sample with respect to heat of wetting it is important 
to consider also that portion which is not extractable by the ordinary means. 
In several instances separation of colloidal material has been made as com¬ 
pletely as present methods will permit. By long and painstaking work of ex¬ 
traction 50 to 90 per cent of the total colloidal material present has been isolated 
from certain soils.. The residues left after these extractions contained the so- 
called unextractable” colloid. The heats of wetting of these residues were ob¬ 
tained; the percentages of colloid present were determined by W. H. Fry micro¬ 
scopically by a method previously described (8); and from these data the heats 
of wetting of the unextracted colloids were calculated. 
Table IV gives a comparison of the heat of wetting of the extracted and of the 
unextracted colloidal material, together with data from which the heat of wetting 
of the unextracted colloid was calculated. 
