Experiments and Observations. Fowler Richard
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At the time I first observed this fact, I was making experiments to ascertain whether it was possible to transmit the influence, which had excited contractions in one leg, into another, removed to some distance from it, and communicating with it, only by means of a single conducting substance, (such as silver, for example). For this purpose, I had separated from the trunk, and from each other, the hind legs of a frog recently killed, and had detached their nerves as far as the knee. I then laid them at some distance from each other, upon a plate of glass, and included the nerve of one leg, and the foot of the other, in the folded ends of a silver chain. With one hand I now raised from the muscles, upon the end of a silver probe, the nerve of the leg, whose foot was folded in the chain; and with a piece of zinc, in the other hand, touched, at the same time, the nerve and the probe. This leg was thrown into strong contractions; but none were excited in the other. I then touched the chain, and nerve of the other, and, to my surprise, both legs instantly contracted. I had observed, in the beginning of November last, that it was not necessary for the metals to be in contact with any thing but nerve, in order to excite contractions in the muscles, to which it was distributed; and had mentioned this fact immediately afterwards to the Medical Society of this place, as a sufficient refutation of the theory, which Dr Valli had formed of Galvani’s discovery. It would not, therefore, have surpassed my expectations, had the influence, excited by the mutual contact of a piece of zinc, and probe, with the nerve, passed through the medium of the chain, from the leg, in which it first excited contractions, and produced contractions in the distant leg. But, I now thought that I had not only passed the influence from one leg to the other, but in one of the legs in a direction contrary to the course of its nerves. The removal of the leg, whose nerve communicated with the chain, convinced me of my error: but, at the same time, discovered to me a fact of much greater importance, than any with which I had hitherto been acquainted. For now, upon touching the chain alone with the zinc, I found that the leg, whose foot it still included, and whose nerve I held suspended upon a probe, contracted as strongly as before. The influence of the two metals, in contact with the nerve of the other leg, had not, therefore, passed into, and excited this.
It had from the first been known, that contractions could be excited by placing two different metals in contact, one with the nerve, the other with the muscles, and making a communication between them: but, in this experiment, the only metal in contact, either with the nerve, or muscle, was silver. Neither had the influence passed through the chain, and up the leg against the course of the nerve, in consequence of a communication by means of moisture subsisting between the zinc, and the foot, as well as between the silver chain, and the foot; for the experiment succeeded equally well when the chain was removed, and the foot laid upon a silver plate made perfectly dry. But when either the zinc, or probe was held by another person not communicating with me; or when either of them was insulated in a stick of sealing wax; no contraction whatever took place. Neither, indeed, were contractions excited in any part of the leg, except the foot, when the probe was withdrawn from the nerve; and the foot, and silver, were both touched with the zinc. It is then clear, that the influence, which, in the former case, excited the whole leg to contraction, must have passed through the medium of my body. It is not necessary that the silver should be laid under the foot; all that is required, is, that it should communicate with it by means of moisture; it may then be laid at almost any distance from it5.
The course of this influence, however, was still undetermined: it might be from the muscles to the nerve: it might be from the nerve to the muscles. To ascertain this, and to prove that the influence, which had excited one limb to contraction, might pass on, through a foreign medium, and excite contractions in another, I made the following experiment.
The leg of a frog was disposed as in the former experiment. The probe, suspending the nerve, was held by myself; the zinc excitor by another person; and the leg of another frog formed the communication betwixt us. So long as I had hold of the nerve, and the person assisting me held the foot of this interposed leg, no contractions were excited in it, by the influence, which passed through it and excited the other leg. But when the person holding the zinc, held the nerve of the interposed leg; and I held the foot, both legs contracted with equal strength. From this experiment it is evident, that Galvani’s influence had passed either from the muscles, or the zinc and silver; and in the direct course of the nerves of both legs.
I was now in possession of an easy method of ascertaining the different substances, which do, or which do not, afford a passage to this new influence.
All the metals when pure appear to be excellent conductors; not quite so good when in the ore; and, I think, least so when combined with acids, forming metallic salts. They are however, in this state, by no means bad conductors, even when so carefully dried, as to leave no suspicion of the slightest degree of moisture adhering to their surface. But, when the metals are calcined, their capacity as conductors is quite destroyed: at least this was the case with the calces of zinc, of bismuth, of iron, and of mercury; the only ones, with which I have had an opportunity of making the experiment. I could not observe that any contractions were excited through the medium of stones, nor ever through barytes.
The different non-conductors of electricity are likewise, I find non-conductors of this influence: even wood, charcoal, and linen, do not conduct except when moist. But all the living vegetables I could procure afforded it a ready passage: probably from the fluids which they contain. While I held the probe which supported the nerve, I touched the shoe of a gentleman, who applied the zinc to the silver under the foot of the frog. Strong contractions were excited, but when he took off his shoe, and we held it between our hands, no contractions could be excited. In the first case, the influence had to pass through no more than the thickness of the shoe: in the second, through its whole length, which might not be all equally moist. This gentleman had on thread stockings. When I touched the foot of another, who had on cotton stockings, no contractions were excited. Cotton is a non-conductor of electricity.
Oils of all kinds are so far from conducting, that if the fingers of the person holding either the probe, or the zinc, have perspired much, even this operates as a complete obstruction to the passage of the influence: the instant the perspired matter has been wiped away, and the fingers have been dipped in water, it again passes, and excites contractions. When the intestines of a frog are removed, and its abdomen is filled with oil, no contraction can be excited by placing one metal upon its sciatic nerves, and bringing another in contact with it, either above or below the surface of the oil.
There is something singular in this respect, with regard to mercury. If the abdomen of a frog be filled with it, a piece of zinc passed through it, so as to touch the sciatic nerves, excites contractions. But a piece of silver, passed to them, excites none. Neither are any excited by touching the silver, beneath the surface of the mercury, with a piece of zinc. But I have before shewn, that, when water is used instead of mercury, contractions may be in this way excited; yet mercury is reckoned a much better conductor of electricity than water. I have repeatedly passed this influence through a great length of thin brass wire, and through the bodies of five persons communicating with each other, by dipping their fingers in basins of water placed between them; yet it did not appear to have lost any of its force, in this long and diffused passage: for the contractions excited in the frog’s leg were equally strong, as when it had passed only through one person. Vitriolic acid, and alcohol appear still better conductors than water.
Wishing to ascertain whether it passed over the surface, or through the substance of metals, I coated several rods of different metals with sealing wax, leaving nothing but their ends, by which they were held, uncovered. Contractions were excited as readily through the media of these, as if they had not been coated. It seems to meet with no obstruction in passing from link to link, of several chains, even when no pressure, except that of their own weight, is used to bring them into contact. I was led from this to hope, that I should be able to make it pass through a very thin plate of air. I, therefore, coated a stick of sealing wax, with a plate of tin-foil, and then made an almost imperceptible division a-cross it with a sharp pen-knife. But even this interruption of continuity in the conductor was sufficient effectually to bar its passage.
The chains, through which it passed most readily, were of gold and silver. It did not pass through a very long and fine brass chain, unless as much force as could be
The contractions produced seemed to be strong in proportion to the extent of the surfaces of the metals in contact, strongest when a large plate of zinc is laid horizontally upon a large plate of silver or gold. If the zinc be suffered to remain in contact with the silver, for a little time, the contractions of the leg cease. The zinc may then be slid over the silver, till it even touch the leg without renewing the contractions: but, in withdrawing the silver, the leg contracts at the instant the silver parts from it!