o the
bottom of a jar containing water, the two portions will at first be
fairly well defined, but if the mixture can exist in all proportions,
the surface of separation will gradually disappear; and the rise of the
colour into the upper part and its gradual weakening in the lower part,
may be watched for days, weeks or even longer intervals. The diffusion
of a strong aniline colouring matter into the interior of gelatine is
easily observed, and is commonly seen in copying apparatus. Diffusion of
gases may be shown to exist by taking glass jars containing vapours of
hydrochloric acid and ammonia, and placing them in communication with
the heavier gas downmost. The precipitation of ammonium chloride shows
that diffusion exists, though the chemical action prevents this example
from forming a typical case of diffusion. Again, when a film of Canada
balsam is enclosed between glass plates, the disappearance during a few
weeks of small air bubbles enclosed in the balsam can be watched under
the microscope.

In fluid media, whether liquids or gases, the process of mixing is
greatly accelerated by stirring or agitating the fluids, and liquids
which might take years to mix if left to themselves can thus be mixed in
a few seconds. It is necessary to carefully distinguish the effects of
agitation from those of diffusion proper. By shaking up two liquids
which do not mix we split them up into a large number of different
portions, and so greatly increase the area of the surface of separation,
besides decreasing the thicknesses of the various portions. But even
when we produce the appearance of a uniform turbid mixture, the small
portions remain quite distinct. If however the fluids can really mix,
the final process must in every case depend on diffusion, and all we do
by shaking is to increase the sectional area, and decrease the thickness
of the diffusing portions, thus rendering the completion of the
operation more rapid. If a gas is shaken up in a liquid the process of
absorption of the bubbles is also accelerated by capillary action, as
occurs in an ordinary sparklet bottle. To state the matter precisely,
however finely two fluids have been subdivided by agitation, the
molecular constitution of the different portions remains unchanged. The
ultimate process by which the individual molecules of two different
substances become mixed, producing finally a homogeneous mixture, is in
every case diffusion. In other words, diffusion is t