May 31,1924 
The Heat of Wetting of Soil Colloids 
933 
slightly higher than the values obtained which were already too high. It would 
seem that the high values of the heat of wetting ratios are due to a reduction in 
the heat of wetting capacity of the colloid produced by extraction. 
Table V. —True colloid content of soil compared with content indicated by heat of 
wetting ratio when ratio is based on a large sample of colloid obtained by almost 
complete extraction of the soil 
Soil type 
Heat 
evolved 
per gram 
of soil 
Heat 
evolved 
per gram 
of colloid 
extracted 
Colloid 
content as 
calcu¬ 
lated by 
heat of 
wetting 
ratio 
Colloid 
extracted 
as per 
cent of 
whole 
soil 
Colloid 
left in 
residues 
as per 
cent of 
whole 
soil 
True 
colloid 
content 
of soil 
TTnnston clay soil __ 
Calories 
9.5 
10.8 
2.2 
Calories 
11.6 
10.5 
5.3 
Per cent 
89.1 
102.9 
41.5 
Per cent 
53 
66 
16 
Per cent 
17 
10 
2 
Per cent 
70 
76 
18 
Houston clay subsoil___ 
Dprham loam subsoil_ 
More conclusive evidence that the process of extraction alters the heat of 
wetting of the colloid is obtainable from some of the data given in Tables IV 
and Y. It can be shown that the sum of the heats of wetting of the extracted and 
unextracted colloids in the soil is in some cases appreciably less than the heat of 
wetting of the untreated soil. This would seem to show quite conclusively that 
in the cases studied the colloidal material as it exists in the untreated soil has a 
higher heat of wetting capacity than the colloid as it exists after isolation. 
Table VI compares the heat of wetting of the untreated soil with the heats of 
wetting of the combined fractions into which the soil is separated by the process 
of extraction. The data for this comparison are given in Tables IV and V. 7 
Table VI.— Influence of extraction upon the heat of wetting of soil colloids 
Soil type 
Heat 
evolved 
by 1 gm. 
of un¬ 
treated 
soil 
Heat 
evolved 
by the 
extracted 
and un¬ 
extracted 
colloid 
contained 
in 1 gm. 
of soil 
Loss in 
heat of 
wetting 
per gram 
of soil 
due to 
process 
of ex¬ 
traction 
Loss ex¬ 
pressed 
as per 
cent of 
heat 
evolved 
by un¬ 
treated 
soil 
TTonston olav loam soil ..._.... 
Calories 
9.5 
Calories 
7.5 
Calories 
2.0 
Per cent 
21 
Houston clay loam subsoil _ ___ 
10.8 
7.3 
3.5 
32 
Tlnrham lnam fiiihsnil _ _...___ 
2.2 
1.2 
1.0 
46 
Vflira Baia piav loam a ____ 
4.0 
3.5 
.5 
13 
a The colloid of the Vega Baja soil was not so completely extracted as in the case of the other three soils 
given in this Table. Details regarding the extraction of this sample are given in a previous publication of 
this bureau (4). The extracted colloid amounted to 29.3 per cent of the soil while the soil contained a total 
of 72.4 per cent of colloid. 
The process of extraction in each case reduced the heat of wetting values of 
the soil material. In two cases the reduction in heat of wetting was of about the 
same order as the probable error. In the other two cases, however, the differ¬ 
ences were sufficiently great to be of undoubted significance. These results are 
7 For example, the heat of wetting of the Houston soil is 9.-6 calories per gram and the colloid extracted 
from it 11.6 calories per gram (Table V). This soil contains 53 per cent extractable colloid (Table V). The 
colloid extracted from 1 gm. of soil would, therefore, give 6.6 calories. This soil also contains 17 per cent of 
unextracted colloid (Table V) which gives a heat value of 5.3 calories per gram (Table IV). The unextracted 
colloid in 1 gm. of soil, therefore, amounts to 0.9 calories, making a total of 7.5 calories for the fractions 
from 1 gm. of soil after extraction, as compared with 9.5 calories for the untreated soil. 
