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, ([Plate IV.] fig. 4.) and two metallic chains were suspended in such a manner, that by pushing
down 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 phænomena
exhibited 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 in [Plate IV.] 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 ([Plate IV.] fig. 3.), terminating at the upper extremity in a neck, to which could be closely fitted, when necessary, a metallic cover, having a perforation in the centre to receive a moveable rod, which was connected with a transverse metal conductor supporting a frog prepared in the usual manner. Having filled the vessel with water, or mercury, which was still better, I placed it on the shelf of a pneumatic tub, and introduced into it, according to Priestley’s method, any particular gas. Some water or mercury was put into the dish to which it was afterwards removed; and the metal cover, having a frog suspended from it, being then fitted to the neck of the vessel, the frog by means of this apparatus could be immersed
at pleasure in carbonic acid gas, hydrogen, or any other kind of gas; so that the constancy of animal electricity might be tried in either. It is evident that, by letting down the spinal marrow of the frog, and the end of the conductor to the surface of the water or mercury, by means of the rod, an arc will be formed, and that muscular contractions must then be produced. The same experiment may be performed by means of a bottle ([Plate IV.] fig. 6.) furnished with a cover like the former. After being filled with water or mercury, it is inverted on the shelf of the pneumatic tub, and the gas is then introduced in the usual manner; but care must be taken not to displace the whole of the water or mercury, as a portion must be left to cover the bottom when the jar is turned up. This experiment I tried only with oxygen gas, reserving the other kinds of air till a more convenient opportunity.
X. To give more weight to these experiments in vacuo, I endeavoured to prevent all those errors which arose, or might be suspected to arise, from the introduction of air; for I supposed that some opponent of the theory of animal electricity, while endeavouring to find out objections against it, since there was no foundation for asserting that the vacuum afforded any electricity to the animals, might pretend that, the plate of the pneumatic machine being metal, according to custom, some electricity from the atmosphere might be attracted by it, and be thus conveyed even through a vacuum to the animals. On this account it was necessary that a
vacuum should be formed, without placing the receiver on a metallic plate; without the contact of any conducting bodies, and in such a manner as to show that the electricity excited was that really existing in the animals. I therefore employed a glass receiver ([Plate IV.] fig. 9.) cut into two parts above its middle, between which was placed a horizontal partition, in order that the upper part might be filled with oil, or some non-conducting body. The partition was perforated with a large aperture, the superior edges of which projected a little upwards; and the lower, projecting downwards, were furnished with small circular grooves, so that the hole could be shut by a piece of bladder tied over it with thread. A sharp-pointed rod was placed over the hole, so that, being let down by a non-conducting handle, it could be made to pierce the bladder. This, however, was to be done only when the lower part of the receiver was exhausted of air; for the oil, when the air is withdrawn, speedily falls down: by these means a vacuum is formed in the upper part, and, in consequence of the interposed stratum of the oil, remains insulated, as it can receive no electricity from the metallic plate of the machine, nor from other conducting bodies. But it will be in vain to attempt to form a vacuum in this manner, unless the parts of the cut glass be so fitted as to prevent entirely the admission of external air. A very simple and ingenious apparatus, for the same purpose, was invented by F. Borelli: in order to stop the efflux of the oil at pleasure, he adapted to the hole a glass cone, by which means he was enabled to produce an insulated vacuum of greater or less
capacity in the upper part of the receiver. Here then I had a vacuum every where surrounded with non-conducting bodies, so that, if animal electricity were excited in it, there could be no reason for ascribing it to electricity borrowed from the atmosphere.
XI. I adapted to a glass rod, in a longitudinal direction, a metallic plate, ([Plate IV.] fig. 7.) which at the upper extremity was bent into a right angle, and supported the spinal marrow of a frog, so that it might be considered as a lengthened conductor of the nerves. This plate was inserted into a glass receiver, so as to move in it with ease; and by means of a screw could be fixed at any altitude whatever. An accurate representation of this apparatus, with the glass rod annexed, is seen in [Plate IV.] fig. 8. Iron hooks fixed to the feet of the suspended frog acted the part of a conductor to the muscles; and the upper part of the receiver being exhausted of air, as above described, a stratum of oil was still left to separate the frog from the partition. To guard against all danger from the action of any internal metallic body, a strong magnet was applied to the iron conductor of the muscles, which, immediately obeying the power of attraction, fell upon the conductor of the nerves; and thus a circuit of animal electricity from the muscles to the nerves being speedily effected[10], contractions were immediately produced. When I communicated
this experiment to the Institute, I was extremely desirous that the apparatus I had here invented for the purposes of animal electricity might be of some advantage to the science of philosophy in general, of which I was always fond, and which formed the chief object of my study. But it is necessary that I should mention to what I more particularly allude.