PROPERTIES OF THE COLLOIDAL SOIL MATERIAL / 
cooling, liquid was added to the point of overflow. A piece of cover 
glass was placed over the opening in the stopper to reduce evapora- 
tion and the pycnometer was placed in a desiccator containing some 
of the same liquid as used in the determination. The desiccator was 
kept in a constant temperature bath a little above room temperature 
for four hours. When constant temperature was attained, the cover 
glass was taken off and any liquid that had overflowed through the 
stopper was removed. A weighing was then quickly made. 9 Dupli- 
cate determinations of specific gravity made in this way usually 
checked within 0.003. 
Table 3 shows the specific gravities of six colloids in water and in 
toluene, determined by the procedure described above : 
Table 3. — Specific gravities of soil colloids in water and toluene 
' Kind of colloid 
Specific 
gravity 
in 
water 
Specific 
gravity 
in 
toluene 
Differ- 
ence 
Fallon 
2.766 
2.748 
2.718 
2.715 
2.708 
2.627 
2.616 
2.642 
2.582 
2.554 
2.639 
2.412 
0. 150 
.106 
Sharkey.- ..- . . - 
.136 
. 161 
Norfolk 
.069 
Marshall . . - ... 
.215 
The specific gravity of each colloid is higher in water than in 
toluene, and in each case the difference in the specific gravities is 
considerably greater than the experimental error. Obviously, 
neither the specific gravity in water nor that in toluene can be 
accepted as being necessarily the true specific gravity of the material, 
although possibly both series of values may be near the truth. 
If the apparent specific gravities in the table approximate the true 
values, the colloidal materials would have specific gravities about the 
same as those of the commoner soil minerals, quartz being 2.65, the 
feldspars 2.55 to 2.75, and the micas 2.7 to 3.1. Also, if the apparent 
specific gravities parallel the true values, it would appear that the 
different colloids do not differ greatly. Variations in the apparent 
specific gravities of the different colloids show no relation to varia- 
tions in chemical compositions of the materials, unless possibly the 
low specific gravity of the Marshall colloid may have been due to 
its relatively high content of organic matter. This apparent absence 
of relationship between specific gravity and chemical composition 
may possibly be due to the number of constituents present, which 
vary somewhat independently, or to a fault in the procedure of 
the determination. 10 
The fact that each of the colloids has a lower specific gravity in 
toluene than in water indicates that, in this respect at least, the 
soil colloidal material is to be classed with finely porous materials. 
The pores, however, need not necessarily be rigid, as in charcoal, 
9 The temperature in the desiccator being higher than room temp'erature, there was no 
danger of overflow through the .capillary while the pycnometer was being weighed. 
10 Although the same procedure was followed closely in the determination of the specific 
gravity of each colloid, there is no assurance that drying at 110° C. constituted exactly 
comparable conditions in each case. In the subsequent treatments with liquid there 
may have been varying quantities of air not eliminated. 
