ion will take place from
the modified zero to which the spot returns during recovery. On now
stimulating the wire as a whole by, say, torsional vibration, the
current of response will be found towards the more excitable, i.e. from
B to A (fig. 52, _a_).

[Illustration: FIG. 52.--RESPONSE BY METHOD OF DEPRESSION (WITHOUT
BLOCK)
When the wire is stimulated as a whole the current of response is
towards the more excitable.
In (_a_) A is a normal contact, B has been depressed by oxalic acid;
current of response is towards the more excitable A.
In (_b_) the same wire is used, only A is depressed by oxalic acid and a
normal contact is made at a fresh point B', a little to the left of
B in (_a_). Current of response is now from A towards the more
excitable B'.]

A corroborative reversal experiment may next be made on the same piece
of wire. The normal contact, through water or salt solution, is now made
at B', a little to the left of B. The excitability of A is now depressed
by oxalic acid. On stimulation of the whole wire, the current of
response will now be found to flow in an opposite direction--i.e. from A
to B'--but still from the relatively less to the relatively more
excitable (fig. 52, _b_).

From these experiments it will be seen how in one identical piece of
wire the responsive current flows now in one direction and then in the
other, in absolute conformity with theoretical considerations.

#Method of exaltation.#--A still more striking corroboration of these
results may, however, be obtained by the converse process of relative
exaltation of the responsiveness of one contact. This may be
accomplished by touching one contact, say B, with a reagent which like
Na_2CO_3 exalts the electric excitability. On stimulation of the
wire, the current of response is towards the more excitable B (fig. 53).

[Illustration: FIG. 53.--METHOD OF EXALTATION
The contact B is made more excitable by chemical stimulant
(Na_2CO_3). The current of response is towards the more
excitable B.]

I give four records (fig. 54) which will clearly exhibit the responses
as obtained by the methods of relative depression or exaltation. In
(_a_) B is touched with the excitant Na_2CO_3, a permanent current
flows from A to B, response to stimulus is in the same direction as the
permanent current (positive variation). In (_b_) B is touched with a
trace of the depressant oxalic acid, the permanent current is in the
sa