hat relative motion
of the molecules of two different substances by which the proportions of
the molecules in any region containing a finite number of molecules are
changed.

In order, therefore, to make accurate observations of diffusion in
fluids it is necessary to guard against any cause which may set up
currents; and in some cases this is exceedingly difficult. Thus, if
gas is absorbed at the upper surface of a liquid, and if the gaseous
solution is heavier than the pure liquid, currents may be set up, and
a steady state of diffusion may cease to exist. This has been tested
experimentally by C. G. von Hufner and W. E. Adney. The same thing may
happen when a gas is evolved into a liquid at the surface of a solid
even if no bubbles are formed; thus if pieces of aluminium are placed
in caustic soda, the currents set up by the evolution of hydrogen are
sufficient to set the aluminium pieces in motion, and it is probable
that the motions of the Diatomaceae are similarly caused by the
evolution of oxygen. In some pairs of substances diffusion may take
place more rapidly than in others. Of course the progress of events in
any experiment necessarily depends on various causes, such as the size
of the containing vessels, but it is easy to see that when experiments
with different substances are carried out under similar conditions,
however these "similar conditions" be defined, the rates of diffusion
must be capable of numerical comparison, and the results must be
expressible in terms of at least one physical quantity, which for any
two substances can be called their coefficient of diffusion. How to
select this quantity we shall see later.

2 _Quantitative Methods of observing Diffusion._--The simplest plan of
determining the progress of diffusion between two liquids would be to
draw off and examine portions from different strata at some stage in the
process; the disturbance produced would, however, interfere with the
subsequent process of diffusion, and the observations could not be
continued. By placing in the liquid column hollow glass beads of
different average densities, and observing at what height they remain
suspended, it is possible to trace the variations of density of the
liquid column at different depths, and different times. In this method,
which was originally introduced by Lord Kelvin, difficulties were caused
by the adherence of small air bubbles to the beads.

In