#Action of stimulants on metals.#--We shall first study the stimulating action of various chemical reagents. The method of procedure is to take a series of normal responses to uniform stimuli, the electrolyte being water. The chemical reagent whose effect is to be observed is now added in small quantity to the water in the cell, and a second series of responses taken, using the same stimulus as before. Generally speaking, the influence of the reagent is manifested in a short period, but there may be occasional instances where the effect takes some time to develop fully. We must remember that by the introduction of the chemical reagent some change may be produced in the internal resistance of the cell. The effect of this on the deflection is eliminated by interposing a very high external resistance (from one to five megohms) in comparison with which the internal resistance of the cell is negligible. The fact that the introduction of the reagent did not produce any variation in the total resistance of the circuit was demonstrated by taking two deflections, due to a definite fraction of a volt, before and after the introduction of the reagent. These deflections were found equal. [Illustration: FIG. 89.--STIMULATING ACTION OF Na_2CO_3 ON TIN] I first give a record of the stimulating action of sodium carbonate on tin, which will become evident by a comparison of the responses before and after the introduction of Na_2CO_3 (fig. 89). The next 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]