212

FLUIDITY AND PLASTICITY

of the particles on standing, analogous to the "annealing" of
metals, so it appears that an equilibrium results and the maxi-
mum in viscosity in emulsions may depend upon the rate of shear.
As the lamellae of the simple case, which we have taken for
consideration, are broken up, the viscosities are no longer
strictly additive. The droplets become smaller and smaller,
the surface tension becomes more and more effective, the droplets
become true spheres with an inappreciable amount of flow within
the spheres, so that finally the distinction between emulsion and
suspension disappears.
We pass finally to that class of polar colloids typified by
gelatine, soap and rubber. In some ways they are in sharp con-
trast with the type which we have just been considering, because
their viscosity increases tremendously on standing and decreases
as a result of shear, but they are alike in the more fundamental
respect of exhibiting the properties of rigidity and elasticity.
It is assumed that the process of gelatinization is the result
of polar forces producing a network of crystals or crystal-like
material interlacing throughout liquid, without necessarily
taking up more than a small portion of the space. The solid
network performs the function of the lamellae at right angles to
the direction of shear in our simple case. The cohesion of the
solid opposes the shear and gives rise to the rigidity of the gel.
The ability of the solid to be deformed without fracture deter-
mines its elasticity. This property of elasticity is enormously
developed in rubber, and we have seen that it is noticeable in
foams and emulsions. Barus (1893) has noted the considerable
degree of elasticity in marine glue which may be regarded as
a very viscous liquid. It also is of importance in suspensions,
as for example in the manufacture of pencils, the "leads" expand
considerably, as they are forced out of the die previous to baking.
If gelatinization is analogous to crystallization, we should
expect the viscosity to increase on standing and that it would
be hastened by "seeding" the solution with a more viscous
colloids. We can readily see that shearing the material would
result in the destruction of the polar structure of the material
and consequently in a decrease in the viscosity. We refer the
reader to the rich material furnished by Garrett (1903).
When a hydrogel is exposed to dry air, it loses moisture