h of the protoplasm, others do not easily
penetrate the semi-permeable colloidal mass, others are themselves changed
by the oxidizing or reducing action of the protoplasm, etc. Again,
colloidal adsorption effects often lead to the so-called "capillary
segregation" of added staining materials. So that this method of study must
be used with great care, or wholly erroneous conclusions will be reached,
and many of the earlier reports have subsequently been found to be
incorrect.

The recent improvements in the apparatus and methods for the determination
of hydrogen-ion concentration have afforded a much more trustworthy method
of determining the actual acidity or alkalinity of such materials than is
obtained by color reactions, and this method is now being extensively used
in the study of the reaction of active protoplasm.

It must be kept in mind that protoplasm is an heterogeneous mass and not an
homogeneous solution, so that it is not always possible to determine the
actual conditions as to neutrality of different parts of the protoplasm of
a single cell, for example. Hence, one of the best methods of determining
the reaction which is favorable to the life and activity of any given type
of protoplasm is to investigate the reaction of a liquid medium in which
the cells live and grow; this plan being based upon the assumption that a
cell is not likely to have a reaction different from that of the medium
which is favorable to its growth.

The results of all of the many investigations which have dealt with this
problem point to the conclusion that the normal reaction for living
protoplasm is either neutral or very faintly alkaline; but that it becomes
acid when the cell is working in the absence of sufficient oxygen, and
after the death of the cell.

The first effect of a change in the reaction toward acidity of the
protoplasm is a decrease in the rate of respiration of the tissue, while
increased alkalinity stimulates respiratory activity. Whet carried to the
point of actual acidity, the respiratory coefficient becomes negative, and
the cell actually gives off carbon dioxide because of the stoppage of the
synthetic processes.

A second effect of change in reaction of protoplasm is to alter the enzymic
activity of the cell. As has been pointed out, enzymes are extraordinarily
sensitive to minute changes in the reaction of the medium in which they are
working. A change toward acidity in protoplasm immediately results in t