CHAPTER VII
COLLOIDAL SOLUTIONS
If it is highly important to discover the relation between
fluidity and conductivity, it is vastly more important to have a
solution of the numerous problems in connection with the
viscosity of colloidal solutions. Indeed it has been said that
the viscometer is to colloid chemistry what the galvanometer
is to the subject of electricity, and Graham referred to the vis-
cometer as a colloidoscope. Since 1 per cent of colloid like agar
agar may give water the properties of a stiff solid, the advantage
of employing this property in recognizing the colloid state is
clearly apparent.
A pure liquid, at a given temperature and pressure, can
have but a single fluidity, but in our study of liquid mixtures we
have seen that a mixture of liquids may have an indefinite number
of fluidities dependent upon the method of mixing, in other words,
upon the structure of the liquid. Since colloidal solutions are
always heterogeneous, they always possess structure, and there-
fore we have this variable always entering into our consideration,
whereas heretofore we have given it but scant attention. There
is, however, every gradation from a pure liquid, to an incom-
pletely mixed solution, an emulsion, suspension or typical gel.
The Two Types of Colloid Structure.—The structures which
may occur are of two kinds, which must be clearly differentiated
from each other, because they give rise to phenomena which are
in some respects exactly opposite, and this is true in spite of the
fact that the two structures may in certain cases merge into each
other.
In the one case typified by gelatine, the structure requires
time to form and the fluidity at a given moment depends upon
the previous history of the solution. When moreover the
solution is agitated by shaking or stirring or when it is heated,
the structure is damaged and the fluidity is affected. This
structure is similar in results to that which would be produced