m of either animal or plant origin.

The effects which salts produce in protoplasm are undoubtedly due to the
fact that, when in solution, they readily ionize and conduct the electric
current. A discussion of the nature and importance of the theory of
dissociation of electrolytes in solution, or the so-called "ionization
theory," which has done so much to clear up otherwise unexplainable
properties of solutions, would be out of place here. But it may be noted
that the ionized condition of salts in solution accounts for the avidity,
or "strength," of acids and bases; for the increased osmotic pressure of
such solutions; for the conduction of the electric current through
solutions; and for the effects of these dissolved electrolytes upon the
colloidal condition of many substances, since this is due to the electric
charge on the dispersed particles.

Hence, the presence of salts in solution in the water of the protoplasm has
a tremendous influence upon the osmotic pressure (which governs the
movement of dissolved materials into and out of the cell protoplasm); upon
the colloidal condition of the cell contents (which controls all the
effects due to the surface boundary phenomena which are discussed below and
which are responsible for a large part of the remarkable chemical activity
of the protoplasm); upon the electrical phenomena (which constitute many of
the stimulations which the protoplasm receives); and upon the acidity or
alkalinity of the cell contents (which determine the nature of the
respiratory, or oxidation, reactions of the protoplasm and, indirectly, its
life or death).

The general nature of these physical-chemical properties of the protoplasm
and of the relation of electrolytes in solution to them may now be
considered in some detail.

OSMOTIC PRESSURE

Osmotic pressure is one of the chief factors in controlling the amount of
water in the protoplasm. As is well known, the phenomenon known as
"osmosis" is the passage of solvents, or of dissolved substances, into or
out of any tissue, or substance, through the membrane which surrounds it.
In the case of a cell, the membrane in question may be either the cell-wall
or the internal colloidal films which are distributed throughout the entire
mass of the cell contents.

From the standpoint of their relation to osmosis, membranes may be either
_impermeable_, in which case neither solvent nor dissolved materials can
pass through t