Galvani endeavoured to maintain his opinion of an animal electricity in opposition to the Profes sor of Pavia; objecting to him the convulsions ex cited by an arc of a single metal, and trying to vary the circumstances of this case. After having, for example, quickly prepared a frog, as described above, if we throw it immediately into a vessel of mercury well cleaned, so that it may touch the metal with the muscles of its thighs, and with the lumbar nerves, it will commonly exhibit convul sions. Volta replied, that, even in this case, there might have been something heterogeneous in the different parts of the conducting arc, either lying on the surface of the mercury itself, or produced by the contact of the metals employed in preparing the animal. In reality, the slightest differences in the substances employed to form the chain, are sufficient to excite convulsions which were not produced be fore. If we cover, for example, the lumbar nerves of the frog with a coating of impure lead, such as is used by glaziers, and complete the communication with the thighs by an arc of the same metal taken from the same sheet, and consequently of a nature exactly similar, we shall scarcely produce any effects ; but. if we establish the communication with purified lead, such as is used by assayers, the covering of the nerves remaining the same, the convulsions imme diately begin to appear; and it is even sufficient to rub the arc of a single metal against another metal, to communicate to it a nature sufficiently heteroge neous. Galvani, however, did not as yet accede to these remarks. He even carried his doubts so far as to prepare the organs of the frog with thin and sharp-edged plates of glass,—he still obtained con vulsions by an arc of a single metal, but only soon after the death of the animal, and when the irritability was extremely powerful. Lastly, after having pre pared the frog with all those precautions, he succeeded in producing contractions by the mere contact of the muscles of the thighs and of the lumbar nerves of the animal itself, without using any other substance what ever to complete the conducting arc. But if, as Volta said, and as we shall afterwards prove, electricity be developed by the mere contact of two metals, it is possible that it may be developed by the contact of any two heterogeneous substances whatever, such as muscles and nerves. If the action be much feebler than that of metal on metal, it is necessary, in order to show it, to employ an electrometer of still greater sensibility, and such as the organs of the frog appear to be immediately after death. The new fact ob served by Galvani, therefore, although extremely re markable, so far from overturning the idea of Volta, only renders it more general.
It was desirable, however, to establish this idea by substantial proofs, and this Volta has done by a series of experiments, repeated by himself in presence of a Commission of the members of the Institute of France, and which have since been invariably con firmed, as well by the members of this Commission as by a number of other philosophers. As these experiments are much simpler and more easily exe cuted than those treated of in the article GALVANISM, which were made with Nicholson's Doubler; and as the publicity then given to them furnishes a reply to the suspicions which had been thrown out in seve ral esteemed works, we shall here give a short a bridgment of them. • In these experiments Volta employed two metal lic discs, the one of zinc, the other of copper, two, or two and one-half inches in breadth, very plane, not varnished, and having in their centres insulating handles perpendicular to their surfaces, by means of which he could bring them into contact with each other without actually touching any of them with the hand. In this manner he made these discs approach until they touched each other, as in fig. 2, Plate LXXXI. • and then separated them, keeping them parallel as he drew them back ; but as the electricity developed by a single contact is always extremely feeble, he did not immediately try it with the electro meter. He armed it with a little condenser, fig. 3, in
which he accumulated the electricity of several con tacts, by making its upper plate communicate with the ground, and with the metallic disc whose elec tricity he wished to estimate, touching the under plate, which communicated with the leaves of the electrometer. This being done, he drew back the metallic disc, and touched it as well as the other in order to restore them both to their natural state; he then brought them again insulated into contact, again separated them, and applied to the condenser the one under examination. After seven or eight contacts of this kind, on raising the upper plate of the con denser the leaves diverged very much, in consequence of the electricity deposited in the under plate by the successive contacts of the metallic disc ; and he was hence enabled to determine the nature of this electricity.
In the case, for example, of the two discs of cop per or zinc, if it is the copper disc which touches the under plate of the electrometer, the electricity which makes the leaves diverge is resinous; if we touch it with the zinc it is vitreous : thus these two metals, insulated and in the natural state, are brought by their simple contaet into different electri cal states; the zinc acquiring an excess of vitreous electricity, and the copper the complementary ex cess of resinous electricity. For the success of the experiment, it is useful, that the metallic surface of the condenser be covered, at the point of con tact, with a small leaf of paper, to prevent any new contact of metal with metal on the condensing plates.
We may repeat this experiment in a different manner. Instead of making one of the plates of the condenser communicate with the ground, leave them both insulated upon the electrometer ; but every time that -the two discs of contact are separated, touch with each of them, and always with the same, each of the plates of the condenser, covered, at the point of contact, with a leaf of paper. As the free electricities which they possess are of a contrary na ture, they will mutually attract each other, and at., tad; themselves to the contiguous surfaces of the plates. After several contacts of this kind, separate the plates, and each of them will be found charged with that species of electricity belonging to the plate with which it was touched.
It might be imagined that the electricity which is produced in these circumstances, is owing to a sort of compression of the plates against each other, like that which arises when we press gummed taffeta against a metallic plate. But it is easy to prove that the action which is developed during the contact of metals is quite of a different kind, and is excited by a reciprocal influence which decomposes their natural electricities. To establish this capital fact, Volta made the following experiment : He formed a thin metal lic plate with two pieces, C, Z, fig. 4, the one Z of zinc, the other C of copper, soldered end to end ; then taking between the fingers the extremity Z, composed of zinc, he touched with the copper extre mity the upper plate of a condenser, which is also of copper, and the under plate of which communi cated with the ground. After the contact, the plate, which has been touched, was found to be electrified resinously. This is entirely conformable to the pre ceding experiments; only we need not here appre hend any compression or any separation between the mollecules of zinc and those of copper, since their juxtaposition is permanently established, and the contact upon the condenser takes place between copper and copper, which cannot develope any new electricity : in order that the electricity thus pro duced by a single contact may be very apparent, the condenser must be much larger than that of the electrometer, and its condensing power consider able.