PROPERTIES OF THE COLLOIDAL. SOIL MATERIAL 11 
loids may therefore be partially distinguished by their adsorptive 
capacities. Since the adsorption of gases and vapors is considered 
by many to be a somewhat different type of phenomenon from that 
of the adsorption of electrolytes, the capacities of the colloids for 
adsorping vapors and electrolytes will be considered separately. 
Many theories have been evolved regarding the mechanism of the 
adsorption process and regarding the characteristics of the materials 
which may be responsible for this phenomenon. According to ad- 
sorption theories, the quantities of a given vapor adsorbed by dif- 
ferent solid adsorbents would depend on the extent of the surface 
exposed in submicroscopic capillaries and also on the specific ad- 
sorbing power of the surface (the nature of the surface). However, 
estimations free from assumptions for either of these values are lack- 
ing in the case of highly adsorptive materials. Some adsorbents, such 
as charcoal and silica gel, give little evidence of specificity in ad- 
sorbing vapors, the quantities of different vapors adsorbed by a given 
preparation frequently following approximately the condensabilities 
of the vapors (11±). According to one theory, the quantities of vapor 
adsorbed by such adsorbents are regarded as indicative of the vol- 
umes of the submicroscopic capillaries, the limits of whose diameters 
do not exceed a certain magnitude. There are, however, many in- 
stances where the specific nature of the surface appears to determine 
the quantity of gas adsorbed far more than does the estimated 
volume of the capillaries or the extent of the surface. 
The adsorptive capacity of colloidal soil material for a series of 
vapors has not been determined ; hence it is not known whether this 
material adsorbs vapors in the order of their condensabilities or 
whether the specific nature of the surface significantly affects ad- 
sorption. The unlike chemical compositions of the different soil 
colloids suggests that they may present different kinds of surfaces 
to a vapor and may therefore be specific in their adsorptions, On 
the other hand, man}^ adsorbents of very unlike chemical composi- 
tion, such as charcoal, platinum black, and silica gel, show little 
specificity in adsorption. 
A complete picture of the differences in adsorbing power for 
vapors exhibited by the various soil colloids would involve deter- 
minations of the adsorption of a variety of vapors under different 
conditions. Such a detailed study was beyond the scope of this 
investigation. This bulletin gives only the quantities of ammonia 
and water vapors adsorbed at a single temperature and pressure. 
The adsorption of ammonia was determined by the method de- 
scribed in a recent publication of this bureau (20). The vapor was 
maintained at 1 centimeter above atmospheric pressure and the 
temperature of adsorption was 0° C. The quantity of adsorbed am- 
monia determined was not the total quantity adsorbed bv the col- 
loid, but was that portion of the total adsorbed at 0° C, which was 
liberated at* 100° C. by passing a current of air over the samples. 13 
The adsorption of water vapor was conducted in an evacuated 
desiccator over 30 per cent sulphuric acid at 30° C. This procedure 
was essentially the same as that described in a previous publication 
(20), except that previously 3.3 per cent (by weight) sulphuric 
13 Determinations made by H. E. Middleton showed that about 15 per cent of the total 
quantity or ammonia adsorbed by soil colloids was retained after the vapor had been 
driven off at 100° C. by aspiration. 
