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.

[Ill.u.s.tration: 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.]

The record was then taken as follows. The recording drum had a fast speed of six inches in a minute, one of the small subdivisions representing a second. The battery contact in the main potentiometer circuit was made for a quarter of a second as just mentioned and a record taken of the effect of a short-lived E.M.F. on the circuit containing the cell. (2) A record was next taken of the E.M. variation produced in the cell by a single stimulus. It will be seen on comparison of the two records that the maximum effect took place relatively later in the case of mechanical stimulus, and that whereas the galvanometer recovery in the former case took place in 11 seconds, the recovery in the latter was not complete till after 60 seconds (fig. 60, _b_). This shows that it takes some time for the effect of stimulus to attain its maximum, and that the effect does not disappear till after the lapse of a certain interval. The time of recovery from strain depends on the intensity of stimulus. It takes a longer time to recover from a stronger stimulus. But, other things being equal, successive recovery periods from successive stimulations of equal intensity are, generally speaking, the same.

We may now study the influence of any change in external conditions by observing the modifications it produces in the normal curve.

[Ill.u.s.tration: FIG. 61.--PROLONGATION OF PERIOD OF RECOVERY AFTER OVERSTRAIN Recovery is complete in 60" when the stimulus is due to 20 vibration.

But with stronger stimulus of 40 vibration, the period of recovery is prolonged to 90".]

#Prolongation of period of recovery by overstrain.#--The pair of records given in fig. 61 shows how recovery is delayed, as the effect of overstrain. Curve (_a_) is for a single stimulus due to a vibration of amplitude 20, and curve (_b_) for a stimulus of 40 amplitude of vibration. It will be noticed how relatively prolonged is the recovery in the latter case.

[Ill.u.s.tration: FIG. 62.--MODEL SHOWING THE EFFECT OF FRICTION]

#Molecular Model.#--We have seen that the electric response is an outward expression of the molecular disturbance produced by the action of the stimulus. The rising part of the response-curve thus exhibits the effect of molecular upset, and the falling part, or recovery, the restoration to equilibrium. The mechanical model (fig. 62) will help us to visualise many complex response phenomena. The molecular model consists of a torsional pendulum--a wire with a dependent sphere. By the stimulus of a blow there is produced a torsional vibration--a response followed by recovery. The writing lever attached to the pendulum records the response-curves. The form of these curves, stimulus remaining constant, will be modified by friction; the less the friction, the greater is the mobility. The friction may be varied by more or less raising a vessel of sand touching the pendulum. By varying the friction the following curves were obtained.

(_a_) When there is little friction we get an after-oscillation, to which we have the corresponding phenomenon in the retinal after-oscillation (compare fig. 105).

(_b_ and _c_) If the friction is increased, there is a damping of oscillation. In (_c_) we get recovery-curves similar to those found in nerve, muscle, plant, and metal.

(_d_) If the friction is still further increased the maximum is reached much later, as will be seen in the increasing slant of the rising part of the curve; the height of response is diminished and the period of recovery very much prolonged by partial molecular arrest. The curve (_d_) is very similar to the 'molecular arrest' curve obtained by small dose of chemical reagents which act as 'poison' on living tissue or on metals (compare fig. 93, _a_).

(_e_) When the molecular mobility is further decreased there is no recovery (compare fig. 93, _b_).

Still further increase of friction completely arrests the molecular pendulum, and there is no response.

From what has been said, it will be seen that if in any way the friction is diminished or mobility increased the response will be enhanced. This is well exemplified in the heightened response after annealing (fig. 58) and after preliminary vibration (figs. 81, 82).

Possibly connected with this may be the increased responses exhibited by the action of stimulants (figs. 89, 90).

#Reduction of molecular sluggishness attended (1) by quickened recovery.#--Sometimes, after a cell has been resting for too long a period, especially on cold days, the wire gets into a sluggish condition, and the period of recovery is thereby prolonged. But successive vibrations gradually remove this inertness, and recovery is then hastened. This is shown in the accompanying curves, fig. 63, where (_a_) exhibits only very partial recovery even after the expiration of 60 seconds, whereas when a few vibrations had been given recovery was entirely completed in 47 seconds (_b_). There was here little change in the height of response.

[Ill.u.s.tration: FIG. 63 (_a_) Slow recovery of a wire in a sluggish condition.

(_b_) Quickened recovery in the same wire after a few vibrations.]

#Or (2) by heightened response.#--The removal of sluggishness by vibration, resulting in increased molecular mobility, is in other instances attended by increase in the height of response, as will be seen from the two sets of records which follow (fig. 64). Cold, due to prevailing frosty weather, had made the wires in the cell somewhat lethargic. The records in (_a_) were the first taken on the day of the experiment. The amplitudes of vibration were 45, 90, and 135. In (_b_) are given the records of the next series, which are in every case greater than those of (_a_). This shows that previous vibration, by conferring increased mobility, had heightened the response. In this case, removal of molecular sluggishness is attended by greater intensity of response, without much change in the period of recovery. In connection with this it must be remembered that greater strain consequent on heightened response has a general tendency to a prolongation of the period of recovery.

[Ill.u.s.tration: FIG. 64 (_a_) Three sets of responses for 45, 90, and 135 vibration in a sluggish wire.

(_b_) The next three sets of responses in the same wire; increased mobility conferred by previous vibration has heightened the response.]

It is thus seen that when the wire is in a sluggish condition, successive vibrations confer increased molecular mobility, which finds expression in quickened recovery or heightened response.

#Effect of temperature.#--Similar considerations lead us to expect that a moderate rise of temperature will be conducive to increase of response.

This is exhibited in the next series of records. The wire at the low temperature of 5 C. happened to be in a sluggish condition, and the responses to vibrations of 45 to 90 in amplitude were feeble. Tepid water at 30 C. was now subst.i.tuted for the cold water in the cell, and the responses underwent a remarkable enhancement. But the excessive molecular disturbance caused by the high temperature of 90 C. produced a great diminution of response (fig. 65).

[Ill.u.s.tration: FIG. 65.--RESPONSES OF A WIRE TO AMPLITUDES OF VIBRATION 45 AND 90 (_a_) Responses when the wire was in a sluggish condition at temperature of 5 C.

(_b_) Enhanced response at 30 C.

(_c_) Diminution of response at 90 C.]

#Diphasic variation.#--It has already been said that if two points A and B are in the same physico-chemical condition, then a given stimulus will give rise to similar excitatory electric effects at the two points. If the galvanometer deflection is 'up' when A alone is excited, the excitation of B will give rise to a downward deflection. When the two points are simultaneously excited the electric variation at the two points will _continuously_ balance each other. Under such conditions there will be no resultant deflection. But if the intensity of stimulation of one point is relatively stronger, then the balance will be disturbed, and a resultant deflection produced whose sign and magnitude can be found independently by the algebraical summation of the individual effects of A and B.

It has also been shown that a balancing point for the block, which is approximately near the middle of the wire, may be found so that the vibrations of A and B through the same amplitude produce equal and opposite deflection. Simultaneous vibration of both will give no resultant current; when the block is abolished and the wire is vibrated as a whole, there will still be no resultant, inasmuch as similar excitations are produced at A and B.

After obtaining the balance, if we apply an exciting reagent like Na_2CO_3 at one point, and a depressing reagent like KBr at the other, the responses will now become unequal, the more excitable point giving a stronger deflection. We can, however, make the two deflections equal by increasing the amplitude of vibration of the less sensitive point. The two deflections may thus be rendered equal and opposite, but the time relations--the latent period, the time rate for attaining the maximum excitation and recovery from that effect--will no longer be the same in the two cases. There would therefore be no continuous balance, and we obtain instead a very interesting diphasic record. I give below an exact reproduction of the response-curves of A and B recorded on a fast-moving drum. It will be remembered that one point was touched with Na_2CO_3 and the other with KBr. By suitably increasing the amplitude of vibration of the less sensitive, the two deflections were rendered approximately equal. The records of A and B were at first taken separately (fig. 66, _a_). It will be noticed that the maximum deflection of A was attained relatively much earlier than that of B.

The resultant curve R' was obtained by summation.

[Ill.u.s.tration: FIG. 66.--DIPHASIC VARIATION (_a_) Records of A and B obtained separately. R' is the resultant by algebraical summation. (_b_) Diphasic record obtained by simultaneous stimulation of A and B.]

After taking the records of A and B separately, a record of resultant effect R due to simultaneous vibration of A and B was next taken. It gave the curious two-phased response--positive effect followed by negative after-vibration, practically similar to the resultant curve R'

(fig. 66, _b_).

The positive portion of the curve is due to A effect and the negative to B. If by any means, say by either increasing the amplitude of vibration of A or increasing its sensitiveness, the response of A is very greatly enhanced, then the positive effect would be predominant and the negative effect would become inconspicuous. When the two const.i.tuent responses are of the same order of magnitude, we shall have a positive response followed by a negative after-vibration; the first twitch will belong to the one which responds earlier. If the response of A is very much reduced, then the positive effect will be reduced to a mere twitch and the negative effect will become predominant.

I give a series of records, fig. 67, in which these three princ.i.p.al types are well exhibited, the two contacts having been rendered unequally excitable by solutions of the two reagents KBr and Na_2CO_3. A and B were vibrated simultaneously and records taken.

(_a_) First, the relative response of B (downward) is increased by increasing its amplitude of vibration. The amplitude of vibration of A was throughout maintained constant. The negative or downward response is now very conspicuous, there being only a mere preliminary indication of the positive effect. (_b_) The amplitude of vibration of B is now slightly reduced, and we obtain the diphasic effect. (_c_) The intensity of vibration of B is diminished still further, and the negative effect is seen reduced to a slight downward after-vibration, the positive up-curve being now very prominent (fig. 67).

[Ill.u.s.tration: FIG. 67.--NEGATIVE, DIPHASIC, AND POSITIVE RESULTANT RESPONSE]

#Continuous transformation from negative to positive.#--I have shown the three phases of transformation, the intensity of one of the const.i.tuent responses being varied by altering the intensity of disturbance.

In the following record (fig. 68) I succeeded in obtaining a continuous transformation from positive to negative phase by a continuous change in the relative sensitiveness of the two contacts.

I found that traces of after-effect due to the application of Na_2CO_3 remain for a time. If the reagent is previously applied to an area and the traces of the carbonate then washed off, the increased sensitiveness conferred disappears gradually. Again, if we apply Na_2CO_3 solution to a fresh point, the sensitiveness gradually increases. There is another further interesting point to be noticed: the beginning of response is earlier when the application of Na_2CO_3 is fresh.

[Ill.u.s.tration: FIG. 68.--CONTINUOUS TRANSFORMATION FROM NEGATIVE TO POSITIVE THROUGH INTERMEDIATE DIPHASIC RESPONSE Thick dots represent the times of application of successive stimuli.]

We have thus a wire held at one end, and successive uniform vibrations at intervals of one minute imparted to the wire as a whole, by means of a vibration head on the other end.

Owing to the after-effect of previous application of Na_2CO_3 the sensitiveness of B is at the beginning great, hence the three resultant responses at the beginning are negative or downward.

Dilute solution of Na_2CO_3 is next applied to A. The response of A (up) begins earlier and continues to grow stronger and stronger. Hence, after this application, the response shows a preliminary positive twitch of A followed by negative deflection of B. The positive grows continuously. At the fifth response the two phases, positive and negative, become equal, after that the positive becomes very prominent, the negative being reduced as a feeble after-vibration.

It need only be added here that the diphasic variations as exhibited by metals are in every way counterparts of similar phenomena observed in animal tissues.

CHAPTER XIV

INORGANIC RESPONSE--FATIGUE, STAIRCASE, AND MODIFIED RESPONSE

Fatigue in metals--Fatigue under continuous stimulation--Staircase effect--Reversed responses due to molecular modification in nerve and metal, and their transformation into normal after continuous stimulation--Increased response after continuous stimulation.

[Ill.u.s.tration: FIG. 69.--FATIGUE IN MUSCLE (WALLER)]