ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 35 
internal surface produced by their "web" or '"brush-heap" struc- 
ture. 17 Obviously macroscopic particles of such material have much 
the same properties and are just as truly colloidal as particles a 
micron or less in size. Data given in Table 6 show that lumps of 
soil colloids from 1 to 50 microns or so in diameter may in some 
instances have just as high an adsorptive capacity as the material 
which is dispersed into particles of 0.3 micron or less in size (adsorp- 
tive capacities of Vega Baja soil residues, Table 6.) If these lumps 
contain crystalline or discrete particles, the particles are so small 
that they can not be recognized as crystalline by the petrographic 
microscope, i. e., they must be at any rate less than 1 micron in 
diameter. Under the ultramicroscope they appear to consist of 
aggregates of particles less than a micron in size. 
Obviously a soil in the air-dried condition contains practically no 
colloidal material in the sol condition as there is not sufficient water 
present to act as the dispersing medium. Also, as pointed out in 
the first part of this paper, the amount of colloidal material that is 
present as a sol when the soil is merely agitated once in a certain 
volume of water is very indefinite and uncharacteristic of the total 
quantity of colloidal material in the soil which is capable of assuming 
this form. Many of the low estimates of the colloidal content of 
soils, such as that of Gedroiz (14), are based on determinations of 
this kind. 
It must be recognized that much of the material in soils which 
has been classed as colloidal in this paper does not have a definite 
size of ultimate or " primary" particle in the sense that a single 
crystal has a definite or ultimate size. Size as ordinarily apparent 
without a consideration of structure means little. 18 Lumps of par- 
tially dry gels may appear as large particles, yet in the presence of 
sufficient water they may disperse into particles or "droplets" of 
submicroscopic size. The exact size of particles into which such 
material will disperse depends chiefly upon the conditions of dis- 
persion. A single crystalline particle, however, is definite as regards 
size. Heretofore, recognition of the gel-like material in soils as col- 
loidal has been dependent almost entirely upon conversion of the gel 
into the sol condition, where the particles or aggregates are small. 
This is an uncertain method. In this study the gel material has 
been recognized partly in this way, partly through its adsorptive 
capacity, and partly by microscopic observation of structure. 
Placing the maximum quantity of colloidal material in soils at 2 
per cent is obviously wrong, even if we were to classify as colloids 
only that material which could readily be deflocculated or peptized 
into particles of submicroscopic size. Data given in Table 3 show 
that from 6 to 38 per cent of such material was isolated from five 
soils^ by no more drastic treatment than rubbing the soil with water 
containing a trace of ammonia. The probable reason for the low 
results obtained by other investigators was given in the first part of 
this paper. 
If the definition of colloidal soil material given in the first part of 
this paper be accepted, it can be seen from Table 10 that six different 
soils contained from 17 to 70 per cent of colloids. These values, 
17 For a development of this idea, see Freundlich (10, p. 905-908). 
13 Ormandy (25) points out that the size of a particle of ball clay is dependent on the time it is in contact 
with water and on the volume of water; hence the system of mechanical analysis suggested by Professor 
Oden measures nothing definite when dealing with clav. 
