ns going on in the interior of
the solution. This latter consideration seems to be the factor of largest
influence in colloidal catalysis.

But in addition to the increased rate of reaction in the surface layer due
to the increased energy available there and to the increased concentration
of dissolved substances, there is the possibility that the act of
concentration itself bring into play molecular forces which give rise to a
resultant increase in chemical potential, or chemical affinity, of the
reacting materials, such as has been observed to result in other
concentrated solutions. A discussion of the theoretical and mathematical
considerations upon which this conception is based would be out of place
here, but there is ample experimental evidence to indicate its soundness.

Further, as has been pointed out, colloidal phenomena are essentially due,
in large part at least, to the electric charges on the dispersed particles.
Electric charges accumulate at the surface of any charged body. Hence, the
surface layers in any colloidal system carry its electric charges in
highest concentration, and all of the chemical changes which are stimulated
by electrical phenomena are most strongly influenced at the surface
boundaries between the different phases of the system. This latter
consideration affords a satisfactory explanation of the well-known
depressing, or stimulating, action of electrolytes, especially acids and
bases, upon the enzymic catalysis of protoplasmic reactions.

These few, brief statements are sufficient to indicate how extensively the
chemical activities of colloidal protoplasm are influenced by the phenomena
arising from the surface boundaries between different materials, which are
present in such enormous extent in a colloidal gel. Surface boundary
phenomena in a heterogeneous system, such as we have seen protoplasm to be,
provide the possibilities for many reactions which would otherwise take
place very slowly, if at all. Mere subdivision of the protoplasmic
materials into the film, or foam, structure brings into play energies which
may predominate over all other types of energy in the system. Here, too,
effects may be extraordinarily modified by slight changes in environment,
which effects could not be explained by any considerations which govern
ordinary chemical reactions. Here, we deal with adsorption and other
colloidal phenomena, rather than with ordinary stoichiometric combinations.

Indeed, it is