Hitherto, as already said, electrical response in animals has been regarded as a purely physiological phenomenon. We have proved by various tests that response in plants is of the same character. And we have seen that by physiological phenomena are generally understood those of which no physical explanation can be offered, they being supposed to be due to the play of some unknown vital force existing in living substances and giving rise to electric response to stimulation as one of its manifestations.
#Is response found in inorganic substances?#[14]--It is now for us, however, to examine into the alleged super-physical character of these phenomena by stimulating inorganic substances and discovering whether they do or do not give rise to the same electrical mode of response which was supposed to be the special characteristic of living substances. _We shall use the same apparatus and the same mode of stimulation as those employed in obtaining plant response, merely substituting, for the stalk of a plant, a metallic wire, say 'tin'_ (fig. 50). Any other metal could be used instead of tin.
#Experiment on tin, block method.#--Let us then take a piece of tin wire[15] from which all strains have been previously removed by annealing, and hold it clamped in the middle at C. If the strains have been successfully removed A and B will be found iso-electric, and no current will pass through the galvanometer. If A and B are not exactly similar, there will be a slight current. But this will not materially affect the results to be described presently, the slight existing current merely adding itself algebraically to the current of response.
If we now stimulate the end A by taps, or better still by torsional vibration, a transitory 'current of action' will be found to flow in the wire from B to A, from the unstimulated to the stimulated, and in the galvanometer from the stimulated to the unstimulated. Stimulation of B will give rise to a current in an opposite direction.
[Illustration: FIG. 50.--ELECTRIC RESPONSE IN METALS (_a_) Method of block; (_b_) Equal and opposite responses when the ends A and B are stimulated; the dotted portions of the curves show recovery; (_c_) Balancing effect when both the ends are stimulated simultaneously.]