record
shows the effect of the same reagent on platinum (fig. 90).

[Illustration: FIG. 90.--STIMULATING ACTION OF Na_2CO_3 ON PLATINUM]

#Action of depressants.#--Certain other reagents, again, produce an
opposite effect. That is to say, they diminish the intensity of
response. The record given on the next page (fig. 91) shows the
depressing action of 10 per cent. solution of KBr on tin.

[Illustration: FIG. 91.--DEPRESSING EFFECT OF KBR (10 PER CENT.) ON THE
RESPONSE OF TIN]

#Effect of 'poison.'#--Living tissues are killed, and their electric
responses are at the same time abolished by the action of poisons. It is
very curious that various chemical reagents are similarly effective in
killing the response of metals. I give below a record (fig. 92) to show
how oxalic acid abolishes the response. The depressive effect of this
reagent is so great that a strength of one part in 10,000 is often
sufficient to produce complete abolition. Another notable point with
reference to the action of this reagent is the persistence of
after-effect. This will be clearly seen from an account of the following
experiment. The two wires A and B, in the cell filled with water, were
found to give equal responses. The wires were now lifted off, and one
wire B was touched with dilute oxalic acid. All traces of acid were next
removed by rubbing the wire with cloth under a stream of water. On
replacing the wire in the cell, A gave the usual response, whereas that
of B was found to be abolished. The depression produced is so great and
passes in so deep that I have often failed to revive the response, even
after rubbing the wire with emery paper, by which the molecular layer on
the surface must have been removed.

[Illustration: FIG. 92.--ABOLITION OF RESPONSE BY OXALIC ACID]

We have seen in the molecular model (fig. 62, _d_, _e_) how the
attainment of maximum is delayed, the response diminished, and the
recovery prolonged or arrested by increase of friction or reduction of
molecular mobility.

It would appear as if the reagents which act as poisons produced some
kind of molecular arrest. The following records seen to lend support to
this view. If the oxalic acid is applied in large quantities, the
abolition of response is complete. But on carefully adding just the
proper amount I find that the first stimulus evokes a responsive
electric twitch, which is less than the normal, and the period of
recovery is very much prolonged from the