Experiments and Observations. Fowler Richard
without breaking the chain, was employed to bring its links into close contact.
I next proceeded to examine if the capacity of different substances, as conductors, or non-conductors, was at all affected by differences of their temperature. But this was not the case with zinc, iron, water, coal, or a common crucible, the only substances with which I tried the experiment. A red hot iron, and boiling water, conducted equally as well as iron and water that had not been heated: and neither the crucible, nor the coal, became conductors from any addition of heat.
I at first thought that ice conducted; but as, on some trials, no contractions were excited through its medium; and as it appeared uniformly to conduct ill in proportion to the dryness of its surface; I suspect that, if perfectly dry, it would not conduct at all. The instant a part of its surface had been dissolved by the heat of the room, contractions were excited with as much ease, as they usually are through a bason of water. It would appear, therefore, that neither very hot, nor very cold water disperse this influence, as has been asserted by Dr Valli, nor do they seem in the least degree to diminish its power of producing contractions6.
It appears upon the whole to be necessary, that this influence should pass to a part in a very condensed state, in order to excite contractions: although there are some facts, which, without reflecting, might lead one to suppose, that, passing even in a diffused state, it would excite them. In making that experiment, in which the piece of zinc under the foot of a frog is touched with zinc, while its crural nerve is supported by a silver probe; no contraction takes place, if the probe be either lowered, so as to come in contact with the muscles of the thigh, or if it be made to touch the silver under the foot.
If again, two persons, one of whom holds the probe, the other the zinc, communicate with each other by dipping their unemployed hands in a bason of water; and the person using the zinc holds another leg of a frog, suspended between his fingers by its nerve beneath the surface of the water; no contraction will take place in this leg, when the silver under the other is touched with zinc, at the same time that strong ones are excited in that other. But, if its nerve be raised above the surface of the water, it then contracts as vividly as the other. It appears that in the last of these instances, at least the greater part of the influence had diffused itself through the water, instead of passing directly through the nerve, from the fingers of the person holding it, and that in both it had passed into the legs, in too diffused a state to excite them to contraction.
I have often likewise observed, that when the nerve of a nearly exhausted leg of a frog had been laid upon a piece of zinc, and both were touched with silver, the contractions excited were very distinct: but when the zinc was placed in contact with the muscle, as well as with the nerve, either no contractions could be excited, or such feeble ones that they were scarcely perceptible.
Contractions, however, certainly may be excited in different parts of a frog, without making any division of its skin, by laying the part of the frog to be excited upon a plate of zinc, or tin-foil, and passing a piece of silver over it, till all three are in contact with each other7. Yet even here the influence does not pass into the part in so diffused a state as it may at first appear to do. For the skin of these animals is abundantly supplied with nerves, whose trunks communicate, at different places, with those which supply the muscles. And the contractions are always strongest, and most readily excited, when the silver is passed over the course of any of the nerves, which go to the muscles.
From the fact, which I have before mentioned, that a limb may be made to contract, when the metals have apparently no communication with any part of it except its nerve; it might reasonably be doubted, whether, in any case, a communication between the muscles, as well as the nerve, and the metals, were necessary, in order that contractions may be excited.
Several considerations, however, induce me to believe, that such communication is absolutely requisite. If the contact of two different metals were alone sufficient to excite contractions; contractions should always take place, whenever a good conductor is interposed between the metals, and the nerve alone. But I have, in no instance, observed this to be the case. In the experiment, where the crural nerve must be supported upon a silver probe, it is necessary that the piece of silver, with which the zinc is brought in contact, should communicate either immediately, or through some good conducting medium, with the muscles of the foot, or leg, before any contraction takes place. And even in the experiment, where water forms the only communication between the metals, and the origin of the sciatic nerves, that same water, it must be observed, forms likewise a communication between the metals and the muscles, to which these nerves are distributed. But the fact, which appears to me most decisive of this question, is the following: When a nerve, which for some time has been detached from surrounding parts, is either carefully wiped quite dry with a piece of fine muslin, or (lest this should be thought to injure its structure,) suffered to remain suspended till its moisture has evaporated; no contractions can be excited in the muscles, to which it is distributed, by touching it alone with any two metals in contact with each other. But, if it be again moistened with a few drops of water, contractions instantly take place: and, in this way, by alternately drying and moistening the nerve, contractions may, at pleasure, be alternately suspended and renewed for a considerable time. It may, indeed, be contended, that the moisture softened, and thus restored electricity and free expansion to the dried cellular membrane surrounding the fibres, of which the trunk of a nerve is composed; and thus, by removing constraint, gave free play to their organization8.
But from observing, that, in every other instance, where contractions are produced by the mutual contact of the metals, a conducting substance is interposed between them and the muscles, as well as between them and the nerve; I think it would be unphilosophical not to allow, that, in the instance in question, the moisture, adhering to the surface of the nerve, formed that requisite communication between the metals and the muscles.
I relate the following fact, in this place, because at the same time that it gives further confirmation to the above opinion, it affords an instance in which insulation diminished the effect of the metals. I had one day laid the nearly exhausted leg of a frog upon my hand, with a piece of zinc in contact with its nerve only; and, when I touched these with a silver probe, tolerably strong contractions were excited, even when the nerve appeared dry: but when both the leg and the metals, thus disposed, were insulated by means of glass and sealing wax, the contractions were scarcely perceptible. My hand, it would appear, had, in these instances, supplied the place of the moisture in the other; and been the conducting medium between the muscles and the metals.
This communication of the muscles with the nerve, through the medium of the metals, had appeared to Dr Valli a circumstance so essential to the production of Galvani’s phenomena, that (taking it for granted they were occasioned by the action of the electrical fluid), it seems to have suggested the hypothesis, which he has offered in order to account for them.
Aware that no electrical phenomenon can possibly have place, except between the opposite states of positive and negative electricity, or, in other words, where there is a breach of equilibrium in the distribution of the electrical fluid; he supposes it to be one office of the nerves, to produce this breach of equilibrium, by continually pumping (to use his own expression) the electrical fluid from the internal parts of muscles, and in this way rendering them negative, with respect to the external surface. The brain, he makes the common receptacle for this fluid. The metals, he seems to consider in the light of a conductor, interposed between the outside of muscles and their nerves. And the rapid transmission of the fluid to restore the equilibrium, as the cause of the contractions.
He presumes his hypothesis proved from the following considerations:
I. The interval which commonly takes place between the contractions; which interval, according to him, is necessary for the restoration of the breach of equilibrium.
II. From observing, that fishermen, in order to preserve their fish from putridity, crush their brains; and thus, by interrupting the medium between the external and internal surfaces of muscles, prevent these repeated discharges of the electrical fluid, which, according to Dr Valli, hastens their putridity.
III. From finding that in general, when the sciatic nerve on one side of a living frog was
‘L’eau trop échauffée, ou qui est en éboullition, disperse l’électricité, de manière à en détruire les phenomènes.’
‘L’excès du froid prive l’eau même de la propriété de conduire le fluide en question.’ —
It was in this way, indeed, that I have always excited contractions, when I have employed this new mode of influencing animals, as a test of remaining life in any part of them.
They were constantly kept in fresh water, as the situation most natural to them, during the whole of the time they were under experiment; and their skins were suffered to remain as entire as possible, since I found their muscles lost their contractile power, in a few hours, and became rigid when exposed, deprived of their skins, to the action of the water.
M. Fontana, in the first volume of his work on Poisons, mentions some facts, which may, to some, appear to give considerable countenance to this explanation. The microscopical eels found in dry and smutty wheat; the seta equina or gordius of Linnaeus; and the wheal polypus, all, when dry, become apparently dead: but again recover motion and life when moistened with water. One of the latter was put, by M. Fontana, upon a bit of glass, and exposed, during a whole summer, to the noon-day sun. It became so dry that it was like a piece of hardened glue. A few drops of water, however, did not fail to restore it to life. Another was, in this way, recovered after a similar exposure of a year and a half. Father Gumillo, a Jesuit, and the Indians of Peru, are quoted by the same author, on the authority of Bonguer, as speaking of ‘a large and venemous snake, which being dead and dried in the open air, or in the smoke of a chimney, has the property of coming again to life, on its being exposed, for some days, to the sun, in a stagnant and corrupted water.’
But it would almost require the credulity of an Indian to credit the testimony of the Jesuit.