230 FLUIDITY AND PLASTICITY
is increased, which enhances the friction, but this effect reaches a
limit eventually when the particles are so small that their
Brownian movement becomes appreciable and strains in the
material are not permanent.
If, as we have intimated, the friction is subject to the laws
of ordinary external friction, the friction should be closely
dependent upon the adhesion of the particles to each other
but independent upon the nature of the medium so long as it is
inert. In confirmation of this we note that whereas the china
clay referred to above showed zero friction when the volume
concentration was 19.5 per cent, the same clay thoroughly shaken
down in a measuring flask in the dry state showed a pore space of
18.4 per cent, the pore space in this case being filled with air.
The two values are in very close agreement. Infusorial earth
exhibited zero fluidity in water when present to the extent of
12.9 per cent by volume, whereas in ethyl alcohol the corre-
sponding concentration was 12.1 per cent. Finally it has been
observed that the temperature and therefore the fluidity of the
medium is without effect upon the friction.
Adhesion between the particles may be influenced in a marked
degree by the addition of small amounts of substances of the
most diverse character. Generally speaking, substances which
yield hydrogen ions increase the adhesion, i.e., promote floccula-
tion, while substances which yield hydroxyl ions decrease the
adhesion and promote deflocculation. Colloids also have a
noteworthy effect. In flocculation, structure is produced and
therefore the friction is enhanced. In a given instance, using
50 per cent china clay in water, the friction was lowered from
78 to 59.5 by adding merely one-tenth of 1 per cent of potassium
carbonate, which of course yields hydroxyl ions.
The mobility is dependent upon the fluidity of the medium.
This in turn is influenced by the temperature, hence we may
expect that the mobility of a solid will be dependent upon the
temperature. Thus in a 50 per cent clay suspension the mobility
at 25° was found to be 5.11 and at 40°, 7.88. The ratio between
these mobilities is 1.54 which is very close to the ratio of the
fluidities of water at these two temperatures
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