SECOND DISSERTATIONONANIMAL ELECTRICITY:READ IN THEINSTITUTE OF BOLOGNA,IN THE YEAR 1794,ByJ. ALDINI.I.Thephilosophers of the present period are so sanguine in their expectations, that when a new theory is proposed, unless it be presented to them perfect and fully proved, they either attack it in part, or entirely reject it. Such has been the case with animal electricity, discovered by Galvani. It is urged against it by its opponents, that it is subject to variations; and because they do not find it obedient to all those laws established by the laborious researches of a Franklin, a Beccaria and an Æpinus, they assert either that it has no foundation, or that it is contrary to nature. It has thereforebeen conceived that an accurate comparison of animal and common electricity, in order to ascertain whether there be any difference between them, might be the best means of obviating such objections. For this purpose I made various experiments in animal electricity under the air pump, employing proper conductors, and I compared its phænomena with those exhibited by the Leyden flask. In a word, while I endeavoured to pursue my researches agreeably to the general theory of common electricity, my principal object was to prove the constancy of animal electricity, by discovering, if possible, an agreement in the physical laws of both.II.As it had been proved by a great many experiments, that common electricity could be obtained from non-conducting bodies in the most perfect vacuum possible to be formed by means of an air pump, experiments were undertaken in order to ascertain whether the same phænomenon was common also to animal electricity, and with this view attempts were made to excite the latter in vacuo[7]. Muscati indeed had deprived animals of life in vacuo, and afterwards found them susceptible of Galvanism in the open air; but he made no attempt to determine whether the animal electricity could be manifested in vacuo. I employed for my experiments a glass vessel;PlateIV.fig. 6., furnished with a metallic rod, which could be, raised up or pushed down at pleasure. To the extremityof the rod, within the receiver, was affixed at right angles a metallic wire, from one end of which an armed frog was suspended by the muscles, and from the other a small metallic chain a little longer than the frog, a plate of silver being placed below both the frog and the chain. When as perfect a vacuum as possible had been obtained, the metallic rod was pushed down, so that the small chain and the spinal marrow of the suspended frog were made to touch the silver plate, and by these means the latter formed an arc in the exhausted receiver. In this experiment, the power of Galvanism was found to be the same as in the open air, so that as often as the rod was pushed down contractions were excited in the frog. By this method it was easy to ascertain what repeated contact could produce by forming new arcs: for though the small chain and the extremity of the spinal marrow touched the silver plate; yet, when removed from that position ever so little, by moving the rod new contractions took place, which could not have been expected unless new contact on moving the rod had produced as it were new arcs. This kind of apparatus seemed the most convenient for performing in vacuo all those experiments which Galvani had performed in the open air.III.But it was as yet difficult to determine, whether the contractions which took place were stronger in rarefied than in common air; for the difference between the electricity was so small, that it was impossible to say which was the more powerful. I therefore resolved to clear up this point by otherexperiments. For this purpose, having cut in two a prepared frog, I placed in vacuo one part of it, by means of the above apparatus, and after a short period drew it out, and compared it with the half which had not been subjected to the action of a vacuum. On applying an arc, the one exhibited strong and the other faint contractions; from which it appeared that the vacuum had occasioned some loss of the animal electricity; as the muscular and nervous parts subjected to experiment belonged to the same frog, this evidently showed that the whole difference arose from the action of the vacuum.IV.It is well known that a vacuum absorbs common electricity; and therefore it need not excite any wonder that in the present experiment it should have dispersed some of the animal electricity. As this loss took place by insensible degrees, strong contractions were not to be expected; and the case is nearly the same in a Leyden flask loaded with aqueous vapours, which produces no remarkable explosion. But though convinced of the truth of this circumstance, I resolved to confirm it by a new experiment. I therefore charged two Leyden flasks armed in the same manner, applying them at the same time to the same conductor, and by the same number of turns of the machine. One of these flasks was introduced into the glass receiver of the air-pump, and the other was exposed to the atmosphere. At the end of five minutes after the air had been exhausted as much as possible, the flask in the receiver was taken out, and, being discharged by a metallic arc, emitted a weak spark, while the other flask exhibitedstrong signs of electricity. I again charged two flasks by the same turns of the machine till the electrometer indicated in both the same degree of electricity, and kept one of them for half an hour beneath the receiver. When the latter was taken out, it afforded a weak and almost exhausted spark; while the other, which had remained in the open air, emitted a strong one. Had the first flask remained longer under the receiver, it would no doubt have entirely lost its electricity; while the other without the machine retained that with which it was charged.V.In this experiment every possible care was taken to observe the variations produced by the vacuum. The receiver which covered the Leyden flask was of a large size, and the flask, being placed in the middle of it, was at a considerable distance from its sides: the receiver was well fitted to the plate of the machine, and not by means of moist leather, as is usual, so that all communication with the external air was prevented; and therefore there is no reason to suppose that any aqueous vapour introduced into the machine, when the pressure of the air was withdrawn, could perform the office of an arc. That the electricity might not immediately bring itself to a state of equilibrium, the conductor of each flask terminated in a ball: had not this been the case, the whole electricity would soon have been dispersed; for, when the conductors terminate in a point, if the chamber be darkened, coruscations of light will every where be seen on the glass receiver, and afford a most agreeable spectacle. A conductor terminating in aball seemed therefore to be most convenient for my purpose, that, by rendering it more difficult for the electricity to bring itself to an equilibrium, I might be able to produce a greater imitation of the intimate manner in which animal electricity adheres to the animals. But though in the above experiments I ascribe some part of the phænomena to the animal electricity being weakened in vacuo, I am of opinion that more is to be ascribed to the violent perturbation of the principles which the vacuum must have excited in the muscular and nervous parts.VI.As it was now established, that animal electricity could be excited in vacuo, I endeavoured to ascertain whether that excited without the vacuum, and conveyed to the receiver, could be made to pass through a very small space in vacuo. For this purpose I placed the metallic rod at a small distance from a silver plate resting on the bottom of the air pump: the limbs of a chicken or lamb, prepared in the usual manner, were then deposited near it; and the muscles, by means of a metallic chain, were made to communicate with the plate of the machine, while an armed nerve was made to communicate by means of an insulated arc with the metallic rod. The air being exhausted, the rod was pushed down, and gradually brought as near as possible to the plate without coming into actual contact with it. In this state no contractions were produced; but they immediately took place when the rod was brought into contact with the plate. It appears therefore that animal electricity is considerably impededin its progress by a vacuum; and that, like artificial electricity, it does not readily suffer itself to be dissipated, unless transmitted through good conducting bodies. For, when a small quantity of the electric fluid is accumulated in the Leyden flask, either none of it proceeds from the metallic wire to a less perfect conductor, or, if any is transmitted, it must be with difficulty, and with great violence. Common electricity, indeed, is seen to pass quietly through metals or water separately; but a strong electric spark, in proceeding from one metallic conductor to another, if it pass through water interposed between them, does so with such violence, that the glass vessel which contains the water is in danger of being broken to pieces. That electricity, therefore, may be conveyed from a conducting body, to one less endowed with that property, it must be in such abundance as to be able to overcome the resistance of the body through which it has to pass: hence, it need excite no surprise, that the small quantity of electricity which produces contractions should not be able to pass over a very small space in vacuo. Here then we have a proof that animal electricity is not only subject to the laws of non-conducting bodies, but that it is affected different ways by the smallest obstacles of conducting bodies, and by different kinds of them. It is indeed so evident that a vacuum from its nature is unfit for conveying electricity, that, even if one be produced, not by the usual method, which is always attended with some defects, but in the most accurate manner possible, it is totally improper for being a conductorof the electric fluid. This is sufficiently proved by the following experiment of Walsh: If two barometers be joined, and the upper part or bend be carefully deprived of air, when one of these barometers is electrified, the electric fluid will not be communicated to the other, in consequence of the resistance opposed by the intervening vacuum. Adams, however, exhibited by means of a single barometer the same phænomenon as Walsh did with a double one[8]; for, having extracted the air entirely from the upper part of the barometer, no electric light was observed; but on introducing a very small quantity of air the whole barometer became luminous. These observations are sufficient to show that animal electricity, in regard to the property of not being able to pass through a small space in vacuo, is subservient to the general laws of common electricity. But let us proceed to other phænomena respecting animal electricity excited in vacuo.VII.If a prepared frog, furnished with two armatures,be placed in a horizontal position on a non-conducting body, under a glass receiver, (PlateIV.fig. 11.) and if an arc be formed by pushing down a rod, so as to join both the armatures without touching the frog, contractions will immediately take place; but if the smallest non-conducting body intervene, none will be produced. I found it no very difficult matter to exhibit in vacuo, by an apparatus somewhat similar, what may be called an animal alarum. A horizontal arm fixed to a vertical moveable rod (PlateIV.fig. 5.) was adapted to the inside of a glass receiver, in such a manner that, when the receiver was exhausted, any body resting on the horizontal arm could be made to fall down. The leg of a prepared frog was then fixed to the vertical rod; while the other rested on the horizontal arm, and the spinal marrow, with an armature of tin foil, touched a silver plate in the bottom of the receiver. The horizontal arm being turned round a little, by means of the vertical rod, the leg of the frog resting on the arm fell down on the plate below: an arc being thus formed, contractions immediately took place, and were incessantly repeated, until all the animal electricity was restored to an equilibrium.VIII.These experiments were made on dead frogs; but I shall here show that living ones also may be made to exhibit signs of electricity under the same circumstances. A piece of tin foil was applied to the back of a frog tied to a silver plate by means of silk strings, (PlateIV.fig. 4.) and two metallic chains were suspended in such a manner, that by pushingdown the rod the extremity of the one chain could be made to touch the silver plate, and the extremity of the other the tin armature: when an arc was by these means formed, contractions instantly took place. The frog began to breathe with difficulty, to be agitated with convulsive movements, and to be seized with an universal tremor, so that its last moment seemed to be approaching; but on air being admitted into the receiver, it recovered and appeared as lively and active as before. On examining more closely the changes which had taken place in the animal while in this state, which was certainly contrary to nature, I found the muscles red with a superabundance of blood: but when dissected in the usual manner, they exhibited strong signs of animal electricity; for, on applying an arc to the nerves or muscles, without any armature, violent contractions were produced, and continued for a long time, provided care was taken that there should be no deficiency of animal moisture. That such a quantity of electricity should be excited, will not seem astonishing to those who have seen more violent electric commotions excited in the animal machine by the action of a needle. This indeed has been placed beyond all doubt by an observation of Gardini, who says “that having made some experiments with a large torpedo, he remarked that stronger convulsions were produced when the animal was subjected to great pain by any means, such as pricking it with a needle[9].” The phænomenaexhibited by a vacuum or rarefied air, were exhibited also by condensed air, so that very powerful contractions were produced by one homogeneous arc. For, if the same apparatus described inPlateIV.fig. 4. be adapted to a condensing machine, a dead frog introduced into it will readily be contracted. Live frogs also, after being kept for half an hour or a whole hour in air twice as dense as that of the atmosphere, exhibited strong signs of electricity without any armature, and merely by the application of a silver arc to the nerves and muscles.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 (PlateIV.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 immersedat 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 (PlateIV.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 avacuum 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 (PlateIV.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 lesscapacity 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, (PlateIV.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 inPlateIV.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 communicatedthis 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 fallingfluid 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 fluidthere 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 theopinion 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 inhis 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 experimentare 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 thatcorresponding 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, therewas 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.XXI.Some, perhaps, will object to this analogy, that in the above experiments the flask destitute of electricity forms the arc; while, on the contrary, both flasks ought to be charged to represent properly the muscles and nerves of frogs, which both possess electricity. But the very same phænomenon is observed in two charged flasks, provided one of them is charged with more electricity than the other. Hence, in support of our analogy, we need only assume, that the quantity of electricity in the one frog is a little different from that of the other; a supposition which, in forming an hypothesis, the severest philosopher may allow. I shall say nothing of the great variety and connection of the elements of which animal bodies are composed, and which on this account require a difference in the quantity of the animal electricity. It appears by some late experiments of Valli[13], thatanimal electricity is discharged in the same manner as that which is collected in non-conducting bodies. It is likewise proved, that the same arc and the same armatures, according to the various consensus and connection of the nerves, and according to the different positions in which they are applied, elicit a larger or smaller quantity of electricity, and sometimes none at all. Those indeed who consider these observations will hardly think it possible, when animal Leyden flasks consist of so many different parts, that there should not be some difference in the electricity collected. But there is no need of employing conjecture in regard to a point, which is confirmed not only in regard to animals, but in all conducting bodies, by the ingenious observations of Coulomb[14]; for it is fully established that the electric matter is communicated and accumulated different ways on the surface of conducting bodies. But if this phænomenon takes place in conducting bodies the parts of which are homogeneous, there is no reason to deny that it may appear in animals in which provident nature has so intermixed conducting with non-conducting parts, in order that the action of animal electricity might not be short and transient, but constant and durable.XXII.But if the diversity in the structure of animalbodies require that the force and power of the animal electricity, collected in the corresponding muscles, should be different, a singular agreement will appear between the phænomena of the Leyden flask and those of animal electricity. If an insulated person touch two flasks containing equal quantities of electricity, however great, he will experience no shock; but if one of the flasks contain a greater charge than the other, he will receive a shock according to the ratio of the difference of the electricity of the flasks. Though the frogs therefore represent two Leyden flasks, one of them may act the part of an arc, and produce an equilibrium, provided it be allowed that there is a difference between the quantities of the electricities collected. I have here endeavoured to establish the proposed analogy, not because I suppose the muscles to be so many Leyden flasks, such as they are exhibited to us by the ingenuity of philosophers, but in order to show that many phænomena are common to both; nor have I applied animal electricity to explain all the phænomena of muscular motion, with a view of obtaining applause from those who are zealous advocates for this theory. I must also observe, that if in the prosecution of this object I have met with any anomalies, I do not on that account despise the agreement of the laws of philosophy which have been established by so much labour. Several of the phænomena observed by Galvani and others have served me as a foundation for the proposed analogy, and induced me to extend, if possible, its boundaries. But going back to theorigin of animal electricity, since it belongs to the subject, I shall here take a general view of the whole, and express the substance of it in a few corollaries.XXIII.The corollaries I propose will follow the order of time in which they arose, and therefore will express the gradual improvement of animal electricity.1. Animal electricity passes freely through bodies which possess nearly the same degree of conducting power, but it does not pass through non-conducting bodies.2. It is affected by the obstacles which occur, not only in non-conducting but in conducting bodies, as well as by their varieties; and if these obstacles be numerous, its passage is stopped: but if it be possible to overcome them, the impediment causes it to make a more powerful effort to attain to a state of equilibrium. Hence, unlike armatures and arcs are of great effect in exciting a moderate degree of electricity, when the same electricity resists the power of a homogeneous metal.3. Animal electricity obeys the law of equilibrium; for, when the muscles have been brought to a state of equilibrium with the corresponding nerves, no contractions are produced by the application of an arc; but if that which produces the equilibrium be removed, the contractions immediately take place.4. Poisons, mephitic air, aëriform fluids, and condensed air, do not prevent animal electricity from being excited.5. The influence of a vacuum on animal electricity is various. In dead animals, if kept a long time in vacuo, the animal electricity is weakened: in living animals it is considerably increased.6. Though a vacuum does not prevent the electricity from being excited, it will not serve as a conductor of it when it has been excited: if an arc from the nerves to the muscles be intercepted by the smallest vacuum, no contractions take place.7. As metallic armatures are of great effect in attracting and collecting artificial electricity, the case is the same with animal electricity; but care must be taken not to ascribe to them that electricity which the muscles naturally possess.8. Though unlike armatures have a great effect in calling forth animal electricity, we have reason to conclude from several experiments, that they do not contain two kinds of electricity capable of producing muscular motion.9. As natural electricity issues with great force from sharp-pointed bodies, and proceeds to them more readily than to others, the case is the same in regard to animal electricity, as it issues more readily from the pointed parts of the metallic arming applied to the nerves and muscles.10. The nervo-electric fluid is propagated with that rapidity which is required in restoring to an equilibrium two opposite kinds of electricity.11. The same conditions which cause two flasks to be discharged when an arc is established from the exterior coating of the one to the interior coating of the other, excite contractions in two frogs, when an arc is formed from the nerve of one to the muscles of the other.12. The arc applied as above mentioned to the interior coating of two phials, and to the nerves of two frogs, seems to give more force to the proposed analogy; for the electric explosions have a great similarity to the muscular motions excited in the frogs.I might have enlarged the number of these corollaries, had not the well-known fate of various opinions, now consigned to oblivion, rendered me more timid in hazarding conjectures. I, however, did not allow myself to think that I ought so far to give way to my timidity as to check the spirit of inquiry, or to abandon the hope of one day attaining to the truth. But it would be unreasonable to expect in animal electricity, which is yet in its infancy, that precision and those satisfactory results which can be the work only of time, and of the continued labour of philosophers.[7]As this term is improperly used by philosophers, I must here observe, that I shall in future understand by it air highly rarefied by the usual means.[8]In the description of a new air-pump of his invention, where he shows that electricity cannot pass through a vacuum, he adds: “There can be little doubt, from the above experiment, of the non-conducting power of a perfect vacuum; and this fact is still more strongly confirmed by the phænomena, which appear upon the admission of a very minute particle of air into the inside of the gauge. In this case, the whole becomes immediately luminous upon the slightest application of electricity; and a charge takes place, which continues to grow more and more powerful, in proportion as fresh air is admitted, till the density of the conducting medium arrives at its maximum.An Essay on Electricity, explaining the Theory and Practice of that useful Science.Third edit. London, 1787.[9]Josephi Gardini de Electrici Ignis Natura Dissertatio, Regiæ Scientiarum Academiæ Mantuanæ exhibita, Mantuæ1792,p.100.[10]This may be accomplished in a much more simple manner, without the aid of a magnet, by connecting a wire with the lower part of the nerves, and applying the wire to the muscles by turning the rod round.[11]Précis historique et expérimental des Phénomènes électriques, par M. Sigaud de la Fond, Paris1781,sect. i. art.4.[12]Aloysii Galvani de Viribus Electricitatis in Motu musculari Commentarius, Mutinæ iterum editus,p.29.[13]M. Valli Cinquième et Huitième Lettre sur l’Electricité Animale, dans Observations sur la Physique, par M. l’Abbé Rozier,tom. xlii.Paris, 1792.[14]Recherches sur la Distribution de Fluide Electrique entre plusieurs Corps Conducteurs, et la Détermination de la Densité Electrique dans les différens Parties de la Surface de ces Corps.Mem. de l’Acad.Royale des Sciences,An.1788.
ON
ANIMAL ELECTRICITY:
READ IN THE
INSTITUTE OF BOLOGNA,
IN THE YEAR 1794,
ByJ. ALDINI.
I.Thephilosophers of the present period are so sanguine in their expectations, that when a new theory is proposed, unless it be presented to them perfect and fully proved, they either attack it in part, or entirely reject it. Such has been the case with animal electricity, discovered by Galvani. It is urged against it by its opponents, that it is subject to variations; and because they do not find it obedient to all those laws established by the laborious researches of a Franklin, a Beccaria and an Æpinus, they assert either that it has no foundation, or that it is contrary to nature. It has thereforebeen conceived that an accurate comparison of animal and common electricity, in order to ascertain whether there be any difference between them, might be the best means of obviating such objections. For this purpose I made various experiments in animal electricity under the air pump, employing proper conductors, and I compared its phænomena with those exhibited by the Leyden flask. In a word, while I endeavoured to pursue my researches agreeably to the general theory of common electricity, my principal object was to prove the constancy of animal electricity, by discovering, if possible, an agreement in the physical laws of both.
II.As it had been proved by a great many experiments, that common electricity could be obtained from non-conducting bodies in the most perfect vacuum possible to be formed by means of an air pump, experiments were undertaken in order to ascertain whether the same phænomenon was common also to animal electricity, and with this view attempts were made to excite the latter in vacuo[7]. Muscati indeed had deprived animals of life in vacuo, and afterwards found them susceptible of Galvanism in the open air; but he made no attempt to determine whether the animal electricity could be manifested in vacuo. I employed for my experiments a glass vessel;PlateIV.fig. 6., furnished with a metallic rod, which could be, raised up or pushed down at pleasure. To the extremityof the rod, within the receiver, was affixed at right angles a metallic wire, from one end of which an armed frog was suspended by the muscles, and from the other a small metallic chain a little longer than the frog, a plate of silver being placed below both the frog and the chain. When as perfect a vacuum as possible had been obtained, the metallic rod was pushed down, so that the small chain and the spinal marrow of the suspended frog were made to touch the silver plate, and by these means the latter formed an arc in the exhausted receiver. In this experiment, the power of Galvanism was found to be the same as in the open air, so that as often as the rod was pushed down contractions were excited in the frog. By this method it was easy to ascertain what repeated contact could produce by forming new arcs: for though the small chain and the extremity of the spinal marrow touched the silver plate; yet, when removed from that position ever so little, by moving the rod new contractions took place, which could not have been expected unless new contact on moving the rod had produced as it were new arcs. This kind of apparatus seemed the most convenient for performing in vacuo all those experiments which Galvani had performed in the open air.
III.But it was as yet difficult to determine, whether the contractions which took place were stronger in rarefied than in common air; for the difference between the electricity was so small, that it was impossible to say which was the more powerful. I therefore resolved to clear up this point by otherexperiments. For this purpose, having cut in two a prepared frog, I placed in vacuo one part of it, by means of the above apparatus, and after a short period drew it out, and compared it with the half which had not been subjected to the action of a vacuum. On applying an arc, the one exhibited strong and the other faint contractions; from which it appeared that the vacuum had occasioned some loss of the animal electricity; as the muscular and nervous parts subjected to experiment belonged to the same frog, this evidently showed that the whole difference arose from the action of the vacuum.
IV.It is well known that a vacuum absorbs common electricity; and therefore it need not excite any wonder that in the present experiment it should have dispersed some of the animal electricity. As this loss took place by insensible degrees, strong contractions were not to be expected; and the case is nearly the same in a Leyden flask loaded with aqueous vapours, which produces no remarkable explosion. But though convinced of the truth of this circumstance, I resolved to confirm it by a new experiment. I therefore charged two Leyden flasks armed in the same manner, applying them at the same time to the same conductor, and by the same number of turns of the machine. One of these flasks was introduced into the glass receiver of the air-pump, and the other was exposed to the atmosphere. At the end of five minutes after the air had been exhausted as much as possible, the flask in the receiver was taken out, and, being discharged by a metallic arc, emitted a weak spark, while the other flask exhibitedstrong signs of electricity. I again charged two flasks by the same turns of the machine till the electrometer indicated in both the same degree of electricity, and kept one of them for half an hour beneath the receiver. When the latter was taken out, it afforded a weak and almost exhausted spark; while the other, which had remained in the open air, emitted a strong one. Had the first flask remained longer under the receiver, it would no doubt have entirely lost its electricity; while the other without the machine retained that with which it was charged.
V.In this experiment every possible care was taken to observe the variations produced by the vacuum. The receiver which covered the Leyden flask was of a large size, and the flask, being placed in the middle of it, was at a considerable distance from its sides: the receiver was well fitted to the plate of the machine, and not by means of moist leather, as is usual, so that all communication with the external air was prevented; and therefore there is no reason to suppose that any aqueous vapour introduced into the machine, when the pressure of the air was withdrawn, could perform the office of an arc. That the electricity might not immediately bring itself to a state of equilibrium, the conductor of each flask terminated in a ball: had not this been the case, the whole electricity would soon have been dispersed; for, when the conductors terminate in a point, if the chamber be darkened, coruscations of light will every where be seen on the glass receiver, and afford a most agreeable spectacle. A conductor terminating in aball seemed therefore to be most convenient for my purpose, that, by rendering it more difficult for the electricity to bring itself to an equilibrium, I might be able to produce a greater imitation of the intimate manner in which animal electricity adheres to the animals. But though in the above experiments I ascribe some part of the phænomena to the animal electricity being weakened in vacuo, I am of opinion that more is to be ascribed to the violent perturbation of the principles which the vacuum must have excited in the muscular and nervous parts.
VI.As it was now established, that animal electricity could be excited in vacuo, I endeavoured to ascertain whether that excited without the vacuum, and conveyed to the receiver, could be made to pass through a very small space in vacuo. For this purpose I placed the metallic rod at a small distance from a silver plate resting on the bottom of the air pump: the limbs of a chicken or lamb, prepared in the usual manner, were then deposited near it; and the muscles, by means of a metallic chain, were made to communicate with the plate of the machine, while an armed nerve was made to communicate by means of an insulated arc with the metallic rod. The air being exhausted, the rod was pushed down, and gradually brought as near as possible to the plate without coming into actual contact with it. In this state no contractions were produced; but they immediately took place when the rod was brought into contact with the plate. It appears therefore that animal electricity is considerably impededin its progress by a vacuum; and that, like artificial electricity, it does not readily suffer itself to be dissipated, unless transmitted through good conducting bodies. For, when a small quantity of the electric fluid is accumulated in the Leyden flask, either none of it proceeds from the metallic wire to a less perfect conductor, or, if any is transmitted, it must be with difficulty, and with great violence. Common electricity, indeed, is seen to pass quietly through metals or water separately; but a strong electric spark, in proceeding from one metallic conductor to another, if it pass through water interposed between them, does so with such violence, that the glass vessel which contains the water is in danger of being broken to pieces. That electricity, therefore, may be conveyed from a conducting body, to one less endowed with that property, it must be in such abundance as to be able to overcome the resistance of the body through which it has to pass: hence, it need excite no surprise, that the small quantity of electricity which produces contractions should not be able to pass over a very small space in vacuo. Here then we have a proof that animal electricity is not only subject to the laws of non-conducting bodies, but that it is affected different ways by the smallest obstacles of conducting bodies, and by different kinds of them. It is indeed so evident that a vacuum from its nature is unfit for conveying electricity, that, even if one be produced, not by the usual method, which is always attended with some defects, but in the most accurate manner possible, it is totally improper for being a conductorof the electric fluid. This is sufficiently proved by the following experiment of Walsh: If two barometers be joined, and the upper part or bend be carefully deprived of air, when one of these barometers is electrified, the electric fluid will not be communicated to the other, in consequence of the resistance opposed by the intervening vacuum. Adams, however, exhibited by means of a single barometer the same phænomenon as Walsh did with a double one[8]; for, having extracted the air entirely from the upper part of the barometer, no electric light was observed; but on introducing a very small quantity of air the whole barometer became luminous. These observations are sufficient to show that animal electricity, in regard to the property of not being able to pass through a small space in vacuo, is subservient to the general laws of common electricity. But let us proceed to other phænomena respecting animal electricity excited in vacuo.
VII.If a prepared frog, furnished with two armatures,be placed in a horizontal position on a non-conducting body, under a glass receiver, (PlateIV.fig. 11.) and if an arc be formed by pushing down a rod, so as to join both the armatures without touching the frog, contractions will immediately take place; but if the smallest non-conducting body intervene, none will be produced. I found it no very difficult matter to exhibit in vacuo, by an apparatus somewhat similar, what may be called an animal alarum. A horizontal arm fixed to a vertical moveable rod (PlateIV.fig. 5.) was adapted to the inside of a glass receiver, in such a manner that, when the receiver was exhausted, any body resting on the horizontal arm could be made to fall down. The leg of a prepared frog was then fixed to the vertical rod; while the other rested on the horizontal arm, and the spinal marrow, with an armature of tin foil, touched a silver plate in the bottom of the receiver. The horizontal arm being turned round a little, by means of the vertical rod, the leg of the frog resting on the arm fell down on the plate below: an arc being thus formed, contractions immediately took place, and were incessantly repeated, until all the animal electricity was restored to an equilibrium.
VIII.These experiments were made on dead frogs; but I shall here show that living ones also may be made to exhibit signs of electricity under the same circumstances. A piece of tin foil was applied to the back of a frog tied to a silver plate by means of silk strings, (PlateIV.fig. 4.) and two metallic chains were suspended in such a manner, that by pushingdown the rod the extremity of the one chain could be made to touch the silver plate, and the extremity of the other the tin armature: when an arc was by these means formed, contractions instantly took place. The frog began to breathe with difficulty, to be agitated with convulsive movements, and to be seized with an universal tremor, so that its last moment seemed to be approaching; but on air being admitted into the receiver, it recovered and appeared as lively and active as before. On examining more closely the changes which had taken place in the animal while in this state, which was certainly contrary to nature, I found the muscles red with a superabundance of blood: but when dissected in the usual manner, they exhibited strong signs of animal electricity; for, on applying an arc to the nerves or muscles, without any armature, violent contractions were produced, and continued for a long time, provided care was taken that there should be no deficiency of animal moisture. That such a quantity of electricity should be excited, will not seem astonishing to those who have seen more violent electric commotions excited in the animal machine by the action of a needle. This indeed has been placed beyond all doubt by an observation of Gardini, who says “that having made some experiments with a large torpedo, he remarked that stronger convulsions were produced when the animal was subjected to great pain by any means, such as pricking it with a needle[9].” The phænomenaexhibited by a vacuum or rarefied air, were exhibited also by condensed air, so that very powerful contractions were produced by one homogeneous arc. For, if the same apparatus described inPlateIV.fig. 4. be adapted to a condensing machine, a dead frog introduced into it will readily be contracted. Live frogs also, after being kept for half an hour or a whole hour in air twice as dense as that of the atmosphere, exhibited strong signs of electricity without any armature, and merely by the application of a silver arc to the nerves and muscles.
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 (PlateIV.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 immersedat 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 (PlateIV.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 avacuum 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 (PlateIV.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 lesscapacity 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, (PlateIV.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 inPlateIV.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 communicatedthis 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 fallingfluid 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 fluidthere 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 theopinion 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 inhis 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 experimentare 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 thatcorresponding 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, therewas 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.
XXI.Some, perhaps, will object to this analogy, that in the above experiments the flask destitute of electricity forms the arc; while, on the contrary, both flasks ought to be charged to represent properly the muscles and nerves of frogs, which both possess electricity. But the very same phænomenon is observed in two charged flasks, provided one of them is charged with more electricity than the other. Hence, in support of our analogy, we need only assume, that the quantity of electricity in the one frog is a little different from that of the other; a supposition which, in forming an hypothesis, the severest philosopher may allow. I shall say nothing of the great variety and connection of the elements of which animal bodies are composed, and which on this account require a difference in the quantity of the animal electricity. It appears by some late experiments of Valli[13], thatanimal electricity is discharged in the same manner as that which is collected in non-conducting bodies. It is likewise proved, that the same arc and the same armatures, according to the various consensus and connection of the nerves, and according to the different positions in which they are applied, elicit a larger or smaller quantity of electricity, and sometimes none at all. Those indeed who consider these observations will hardly think it possible, when animal Leyden flasks consist of so many different parts, that there should not be some difference in the electricity collected. But there is no need of employing conjecture in regard to a point, which is confirmed not only in regard to animals, but in all conducting bodies, by the ingenious observations of Coulomb[14]; for it is fully established that the electric matter is communicated and accumulated different ways on the surface of conducting bodies. But if this phænomenon takes place in conducting bodies the parts of which are homogeneous, there is no reason to deny that it may appear in animals in which provident nature has so intermixed conducting with non-conducting parts, in order that the action of animal electricity might not be short and transient, but constant and durable.
XXII.But if the diversity in the structure of animalbodies require that the force and power of the animal electricity, collected in the corresponding muscles, should be different, a singular agreement will appear between the phænomena of the Leyden flask and those of animal electricity. If an insulated person touch two flasks containing equal quantities of electricity, however great, he will experience no shock; but if one of the flasks contain a greater charge than the other, he will receive a shock according to the ratio of the difference of the electricity of the flasks. Though the frogs therefore represent two Leyden flasks, one of them may act the part of an arc, and produce an equilibrium, provided it be allowed that there is a difference between the quantities of the electricities collected. I have here endeavoured to establish the proposed analogy, not because I suppose the muscles to be so many Leyden flasks, such as they are exhibited to us by the ingenuity of philosophers, but in order to show that many phænomena are common to both; nor have I applied animal electricity to explain all the phænomena of muscular motion, with a view of obtaining applause from those who are zealous advocates for this theory. I must also observe, that if in the prosecution of this object I have met with any anomalies, I do not on that account despise the agreement of the laws of philosophy which have been established by so much labour. Several of the phænomena observed by Galvani and others have served me as a foundation for the proposed analogy, and induced me to extend, if possible, its boundaries. But going back to theorigin of animal electricity, since it belongs to the subject, I shall here take a general view of the whole, and express the substance of it in a few corollaries.
XXIII.The corollaries I propose will follow the order of time in which they arose, and therefore will express the gradual improvement of animal electricity.
1. Animal electricity passes freely through bodies which possess nearly the same degree of conducting power, but it does not pass through non-conducting bodies.
2. It is affected by the obstacles which occur, not only in non-conducting but in conducting bodies, as well as by their varieties; and if these obstacles be numerous, its passage is stopped: but if it be possible to overcome them, the impediment causes it to make a more powerful effort to attain to a state of equilibrium. Hence, unlike armatures and arcs are of great effect in exciting a moderate degree of electricity, when the same electricity resists the power of a homogeneous metal.
3. Animal electricity obeys the law of equilibrium; for, when the muscles have been brought to a state of equilibrium with the corresponding nerves, no contractions are produced by the application of an arc; but if that which produces the equilibrium be removed, the contractions immediately take place.
4. Poisons, mephitic air, aëriform fluids, and condensed air, do not prevent animal electricity from being excited.
5. The influence of a vacuum on animal electricity is various. In dead animals, if kept a long time in vacuo, the animal electricity is weakened: in living animals it is considerably increased.
6. Though a vacuum does not prevent the electricity from being excited, it will not serve as a conductor of it when it has been excited: if an arc from the nerves to the muscles be intercepted by the smallest vacuum, no contractions take place.
7. As metallic armatures are of great effect in attracting and collecting artificial electricity, the case is the same with animal electricity; but care must be taken not to ascribe to them that electricity which the muscles naturally possess.
8. Though unlike armatures have a great effect in calling forth animal electricity, we have reason to conclude from several experiments, that they do not contain two kinds of electricity capable of producing muscular motion.
9. As natural electricity issues with great force from sharp-pointed bodies, and proceeds to them more readily than to others, the case is the same in regard to animal electricity, as it issues more readily from the pointed parts of the metallic arming applied to the nerves and muscles.
10. The nervo-electric fluid is propagated with that rapidity which is required in restoring to an equilibrium two opposite kinds of electricity.
11. The same conditions which cause two flasks to be discharged when an arc is established from the exterior coating of the one to the interior coating of the other, excite contractions in two frogs, when an arc is formed from the nerve of one to the muscles of the other.
12. The arc applied as above mentioned to the interior coating of two phials, and to the nerves of two frogs, seems to give more force to the proposed analogy; for the electric explosions have a great similarity to the muscular motions excited in the frogs.
I might have enlarged the number of these corollaries, had not the well-known fate of various opinions, now consigned to oblivion, rendered me more timid in hazarding conjectures. I, however, did not allow myself to think that I ought so far to give way to my timidity as to check the spirit of inquiry, or to abandon the hope of one day attaining to the truth. But it would be unreasonable to expect in animal electricity, which is yet in its infancy, that precision and those satisfactory results which can be the work only of time, and of the continued labour of philosophers.
[7]As this term is improperly used by philosophers, I must here observe, that I shall in future understand by it air highly rarefied by the usual means.
[8]In the description of a new air-pump of his invention, where he shows that electricity cannot pass through a vacuum, he adds: “There can be little doubt, from the above experiment, of the non-conducting power of a perfect vacuum; and this fact is still more strongly confirmed by the phænomena, which appear upon the admission of a very minute particle of air into the inside of the gauge. In this case, the whole becomes immediately luminous upon the slightest application of electricity; and a charge takes place, which continues to grow more and more powerful, in proportion as fresh air is admitted, till the density of the conducting medium arrives at its maximum.An Essay on Electricity, explaining the Theory and Practice of that useful Science.Third edit. London, 1787.
[9]Josephi Gardini de Electrici Ignis Natura Dissertatio, Regiæ Scientiarum Academiæ Mantuanæ exhibita, Mantuæ1792,p.100.
[10]This may be accomplished in a much more simple manner, without the aid of a magnet, by connecting a wire with the lower part of the nerves, and applying the wire to the muscles by turning the rod round.
[11]Précis historique et expérimental des Phénomènes électriques, par M. Sigaud de la Fond, Paris1781,sect. i. art.4.
[12]Aloysii Galvani de Viribus Electricitatis in Motu musculari Commentarius, Mutinæ iterum editus,p.29.
[13]M. Valli Cinquième et Huitième Lettre sur l’Electricité Animale, dans Observations sur la Physique, par M. l’Abbé Rozier,tom. xlii.Paris, 1792.
[14]Recherches sur la Distribution de Fluide Electrique entre plusieurs Corps Conducteurs, et la Détermination de la Densité Electrique dans les différens Parties de la Surface de ces Corps.Mem. de l’Acad.Royale des Sciences,An.1788.