10 BULLETIN" 1452, U. S. DEPARTMENT OF AGRICULTURE 
Table 4. — The heats of wetting of colloidal soil materials 
Kind of colloid 
Heat of 
wetting 
per gram 
of colloid 
Kind of colloid . 
Heat of 
wetting 
per gram 
of colloid 
Fallon . . 
Calories 
17.5 
16.3 
14.6 
9.8 
Huntingvon, .... 
Calories 
8.3 
Sharkev- 
Aragon 
Norfolk,. .... 
8.0 
7.6 
Sassafras ... . .. ... 
j Susquehanna - _. 
5.3 
The heats of wetting of the different colloids vary from 5.3 to 
17.5 calories per gram. The higher values are of the same order 
of magnitude as Patrick (44) found for silica gel (19.2 calories per 
gram) and as Harkins (23) gives for bone charcoal when immersed 
in water (18.3 calories per gram). 
If the same assumptions are made for the soil colloids as Patrick 
made for silica gel. the diameters of the particles in the different col- 
loids would range from 3.6 millimicrons for the Fallon colloid to 12 
millimicrons for the Susquehanna colloid. These diameters are only 
about one-tenth those shown by the Zsigmondy count method. The 
values obtained by the Zsigmondy method seem more probable. 
However, the values derived by assuming an application of Pat- 
rick's hypothesis are not beyond the range of possibility, for it must 
be borne in mind that many of the particles counted may in reality 
have been aggregates. 
It is of course doubtful whether Patrick's hypothesis leads to 
absolutely true conclusions regarding the size of silica-gel particles, 
for he points out in a subsequent paper that the values deduced 
seem too small in the light of other data on silica gel (4-5). It is also 
doubtful whether Patrick's hypothesis regarding silica gel should 
be applied in its entirety to soil colloids; particularly since the 
difference between heat of wetting in organic liquids and heat of 
wetting in water is greater for silica gel than for soil colloids. Ac- 
cording to Patrick and Grimm (44), the heats evolved by silica gel 
in benzene and in alcohol are respectively 0.58 and 1.18 times the 
heat evolved in water. On the other hand, the Fallon and Sassa- 
fras colloids evolve only 0.36 and 0.4:0 as much heat in benzene as 
in water and 0.S4: and 0.78 as much heat in alcohol as in water. 
Aside from theoretical considerations, a correspondence obviously 
exists between particle size and heat of wetting. The data given in 
Tables 2 and 1 show that the heat of wetting tends to increase with 
decreasing particle size as determined by the Zsigmondy count 
method, and is roughly proportional to the number of particles per 
gram of material or inversely to the average volume of a single 
particle. 
ADSORPTION OF VAPORS 
Adsorptive caj^acity has generally been regarded as one of the most 
characteristic properties of colloidal soil material. In fact, several 
methods for determining the quantity of colloid in a soil or clay have 
been based upon this property (4^. ~0, 4- 4$)- It has been recently 
shown that the colloidal materials extracted from different soils may 
vary widely in adsorptive capacities (20). The different soil col- 
