: 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
sa