definite surface of separation will exist between the liquid and the
gaseous parts. When no surface of separation ever exists between two
substances they must necessarily be capable of mixing in all
proportions. If they are not capable of mixing in all proportions a
discontinuous change must occur somewhere between the regions where the
substances are still unmixed, thus giving rise to a surface of
separation.

The phenomena of mixing thus involves the following processes:--(1) A
motion of the substances relative to one another throughout a definite
_region_ of space in which mixing is taking place. This relative motion
is called "diffusion." (2) The passage of portions of the mixing
substances across the _surface_ of separation when such a surface
exists. These surface actions are described under various terms such as
solution, evaporation, condensation and so forth. For example, when a
soluble salt is placed in a liquid, the process which occurs at the
surface of the salt is called "solution," but the salt which enters the
liquid by solution is transported from the surface into the interior of
the liquid by "diffusion."

Diffusion may take place in solids, that is, in regions occupied by
matter which continues to exhibit the properties of the solid state.
Thus if two liquids which can mix are separated by a membrane or
partition, the mixing may take place through the membrane. If a solution
of salt is separated from pure water by a sheet of parchment, part of
the salt will pass through the parchment into the water. If water and
glycerin are separated in this way most of the water will pass into the
glycerin and a little glycerin will pass through in the opposite
direction, a property frequently used by microscopists for the purpose
of gradually transferring minute algae from water into glycerin. A still
more interesting series of examples is afforded by the passage of gases
through partitions of metal, notably the passage of hydrogen through
platinum and palladium at high temperatures. When the process is
considered with reference to a membrane or partition taken as a whole,
the passage of a substance from one side to the other is commonly known
as "osmosis" or "transpiration" (see SOLUTION), but what occurs in the
material of the membrane itself is correctly described as diffusion.

Simple cases of diffusion are easily observed qualitatively. If a
solution of a coloured salt is carefully introduced by a funnel int