That this influence, however, whatever it be, is not derived from the metals alone, but that animals at least contribute to its production, as well as indicate its presence, is, I think, rendered highly probable, by what I have already urged, relative to the necessity of a communication between the metals, and the muscles, as well as between the metals and the nerves.
I may likewise observe, that animals appear to have a much more complete controul over its effects, than one would expect them to have over an influence wholely external to them.
When living and entire frogs are placed upon a plate of zinc, or tin-foil, and a piece of silver, or of gold, is passed over different parts of their legs, and thighs, till it come into contact with the plate; contractions are very seldom produced, and scarcely ever, if the frogs be healthy and upon their guard. But the instant their sciatic nerves are divided, the contractions produced are as free and vigorous, as if the legs had been completely separated from the body. This difference is not owing to the silver coming in contact with the wound, necessarily made in order to divide the nerve; for I have always taken care that it should not, and indeed when it did, no contractions were produced, unless the nerve had been divided.
Taking off the head of an animal, or intercepting, in any way, the influence of its will upon the muscles of the part excited, has precisely the same effect. But the will is not able to controul the effects of electricity, when the electricity is otherwise sufficiently strong to excite muscles to contraction. I have repeatedly found that even by the strongest voluntary contractions of the muscles of my arm, I have not been able altogether to counteract the involuntary ones, produced by electrical sparks, nor have I found that frogs could ever counteract them.
On attending carefully to the state of the muscles of the legs of living frogs, at the instant the metals were applied, I could perceive by the touch, that, in many frogs, though by no means in all, their muscles were perfectly soft and relaxed: a proof that they have other means of counteracting the involuntary contractions, which the metals have a tendency to produce, besides keeping their muscles in a state of permanent and voluntary contraction.
[1]. Cavallo.
[2]. On this metal Cronstedt has the following very curious remark: “It seems to become electrical by friction, and then its smaller particles are attracted by the loadstone; which effects are not yet properly investigated.” Zinc is an ingredient of the best amalgam for smearing the rubbers of electrical machines: But I have not been able to render a bar of zinc electrical by friction, nor to find that its smaller particles were in any state attracted by the loadstone, unless they had been scraped off by means of an instrument of iron. But, in this way, the dust of any metal is rendered susceptible of the influence of the loadstone.
[3]. If further experiments should establish decidedly, that the mutual contact of two different metals is absolutely necessary for the productions of Galvani’s phenomena, may not this circumstance afford an useful test of the purity of the precious metals? For instance, contractions in an animal produced by the contact of a piece of gold or silver, whose purity we wish to ascertain, with a piece of the same metal known to be pure, would then prove incontestably the presence of alloy.
[4]. In an able lecture, which Dr Monro lately delivered, chiefly upon this subject, he demonstrated the possibility of exciting contractions in the limb of a frog, without either of the metals he employed being in contact with it; or having any other communication with it than through the medium of some moist substance. In varying this experiment, I find, that if a frog be divided in two parts, just above the origin of the sciatic nerves, and put into a bason of water, the hind legs may be thrown into strong contractions, by bringing zinc, or tin-foil, and silver, in contact with each other, at the distance of at least an inch from the divided spine, so long as they are kept nearly in a right line with it. Water, in this case, is the only communication between the metals and the origin of the nerves.