That the excitatory state persists for a time even on the cessation of stimulus can be independently shown by keeping the galvanometer circuit open during the application of stimulus, and completing it at various short intervals after the cessation, when a persisting electrical effect, diminishing rapidly with time, will be apparent. The rate of recovery immediately on the cessation of stimulus is rather rapid, but traces of strain persist for a short time. CHAPTER XIII INORGANIC RESPONSE--MOLECULAR MOBILITY: ITS INFLUENCE ON RESPONSE Effects of molecular inertia--Prolongation of period of recovery by overstrain--Molecular model--Reduction of molecular sluggishness attended by quickened recovery and heightened response--Effect of temperature--Modification of latent period and period of recovery by the action of chemical reagents--Diphasic variation. We have seen that the stimulation of matter causes an electric variation, and that the acted substance gradually recovers from the effect of stimulus. We shall next study how the form of response-curves is modified by various agencies. In order to study these effects we must use, in practice, a highly sensitive galvanometer as the recorder of E.M. variations. This necessitates the use of an instrument with a comparatively long period of swing of needle, or of suspended coil (as in a D'Arsonval). Owing to inertia of the recording galvanometer, however, there is a lag produced in the records of E.M. changes. But this can be distinguished from the effect of the molecular inertia of the substance itself by comparing two successive records taken with the same instrument, in one of which the latter effect is relatively absent, and in the other present. We wish, for example, to find out whether the E.M. effect of mechanical stimulus is instantaneous, or, again, whether the effect disappears immediately. We first take a galvanometer record of the sudden introduction and cessation of an E.M.F. on the circuit containing the vibration-cell (fig. 60, _a_). We then take a record of the E.M. effect produced by a stimulus caused by a single torsional vibration. In order to make the conditions of the two experiments as similar as possible, the disturbing E.M.F., from a potentiometer, is previously adjusted to give a deflection nearly equal to that caused by stimulus. The torsional vibration was accomplished in a quarter of a second, and the contact with the potentiometer circuit was also made for the same length of time. [Illustration: FIG. 60 (_a_) Arrangement for applying a short-lived E.M.F. (_b_) Difference in the periods of recovery: (1) from instantaneous E.M.F.; and (2) that caused by mechanical stimulus.]