me direction as before, but the current of response is in the opposite
direction (negative variation). In (_c_) B is touched with dilute KHO,
the response is exhibited by a positive variation. In (_d_) B is touched
with strong KHO, the response is now exhibited by a negative variation.
The last two results, apparently anomalous, are due to the fact, which
will be demonstrated later, that KHO in minute quantities is an
excitant, while in large quantities it is a depressant.

[Illustration: FIG. 54

+-------------------+-----------+----------+
| | | Current |
| | Permanent | of |
| | Current | Response |
+-------------------+-----------+----------+
|B treated with | | |
|sodium carbonate. | --> | --> |
+-------------------+-----------+----------+
|B treated with | | |
|oxalic acid | --> | <-- |
+-------------------+-----------+----------+
|B treated with very| | |
|dilute potash | --> | --> |
+-------------------+-----------+----------+
|B treated with | | |
|strong potash | --> | <-- |
+-------------------+-----------+----------+

Current of response is always towards the more excitable point.
(_a_) Response when B is treated with sodium carbonate.--An apparent
positive variation.
(_b_) Response when B is treated with oxalic acid.--An apparent negative
variation.
(_c_) Response when B is treated with very dilute potash.--Positive
variation.
(_d_) Response when B is treated with strong potash.--Negative
variation.
The response is up when B is more excitable, and down when A is more
excitable.
Lines thus ------ indicate deflection due to permanent current.]

We have thus seen that we may obtain response (1) by block method, (2)
by the method of injury, or relative depression of responsiveness of one
contact, and (3) by the method of relative exaltation of responsiveness
of one contact. In all these cases alike we obtain a consistent action
current, which in tin is normally positive, or towards the relatively
more excited.

FOOTNOTES:

[14] Following another line of inquiry I obtained response to electric
stimulus in inorganic substances using the method of conductivity
variation (see 'De la Generalite des Phenomenes Moleculaires