say, a series of
observations were taken for maximum effects, starting from amplitude of
vibration of 10 deg. and ending in 100 deg., and backwards from 100 deg. to 10 deg..
Effect of hysteresis is very clearly seen (see A, fig. 87); there is a
considerable divergence between the forward and return curves, the
return curve being higher. On repeating the cycle several times, the
divergence is found very much reduced, the wire on the whole is found to
assume a more constant sensitiveness. In this steady condition,
generally speaking, the sensitiveness for smaller amplitude of vibration
is found to be greater than at the very beginning, but the reverse is
the case for stronger intensity of stimulation.

#Effect of annealing.#--I repeated the experiment with the same wire,
after pouring hot water into the cell and allowing it to cool to the old
temperature. From the cyclic curve (B, fig. 87) it will be seen (1) that
the sensitiveness has become very much enhanced; (2) that there is
relatively less divergence between the forward and return curves. Even
this divergence practically disappeared at the third cycle, when the
forward and backward curves coincided (C, fig. 87). The above results
show in what manner the excitability of the wire is enhanced by purely
physical means.

It is very curious to notice that addition of Na_2CO_3 solution (see
Chap. XV--Action of Stimulants) produces enhancement of responsive power
similar to that produced by annealing; that is to say, not only is there
a great increase of sensitiveness, but there is also a reduction of
hysteresis.

CHAPTER XVI

INORGANIC RESPONSE--EFFECT OF CHEMICAL REAGENT

Action of chemical reagents--Action of stimulants on metals--Action of
depressants on metals--Effect of 'poisons' on metals--Opposite
effect of large and small doses.

We have seen that the ultimate criterion of the physiological character
of electric response is held to be its abolition when the substance is
subjected to those chemical reagents which act as poisons.

[Illustration: FIG. 88.--ACTION OF POISON IN ABOLISHING RESPONSE IN
NERVE (WALLER)]

#Action of chemical reagents.#--Of these reagents, some are universal in
their action, amongst which strong solutions of acids and alkalis, and
salts like mercuric chloride, may be cited. These act as powerful toxic
agents, killing the living tissue, and causing electric response to
disappear. (See fig. 88.) It must, however,