be remembered that there are
again specific poisons which may affect one kind of tissue and not
others. Poisons in general may be regarded as extreme cases of
depressants. As an example of those which produce moderate physiological
depression, potassium bromide may be mentioned, and this also diminishes
electric response. There are other chemical reagents, on the other hand,
which produce the opposite effect of increasing the excitability and
causing a corresponding exaltation of electric response.

We shall now proceed to inquire whether the response of inorganic bodies
is affected by chemical reagents, so that their excitability is exalted
by some, and depressed or abolished by others. Should it prove to be so,
the last test will have been fulfilled, and that parallelism which has
been already demonstrated throughout a wide range of phenomena, between
the electric response of animal tissues on the one hand, and that of
plants and metals on the other, will be completely established.

#Action of stimulants on metals.#--We shall first study the stimulating
action of various chemical reagents. The method of procedure is to take
a series of normal responses to uniform stimuli, the electrolyte being
water. The chemical reagent whose effect is to be observed is now added
in small quantity to the water in the cell, and a second series of
responses taken, using the same stimulus as before. Generally speaking,
the influence of the reagent is manifested in a short period, but there
may be occasional instances where the effect takes some time to develop
fully. We must remember that by the introduction of the chemical reagent
some change may be produced in the internal resistance of the cell. The
effect of this on the deflection is eliminated by interposing a very
high external resistance (from one to five megohms) in comparison with
which the internal resistance of the cell is negligible. The fact that
the introduction of the reagent did not produce any variation in the
total resistance of the circuit was demonstrated by taking two
deflections, due to a definite fraction of a volt, before and after the
introduction of the reagent. These deflections were found equal.

[Illustration: FIG. 89.--STIMULATING ACTION OF Na_2CO_3 ON TIN]

I first give a record of the stimulating action of sodium carbonate on
tin, which will become evident by a comparison of the responses before
and after the introduction of Na_2CO_3 (fig. 89). The next