6 BULLETIX 1452, tf. S. DEPARTMENT OF AGRICULTURE 
microscopically porous, widely different results may be obtained 
when different liquids are used for the determination. 7 Certain in- 
vestigators regard this variation in apparent specific gravity as being 
chiefly due to unequal penetration of different liquids into the 
minute capillaries or micropores. If this is the true explanation, 
penetration is evidently not governed solely by viscosity of the 
liquid nor by the size of its molecules, since penetration does not 
always parallel either viscosity or molecular weight. According 
to one theory, penetration would be governed chiefly by surface ten- 
sions ; that is. by whether or not the liquid tends to displace a solid- 
air or liquid-air interface and thus wet the surface of the capillaries. 
In the absence of a definitely established explanation, penetration 
might be vaguely ascribed to an ' w affinity " of the solid for the liquid. 
A very different explanation for the fact that microscopically 
porous substances show varying specific gravities in different liquids 
is offered by Harkins and Ewing {23). They hold that liquids of 
low viscosity completely fill the micropores, but are so greatly com- 
pressed therein by an adhesional force that the liquid densities are 
markedly increased. Differences in apparent specific gravity are thus 
due to different liquids showing variable increases in density on 
being compressed. It is suggested that, in the micropores of an acti- 
vated charcoal, an adhesional force equivalent to approximately 
:20.000 atmospheres compresses water about 25 per cent and ether 
40 per cent. 
It is obviously important to know whether the specific gravity of 
the soil colloidal material varies in different liquids. A constant 
specific gravity in different liquids would indicate a " true * ? specific 
gravity. Such a constant should serve to distinguish the different 
soil colloids, since they vary considerably in their chief constituents, 
silica, alumina, iron, and organic matter. Different specific gravi- 
ties in different liquids would at least indicate that the soil colloids 
possess a property in common with activated charcoals and other 
substances presumably submicroscopically porous in structure. 
Washburn and Bunting (S3) have determined the porosity of 
certain clays and ceramic products in gases, in water, and in vaseline. 
Porosities indicated by determinations in water and vaseline were 
practically identical: air. helium, and hydrogen in many cases indi- 
cated greater pore volumes than were shown by the liquids. Xo 
data are available in the literature on the specific gravities of isolated 
soil colloids. 
In connection with the present investigation, determinations were 
made of the specific gravities of six colloidal soil materials in both 
water and toluene. The air-dried colloidal material, ground to pass 
a 50-mesh sieve, was dried at 110° C. for 48 hours. It was then 
transferred while hot to a pycnometer s and allowed to cool in a 
desiccator. After weighing, the pycnometer was half filled with 
liquid (either water or toluene) and was placed in an evacuated 
chamber, where the liquid was allowed to boil for several minutes to 
remove air. The pycnometer was then placed on a steam bath over 
night to remove air and to promote penetration of the liquid. After 
7 Harkins and Ewing u'o) give the apparent specific gravity of an activated charcoal 
as 0.8 in mercury. '2.1 in water, and 2.4 in ether. 
- A specific gravity bottle' of Gay Lussae type was used. It was found advisable to 
equip it with a uiass cap ntiing over the stopper to reduce evaporation. 
