lications of the principles of physical chemistry to the
properties and activities of protoplasm. Therefore, it may be profitable to
summarize briefly these into a series of definite statements which may
serve as a review of the principles which have been discussed in the
preceding chapters, as applied to the activities of protoplasm.

Protoplasm is a complex hydrogel, composed of an heterogeneous mixture of
proteins, fats, and carbohydrates, arranged in a foamlike structure, the
compartments of the gel being filled with an aqueous solution of the
soluble organic products of synthesis and of varying proportions of mineral
salts which are of the same general nature as those of sea-water.

The gel is not uniform throughout the volume of any given cell, but is
differentiated in different parts into what are known as the nucleus, the
chloroplasts, the plasma of the cell, etc.

The vital activities of the cell consist in chemical reactions which are
controlled by comparatively slight changes in the electrolyte distribution,
or other environmental changes which affect the colloidal condition of the
mass and, generally speaking, result in changes of the water content of the
plasma, most such chemical changes being essentially reversible hydrolytic
reactions.

The components of active protoplasm are in a condition most favorable to
chemical reactions by reason of the enormous surface area of the colloidal
material, resulting in abundance of available energy, intimate contact of
the reacting materials, and the nearest possible approach to the condition
of true solution which can be obtained without the loss of stable form and
structure.

The reactions which take place in cell protoplasm, as a result of the
action of either physical or chemical stimuli, are accompanied by
electrical disturbances, which may be either caused by, or the result of,
changes in the electrical charges of the mineral salts which are present in
the gel. Such changes, like the chemical reactions which they accompany,
may be regarded as reversible and mutually self-regulatory; so that the
protoplasm has not only the possibilities of enormous chemical reactivity,
but also the mechanism for self-regulation of its actions, the products or
results from any given series of changes generally tending to reverse the
process by which they are proceeding and so to restore the condition of
normal equilibrium.

Finally, the most characteristic difference between t