experiments. 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 exhibited
strong 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 a
ball 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 impeded
in 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 conductor
of 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, ([Plate IV.] 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 ([Plate IV.] 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.