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.
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.
[Illustration: FIG. 69.--FATIGUE IN MUSCLE (WALLER)]
[Illustration: FIG. 70.--FATIGUE IN PLATINUM]
#Fatigue.#--In some metals, as in muscle and in plant, we find instances of that progressive diminution of response which is known as fatigue (fig. 69). The accompanying record shows this in platinum (fig. 70). It has been said that tin is practically indefatigable. We must, however, remember that this is a question of degree only. Nothing is absolutely indefatigable. The exhibition of fatigue depends on various conditions. Even in tin, then, I obtained the characteristic fatigue-curve with a specimen which had been in continuous use for many days (fig. 71). While discussing the subject of fatigue in plants, I have adduced considerations which showed that the residual effect of strain was one of the main causes for the production of fatigue. This conclusion receives independent support from the records obtained with metals.
[Illustration: FIG. 71.--FATIGUE SHOWN BY TIN WIRE WHICH HAD BEEN CONTINUOUSLY STIMULATED FOR SEVERAL DAYS]
In this connection the important fact is that the various typical fatigue effects exhibited in living substances are exactly reproduced in metals, where there can be question neither of fatigue-product producing fatigue effects, nor of those constructive processes by which they might be removed. We have seen, both in muscles and in plants, that if sufficient time for complete recovery be allowed between each pair of stimuli, the heights of successive responses are the same, and there is no apparent fatigue (see page 39). But the height of response diminishes as the excitation interval is shortened. We find the same thing in metals. Below is given a record taken with tin (fig. 72). Throughout the experiment the amplitude of vibration was maintained constant, but in (_a_) the interval between consecutive stimuli was 1', while in (_b_) this was reduced to 30". A diminution of height immediately occurs. On restoring the original rhythm as in (_c_), the responses revert to their first large value. Thus we see that when the wire has not completely recovered, its responses, owing to residual strain, undergo diminution. Height of response is thus decreased by incomplete recovery. If then sufficient time be not allowed for perfect recovery, we can understand how, under certain circumstances, the residual strain would progressively increase with repetition of stimulus, and thus there would be a progressive diminution of height of response or fatigue. Again, we saw in the last chapter that increase of strain necessitates a longer period of recovery. Thus the longer a wire is stimulated, the more and more overstrained it becomes, and it therefore requires a gradual prolongation of the interval between the successive stimuli, if recovery is to be complete. This interval, however, being maintained constant, the recovery periods virtually undergo a gradual reduction, and successive recoveries become more and more incomplete. These considerations may be found to afford an insight into the progressive diminution of response in fatigued substances.