#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, be remembered that there are again specific poisons which may affect one kind of tissue and not others. Poisons in general may be regarded as extreme cases of depressants. As an example of those which produce moderate physiological depression, potassium bromide may be mentioned, and this also diminishes electric response. There are other chemical reagents, on the other hand, which produce the opposite effect of increasing the excitability and causing a corresponding exaltation of electric response. We shall now proceed to inquire whether the response of inorganic bodies is affected by chemical reagents, so that their excitability is exalted by some, and depressed or abolished by others. Should it prove to be so, the last test will have been fulfilled, and that parallelism which has been already demonstrated throughout a wide range of phenomena, between the electric response of animal tissues on the one hand, and that of plants and metals on the other, will be completely established.