12 
acid was used. 14 After equilibrium was reached in the desiccator, 
the samples were dried for 18 hours at 110° C, and the loss in 
weight was taken as the quantity of water adsorbed. 
Table 5 shows the quantities of water and of ammonia adsorbed 
by seven soil colloids and the relation between the adsorption of the 
two vapors. 
Table 5. — Adsorption of water and ammonia vapors oy soil colloids 
H 2 NH 3 
Kind of colloid i adsorbed adsorbed 
jvinu oi umoiu . per gj-am per gram 
of colloid 1 ! of colloid 
Eatio 
H2O adsorbed 
NH3 adsorbed 
Gram Gram 
Fallon 0.1776 0.0532 3.34 
Sharkey- .1605 .0529' 3.03 
Marshall .1305; .0415 3.14 
Sassafras .1143' .0264 1 4.33 
Aragon .0886 ( 2 ) 
Norfolk .0845! . 0266 J 3.18 
Huntington .0633 . 0214 j 2.96 
Susquehanna . 0518 j . 0177 ' 2.93 
1 Determinations made by H. E. Middleton. 2 Not determined. 
There is a considerable range in the a dsorptive- capacities of the 
different colloids for water vapor and for ammonia, the most ad- 
sorptive colloid of the series taking up approximately three times 
as much water or ammonia as the least adsorptive material. 
It is evident from the last column that the quantities of ammonia 
adsorbed by the different colloids are, with one exception, approxi- 
mately proportional to the quantities of water vapor adsorbed under 
the conditions of the determinations. It is thus apparent that, if 
the colloids possess any specificity for the adsorption of either am- 
monia or water vapor, they possess, on the whole, the same degree 
of specificity for both substances. 
The fact that the colloids adsorb more water than ammonia is 
in accord with the capillary theory of adsorption; since under the 
condition of the determination the corresponding. pressure 15 of the 
water vapor was about three times as great as that of the ammonia 
(34. 51f, 55). This indicates that the mechanism of the adsorption 
process in soil colloids is similar to that in such substances as silica 
gel and charcoal, which frequently adsorb vapors in the order of 
their corresponding pressure modified by the liquid volumes of the 
condensed vapors. However, the data for adsorption of soil colloids 
are too meager to establish this similarity with any surety. 
If the mechanism of adsorption were the same in soil colloids 
as in silica gel, it would be reasonable to assume a similar structure 
for the two materials. Many hold that inorganic gels are made up 
of an agglomeration of particles and that capillary spaces are formed 
14 Different soil colloids vary much less in their capacity for adsorbing water vapor 
from a very humid atmosphere than from a drier one., Strong acid was vised, therefore, 
to bring out differences characteristic of the colloids. 
p 
1Z The term " corresponding " presure has been used for expressing the relation p-r 
where P is the pressure of the vapor in equilibrium with the adsorbent and Po the satura- 
tion pressure of the vapor at the temperature of adsorption. In the case 01' water vapor 
P P 
under the conditions employed the value for p- was 0.75, and p - for ammonia was 0.24. 
