XII. There is no reason, therefore, in this case, for ascribing the contractions either to the arc or to the armatures, which, as they consist of mercury alone, cannot produce the two kinds of electricity necessary for exciting contractions. But even if we should allow, with those who form the most absurd suppositions, that mercury alone possesses both kinds of electricity, one contraction only could be expected, and not several in succession. In a word, there is no reason to apprehend that any external electricity is obtained either from the glass vessel which receives the mercury, or from the surrounding atmosphere, which is separated from the spinal marrow by three strata of non-conducting bodies; namely, air, glass, and oil. The simplicity of this process may not be fully apparent to the reader; but I can with truth assert, that simplicity was an object which I had always in view. Having prepared a frog, I laid it to float in mercury immersed in oil, and then endeavoured to excite contractions by the application of an insulated arc. Owing to some inaccuracy in the experiment, however, it did not succeed; and my attachment to simplicity, while endeavouring to discover the
least complex method of exciting contractions, was the cause of my not obtaining the desired result. This want of success was perhaps owing to the mercury not being in proper contact with the nerves, in consequence of the oil adhering to them. That I might exclude all suspicion of atmospheric air having any share in the phænomenon, I was obliged to adopt that method of performing the experiment which I have already described.
XIII. I however readily foresaw, that the advocates of external metallic electricity might object, that no contractions were obtained but by the application of armatures, or, when armatures were excluded, by using in their stead an arc, which is itself an armature. And I must indeed acknowledge that we are as yet acquainted with no substances but metals capable of exciting animal electricity, though nature, so abundant in resources, may no doubt furnish a great many[4]. But I shall here observe that metals are not the cause of the contractions produced in animal bodies, but merely a condition requisite for calling forth the latent innate animal electricity which exists in them. For though armatures are necessary to render non-conducting bodies electric, there is no reason why the shock given by the Leyden flask should be ascribed to the arc or to the armatures; as a charged magic square, or a Leyden flask,
when freed from the armatures, exhibits a great quantity of electricity. If you charge a Leyden flask filled with water, pour the water from it, then pour in other water, and form a communication by an arc between the opposite surfaces, you will experience a shock. This very simple experiment agrees with some made by Mr. Wilson, Mr. Cavallo, and others. Armatures, therefore, have a powerful effect in attracting electricity, and confining it in non-conducting bodies; but they do not supply electricity themselves.
XIV. If this mode of reasoning be admitted in regard to common electricity, it ought not to be rejected when applied to the phænomena of animal electricity. For, as an explosion is produced by a magic square, or Leyden flask, even when the armatures are removed, the case is the same in regard to the contractions in animals. But to prove in a more evident manner that metals have no action in such cases, the following experiment was made. An insulated person, holding in his hand a metallic arc, and a prepared frog, furnished with heterogeneous armatures, was electrified, as well as every thing about him, by means of an electrical machine. The person, the frog, the armatures, and the arc, being all electrified positively, none of the metals employed in this experiment, as their electricity was reduced to a state of equilibrium, could produce contractions. As these are produced only by applying the arc to the armatures, they cannot be ascribed to external electricity, but to the innate electricity of the frog. If only one of the armatures be touched, no contractions take
place; but if the arc be applied to both, they are immediately produced; which is a strong proof that the arc possesses the power of exciting the electricity inherent in animal bodies. Those philosophers would reason very incongruously who should ascribe the cause of these contractions to electricity communicated from the person, since they would thus allow to the person that animal electricity which they deny to the frog; a conclusion which few will admit. But in attempting to remove every suspicion of communicated electricity, it was necessary that the experiments undertaken should be free from all influence of artificial electricity.
XV. I therefore endeavoured to obtain an equilibrium in the armatures by mutual contact. This simple method was borrowed from the principles adopted by philosophers, who, while they endeavour to produce an equilibrium, are accustomed to apply a body electric by excess to one in a contrary state. Immerse in water the spinal marrow of a prepared frog, without armature, and let the muscles rest on a non-conducting body. The vessel must be somewhat in the form of a syphon, and the spinal marrow introduced into the smaller branch must not float on the liquid, but be totally immersed in it: this arrangement is of great importance in regard to the success of the experiment: a small piece of tin foil must be made to float on the surface of the water at a considerable distance from the spinal marrow: if you then touch the muscles with one of your hands moistened a little with water, and with the other apply a silver arc to the tin
foil, a contraction will immediately take place. While struck with the constancy of this phænomenon, I could not help reasoning in the following manner: Muscular motion is produced though the frog is in contact with no metallic body: every thing metallic is separated from the frog; and even if it possessed contrary kinds of electricity, they are in a state of equilibrium. What then is the external agent which produces contractions in the frog? Though the metals, while they acquire an equilibrium, come into contact, which is followed by contractions, there certainly can be no fear of external electricity from them. This is sufficiently proved by the metals themselves being brought into equilibrium before the contractions take place. If you touch the muscles with one hand moistened with water, and with the other immerse into water a piece of gold coin, a small part of which is covered with tin foil, contractions will immediately ensue. Yet both the metals, before the contraction, were brought to a state of equilibrium by being in contact: they can therefore have no share in the contractions, which must arise from innate electricity. If salt water, milk, serum, or the crassamentum of the blood, be substituted for common water, there will still be contractions; and the case will be the same if a bit of gold, silver, or brass, or even iron, covered with tin foil be employed. Here then we have tin brought into a state of equilibrium with various metals before the contractions take place, without these contractions being impeded. Nor is it of any consequence that the metals be touched with
the hand. For, if the tin foil be applied to the tip of the tongue, or to the lips, and if a piece of silver wire be made to touch the tin foil on the one side, and the spinal marrow covered by no metal on the other, contractions will be produced as often as an arc is made to the muscles by means of the moistened hand.
XVI. The object of our researches hitherto has been muscular motion; we shall now direct them to the phænomena of the senses. Let an insulated person be electrified by means of a common electrical machine, and let him apply a silver arc to the tip of his tongue covered with tin foil. The armature and the arc will both become electrified by this new accession of electric matter. We cannot therefore suppose one armature to be positively and the other negatively so; and for this reason no transmission of electricity, and no sensation of taste, can be expected. As the tongue, however, experiences a sensation of acidity, it must have been excited not by the armatures or by the arc, but by the innate electricity of the muscles and nerves. But the necessity of an internal arc, which appears in the above phænomena, is an argument in favour of innate electricity. In order, however, to excite the taste by animal electricity, the application of different metals to the tongue is not sufficient: the arc must be conveyed to the muscles of the tongue, or to others at a distance from it. When I first made this experiment, it gave me no small pleasure to find that it was confirmed