Part 10

IX. Having made these experiments in vacuo and condensed air, it was of some importance to try also what effect would be produced on animal electricity by the action of the aëriform fluids. The apparatus employed for this purpose was as follows: I provided a glass vessel (Plate IV. fig. 3.), terminating at the upper extremity in a neck, to which could be closely fitted, when necessary, a metallic cover, having a perforation in the centre to receive a moveable rod, which was connected with a transverse metal conductor supporting a frog prepared in the usual manner. Having filled the vessel with water, or mercury, which was still better, I placed it on the shelf of a pneumatic tub, and introduced into it, according to Priestley’s method, any particular gas. Some water or mercury was put into the dish to which it was afterwards removed; and the metal cover, having a frog suspended from it, being then fitted to the neck of the vessel, the frog by means of this apparatus could be immersed at pleasure in carbonic acid gas, hydrogen, or any other kind of gas; so that the constancy of animal electricity might be tried in either. It is evident that, by letting down the spinal marrow of the frog, and the end of the conductor to the surface of the water or mercury, by means of the rod, an arc will be formed, and that muscular contractions must then be produced. The same experiment may be performed by means of a bottle (Plate IV. fig. 6.) furnished with a cover like the former. After being filled with water or mercury, it is inverted on the shelf of the pneumatic tub, and the gas is then introduced in the usual manner; but care must be taken not to displace the whole of the water or mercury, as a portion must be left to cover the bottom when the jar is turned up. This experiment I tried only with oxygen gas, reserving the other kinds of air till a more convenient opportunity.

X. To give more weight to these experiments in vacuo, I endeavoured to prevent all those errors which arose, or might be suspected to arise, from the introduction of air; for I supposed that some opponent of the theory of animal electricity, while endeavouring to find out objections against it, since there was no foundation for asserting that the vacuum afforded any electricity to the animals, might pretend that, the plate of the pneumatic machine being metal, according to custom, some electricity from the atmosphere might be attracted by it, and be thus conveyed even through a vacuum to the animals. On this account it was necessary that a vacuum should be formed, without placing the receiver on a metallic plate; without the contact of any conducting bodies, and in such a manner as to show that the electricity excited was that really existing in the animals. I therefore employed a glass receiver (Plate IV. fig. 9.) cut into two parts above its middle, between which was placed a horizontal partition, in order that the upper part might be filled with oil, or some non-conducting body. The partition was perforated with a large aperture, the superior edges of which projected a little upwards; and the lower, projecting downwards, were furnished with small circular grooves, so that the hole could be shut by a piece of bladder tied over it with thread. A sharp-pointed rod was placed over the hole, so that, being let down by a non-conducting handle, it could be made to pierce the bladder. This, however, was to be done only when the lower part of the receiver was exhausted of air; for the oil, when the air is withdrawn, speedily falls down: by these means a vacuum is formed in the upper part, and, in consequence of the interposed stratum of the oil, remains insulated, as it can receive no electricity from the metallic plate of the machine, nor from other conducting bodies. But it will be in vain to attempt to form a vacuum in this manner, unless the parts of the cut glass be so fitted as to prevent entirely the admission of external air. A very simple and ingenious apparatus, for the same purpose, was invented by F. Borelli: in order to stop the efflux of the oil at pleasure, he adapted to the hole a glass cone, by which means he was enabled to produce an insulated vacuum of greater or less capacity in the upper part of the receiver. Here then I had a vacuum every where surrounded with non-conducting bodies, so that, if animal electricity were excited in it, there could be no reason for ascribing it to electricity borrowed from the atmosphere.

XI. I adapted to a glass rod, in a longitudinal direction, a metallic plate, (Plate IV. fig. 7.) which at the upper extremity was bent into a right angle, and supported the spinal marrow of a frog, so that it might be considered as a lengthened conductor of the nerves. This plate was inserted into a glass receiver, so as to move in it with ease; and by means of a screw could be fixed at any altitude whatever. An accurate representation of this apparatus, with the glass rod annexed, is seen in Plate IV. fig. 8. Iron hooks fixed to the feet of the suspended frog acted the part of a conductor to the muscles; and the upper part of the receiver being exhausted of air, as above described, a stratum of oil was still left to separate the frog from the partition. To guard against all danger from the action of any internal metallic body, a strong magnet was applied to the iron conductor of the muscles, which, immediately obeying the power of attraction, fell upon the conductor of the nerves; and thus a circuit of animal electricity from the muscles to the nerves being speedily effected[10], contractions were immediately produced. When I communicated this experiment to the Institute, I was extremely desirous that the apparatus I had here invented for the purposes of animal electricity might be of some advantage to the science of philosophy in general, of which I was always fond, and which formed the chief object of my study. But it is necessary that I should mention to what I more particularly allude.

XII. Such bodies as had hitherto been put into the air pump were gradually subjected to the action of a vacuum. Hence it appeared that the apparatus above described might be attended with a considerable degree of utility; as in future, any body whatever, whether solid or fluid, might be subjected to this action at once. For, the lower part of the receiver being deprived of its internal air, if the bladder be burst by means of the glass rod, the fluid will run down and occupy the space emptied of air, leaving a vacuum in the upper part. When the fluid has thus fallen to the lower part of the receiver, solid bodies even, if any were immersed in it while in the upper part of the receiver, will also experience the action of the vacuum. But liquid bodies are of such a nature, that they have united with them certain aëriform fluids, which, when the pressure of the air is removed, readily expand. A fluid, therefore, when it has fallen to the lower part of the receiver, being agitated and thrown into a state of perturbation by the motion, its most subtle principles will be extricated, and fill the capacity of the bell. The vacuum will then be disturbed by the evaporation, which, acting on the mercury in the barometer connected with the air pump, will cause it to fall. But every one acquainted with the principles of philosophy must know, that the depression of the mercury in the barometer will be greater, according as a greater quantity of aëriform fluids has been disengaged in the receiver; and if the degree of pressure in the barometer varies according to the variety of aëriform principles, it may be readily seen, that this method may be employed to determine the quantity of them, or their elasticity, since they are cut off from all communication with the surrounding atmosphere, though it still exercises its pressure upon them.

XIII. The vacuum here obtained in the upper part of the bell, seems to be far superior to that produced in the lower part, according to the usual method. For it has long been a complaint among philosophers, that by working the air pump the air is only rarefied, till it no longer possesses elasticity capable of raising the valve, so that it is impossible to produce a complete vacuum by this method. If we can credit the followers of Euler, that subtle fluid, which they call ether, and which permeates every thing, still remains; for, adopting the opinion of the antient Peripatetics, they consider a vacuum as beneath the dignity of nature. But, setting aside this question, I shall only observe, that if a vacuum be formed in the upper part of the receiver, by the method above described, it does not appear that it can be disturbed by any thing from without, and the gravity of the falling fluid will not suffer itself to be overcome by the subtle ether, if any really exists. Should it be apprehended that the air contained in the oil may be disengaged, and disturb the vacuum, you may substitute in its stead mercury or water, which can be deprived of air either by boiling or by long exposure to the action of a vacuum. But before any thing certain on this subject can be said, new and repeated experiments will be necessary. As every objection that could be made in regard to a vacuum seems now to be obviated, since a space perfectly free from common air can be produced, and cut off from all communication with the atmosphere, or with conducting bodies, I shall return to animal electricity, from which I was led by a desire of contributing towards the improvement of natural philosophy in general.

XIV. Those who attempt to determine the velocity of the nervous fluid in a given time, undertake a matter of great difficulty, respecting which nothing certain can be known. Haller rejected the suppositions of those who, comparing the tenuity of the nervous tubes of the heart with the large branch of the aorta, were of opinion, that the velocity of the nervous fluid must be two thousand eight hundred and eighty times greater than that of the blood. This celebrated physician, distrusting hypothesis, had recourse to experiment, and found that the velocity of the nervous fluid would be no less than nine thousand feet in the first minute. But in this determination of the velocity of the nervous fluid there seems to be some difficulty, which perhaps ought to be ascribed rather to the period when that celebrated man lived, than to his want of sagacity or accuracy. Had Haller possessed the means of conveying the nervous fluid with his own hands to different parts at pleasure, he would no doubt have given us some more certain ideas respecting its rapid motion. I resolved, therefore, not to neglect those advantages with which the modern philosophy has been enriched, and to employ very long metallic arcs, by which I could direct the animal electricity as I pleased. A staircase which reached from the top to the bottom of the house, with many windings, presented me with an iron plate, exceedingly convenient for the transmission of animal electricity. A metal wire, brought down from the top of the staircase, was connected with the iron plate, and by these means I obtained an arc, the length of which was above one hundred and fifty feet. When this arrangement was made, the two extremities of this very long arc were applied to the armed nerves and muscles of a frog; and the animal electricity being thus excited, instantly proceeded with so much velocity from the one extremity of the arc to the other, that no difference could be perceived between the time when the frog touched the arc, and that when it began to be agitated. But to show that this result was not owing entirely to the metallic conductor, I employed long ropes dipped in salt water, and always with the same effect. This circumstance seems to prove, in a striking manner, a great similarity between the nervous fluid and common electricity, and to overturn the opinion of Haller, who, according to his calculations, makes the nervous fluid require a second for passing over the space of 150 feet.

XV. According to Beccaria, a celebrated observer of the propagation of the electric fluid, there are two ways of its being transmitted; one when it flows through conducting bodies only, and the other when, being collected in non-conducting bodies, it proceeds from the coating electric by excess to that which is in the contrary state. In the one case, Beccaria establishes a certain time for its passage; in the other he allows none. This celebrated man observed, that the electric matter was conveyed from the conductor of a machine, along a metallic wire 500 feet in length, in the course of a second; in its passage along a hempen rope of the same length, it employed seven seconds; but when the rope was moistened with water, it required only two or nearly three vibrations of a pendulum. When he discharged a Leyden flask by the longest conductor, he was never able to observe the least interval of time. The same thing was remarked by Jallabert, Sigaud de la Fond, and other philosophers, who performed their experiments, not in an apartment but in the open air; and conveyed the electric fluid in this manner for a considerable distance along the banks of large rivers. Monnier extended two iron wires[11] in an open field, parallel to each other, for the distance of 5107 feet; and a man placed between them held in his hands the extremity of the conductors, keeping them at a little distance from his body. “But the man, who was in the middle of the arc,” says the author, “while he saw the spark issue from the jar, received the shock: he could have distinguished the smallest interval of time between the explosion and the shock; and if it had amounted to the fourth part of a second, it could have easily been remarked.”

XVI. While reflecting on these facts, I formed a conjecture from the great celerity with which animal electricity is conveyed, respecting the manner in which it is evolved. If the animal electricity were conveyed from a muscle to a nerve, or vice versa, in the same manner as the common electric fluid is conveyed from the machine by the chain, it would have been observed to employ some time, however small, in its passage. As I at first ascribed this to the shortness of the conductor I had used, I extended it to more than 250 Parisian feet, and applied the nerves and muscles of a frog to this new conductor in the manner above described, without observing the least obstacle to the passage of the electric fluid. As this arc formed a half of that employed by Beccaria, the space of half a second would have been required, if we consider in this passage only one kind of electricity. But the half second required according to the observations of Beccaria was not observed: it therefore appears, that this propagation of animal electricity ought not to be referred to the first-mentioned case, but to that where equilibrium is restored between the negative and positive state.

XVII. This rapid conveyance of animal electricity, however, is entirely stopped, if the metallic arc be intercepted, not only by non-conducting but by certain conducting bodies. Here then we have again occasion for the action of two contrary kinds of electricity. For the electric matter, whether positive or negative, when conveyed from the machine to the chain, pervades all bodies in the same manner, provided they be conductors. Thus the metallic conductor of the machine, the insulated person, and all other bodies that may be connected with it, become electric in the same manner. But the same electric matter collected in non-conducting bodies requires, before it can be discharged, certain conditions in the conducting bodies by which it is discharged. When the metallic arc is interrupted by a little water, the Leyden flask, if it contain a moderate quantity of electricity, cannot be discharged; for the two contrary kinds of electricity to be discharged require that every part of the arc should be equally endowed with the property necessary for conducting the electric fluid. But in the case of only one kind of electricity, it would pass with great readiness either through water or metal. Let us now apply these phænomena of general electricity to the theory of animal electricity. In the experiment mentioned in the sixth section, if only one kind of electricity proceeded from the nerves or the muscles, it would be immediately conveyed from the nerve to the metal, then to the vacuum back to the metal, and thence to the muscle, as being the place from which it issued. Besides, the electricity propagated in this manner would have produced contractions, which however were not observed. The progress of the animal electricity, therefore, experienced considerable obstacles, not only from non-conducting but also from conducting bodies; which affords a strong proof that electricity exists and is collected in the muscular fibre, in the same manner as in non-conducting bodies. But the remarkable quickness of the progress of animal electricity leads me to the Leyden flask; and therefore I shall here say a few words respecting the analogy between the phænomena it exhibits, and those of animal electricity; and give an account of the reasons which first induced me to enter on this comparison.

XVIII. When public meetings were held at the house of Galvani, for the purpose of discussing the theory of animal electricity, great doubts were entertained respecting two contrary kinds of electricity acting in animals. The reasoning on this subject, which displays acuteness and ingenuity, was as follows: If two kinds of electricity, one positive and the other negative, prevailed in the nerves and muscles of animals, on applying the muscles of one frog to the armed nerve of another, we should observe contractions; which however is not found to be the case. The proposed doubt, however, gave me considerable uneasiness, as the dispute on that subject seemed to lay a foundation for many objections against the theory of animal electricity. But the novelty of this event will excite no astonishment in those who consider the subject with attention: nay, it would rather seem astonishing if the matter were otherwise. The phænomena in the above experiment are perfectly agreeable to the laws of general electricity, and to the theory of the new animal Leyden flask. For, if we suppose the muscles of the frog furnished with nerves to represent so many Leyden flasks, no contractions ought to be expected from them, in circumstances under which Leyden flasks themselves would produce no explosion. This we always observe in two electric jars, neither of which is discharged when the arc is established between the exterior coating of the one and the interior coating of the other. If the muscles, therefore, in the above experiment, represent Leyden flasks, in cases in which no explosion could take place in the latter no contractions can be observed in the former.

XIX. For the sake of illustrating the proposed analogy, it will be proper that I should here explain the conditions under which contractions are produced by the application of two frogs to each other, and compare them with the phænomena of the Leyden flasks. I shall therefore show, in a few words, the different methods in which several frogs are made to contract at the same time, and in which Leyden jars are discharged. Place on a glass plate two frogs, one of which has its spinal marrow armed, and let a communication be established between its muscles and the spinal marrow of another frog, by means of a small metallic chain. If the arc be formed from the armed spinal marrow of the one frog to the muscles of the other, contractions will be produced in both. Let us now apply to Leyden flasks the arrangement followed with the frogs. If two electric Leyden jars stand on a glass plate, when an arc is applied, some electricity will be elicited; but an absolute explosion will never be produced. If one extremity of a metallic wire, however, be brought in contact with the inside of one of the Leyden jars, and the other with the exterior coating of the other, on applying an arc to the other two coatings, which have no communication with the metallic wire that has been added, an explosion will take place, and both the jars will be discharged. But the above contractions may be produced in a manner still simpler. If the spinal marrow of one frog be united to the muscles of another, as soon as an arc is formed from the spinal marrow of the armed frog to the remote muscles of the other, strong contractions will be produced in both. The analogy between animal and artificial electricity, which is the object of our research, will always be apparent in this experiment. Two charged Leyden flasks, suspended in such a manner from the conductor of the machine that the exterior coating of the one is connected with the interior coating of the other, form a very happy representation of the frogs; for, the same arc being applied, and in the same manner, both to the frogs and the Leyden flasks, when an explosion is produced by the latter contractions will take place in the former.

XX. Hitherto the contractions have been produced by establishing the arc from the nerve of one frog to the muscles of another: but contractions will take place in both, if the arc be conveyed from the armed spinal marrow of the one to the armed spinal marrow of the other, provided care be taken that corresponding muscles communicate alternately with the conducting body. But it is much more difficult to reconcile this phænomenon than the former to the general laws of electricity. This difficulty, however, may be obviated, if we suppose that the one frog, in consequence of its natural moisture, forms an arc to the other. This indeed was first confirmed by the experiments of Galvani; for, having divided a frog lengthwise, both parts were connected merely by their moisture[12]. Yet, when the arc touched one of the separated parts, the other was immediately contracted. As this explanation is so obvious, nothing further needs be said on the subject. But I was unwilling to leave in a state of uncertainty the analogy between the phænomena of animal electricity and those of the Leyden flask, the wonderful agreement of which had so much excited my astonishment: and indeed I had no cause to repent of my perseverance; for, though it did not enable me to attain to what I proposed, it conducted me to some general phænomena of electricity, which no one perhaps had before made an object of research. I discovered that one Leyden flask may be applied as an arc to another. I provided two insulated Leyden jars of the same capacity, one of them charged and the other uncharged, and established a communication between the exterior coating of both, by means of a conducting body; and having then formed an arc from the interior coating of the one to that of the other, there was an immediate transmission of the electricity with an explosion; and at the same time the flask which at first was uncharged became charged. If I formed an arc with my arms and hands, I experienced a considerable shock during the passage of the electric fluid. When I observed this effect, I conceived it was not contrary to the principles of philosophy to suppose that the one frog, in respect to the other, represented a Leyden flask, and at the same time acted the part of an arc.