must be in either colloidal or true solutions.

Inorganic materials, when dissolved in water, usually ionize very readily.
That is, they are not only disintegrated into individual _molecules_, but a
considerable proportion of these molecules separate into their constituent
_ions_. When solutions containing ionized compounds are brought together,
conditions for chemical interaction are ideal, and the reaction proceeds
with such tremendous rapidity as to be completed almost instantaneously, in
most cases.

Organic compounds, on the other hand, ionize only very slowly, if at all.
Hence, reactions between organic compounds, even when they are in solution,
proceed very slowly unless carried on at high temperatures, under increased
pressure, or under the influence of some catalytic agent. Even under the
stimulation of these reaction-accelerating agencies, most chemical changes
in organic compounds when carried on in the laboratory, require several
hours or even days and sometimes weeks, for their completion. But when
similar reactions take place in living organisms, they proceed with
velocities which resemble those of inorganic compounds in the laboratory.
This difference between the velocity of organic reactions when carried on
under artificial conditions in the laboratory (often spoken of as "_in
vitro_") as compared with that of the same reactions when they take place
in a living organism ("_in vivo_"), is due to the universal presence in
the living protoplasm of certain organic catalysts, known as _enzymes_.

ENZYMES AS CATALYSTS

The phenomenon known as "catalysis" is of common occurrence in both
inorganic and organic chemistry. The effect of a small amount of manganese
dioxide in aiding in the liberation of oxygen from potassium chlorate is an
example which is familiar to all students of elementary chemistry.
Similarly, spongy platinum accelerates the oxidation of sulfur dioxide to
sulfur trioxide, in the commercial manufacture of sulfuric acid. Again, the
hydrolysis of sucrose into fructose and glucose proceeds very slowly in the
presence of water alone, but if a little hydrochloric acid or sulfuric acid
be added to the solution, the velocity of the hydrolysis is enormously
accelerated. Many other examples of the accelerating effect of various
chemicals upon reactions into which they do not themselves enter, might be
cited.

The essential features of all such catalytic actions are: (1)