[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 same 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