chalk when pounded with a hammer. May it not be that the light thus disengaged, indicates, when it is sudden, a decomposition of the natural electricities? For example, when we separate rapidly in the dark, the leaves of a piece of Siberian mica the extremities of which have been previously fixed to nonconducting rods, a vivid bluish flash of light is seen upon the separating surfaces. Now if we present these surfaces to the electroscope after their separation, it is found, as was observed by M. Becquerel, that one is electrified vitreously, and the other resinously. Why may it not be the same in other cases of violent separation? Quantities of electricity too small to be appreciated by our best electroscopes, are yet perhaps capable of disengaging by their developement a visible light.

The account which I have now given of these various experiments, shows that the developement of the electrical principles is still but imperfectly understood; but we must, at the same time, perceive that it affords one of the finest subjects of physical enquiry.

Of the Developement of Electricity by simple Contact.

114. We now proceed to consider the developement of electricity by simple contact. This branch of Natural Philosophy, which dates only about thirty years back, presents the contrast of a great discovery, resulting from an accident, and of one still greater, made directly and carried out by the most rigorous inductions and experiments.

It was about the year 1789 that the first phenomena of this sort presented themselves. Galvani, professor of Natural Philosophy at Bologna, instituted some inquiries on the excitabili ty of the muscular organs by electricity. He employed in his experiments frogs recently killed and skinned, of which he divided the spine in order to insulate and lay bare the lumbar Fig. 50. nerves. That he might manage them conveniently, he introduced into the remaining part E of the spine, a copper wire bent in the form of a hook. It accidently happened one day that several frogs were suspended by these copper hooks from the

iron balcony of a terrace; at that instant their feet and legs, which also lay in part upon the iron, became spontaneously convulsed; and the effect was the same at every new contact. Galvani perceived the importance of this phenomenon, and set himself to determine its essential circumstances. He saw, in the first place, that instead of holding the frog by the hand, it might be laid on an iron plate, and that applying to this plate the copper hook, the convulsions still took place. He next perceived that the whole was reduced to the establishing a communication between the muscles and nerves of the frog by a metallic arc. He observed that the convulsions still took place, when this arc was of a single metal, but that they were then very rare and very feeble, and that to render them strong and permanent, it was necessary to employ two different metals in contact. This condition being fulfilled, the communication might be completed by any substances whatever provided they were conductors of electricity. He introduced into the chain of communication other animal substances, and even living persons who held each other by the hand; convulsions still took place. Now Galvani had recently observed, that the electricity developed by the ordinary methods produced similar effects upon the organs of frogs, when they were exposed to its influence. A most evident analogy seemed therefore to lead directly to the conjecture that the convulsions produced by the contact of the heterogeneous metals were also the effect of some electrical current which this contact developed. Nevertheless he did not draw from it this simple conclusion; he thought he saw in it the extraordinary effect of a new source of electricity, which he called animal electricity, and which, existing originally in the muscles and nerves, circulated when these parts were placed in communication by a metallic arc, or by any good electrical conductor. Galvani vainly attempted to compare this action to that of the Leyden jar; but on looking at the work itself in which this hypothesis is advanced, it is apparent that he was not acquainted with the true theory of electrical influence, and that, explaining the circumstance in this way, he was led to adopt theories

De Viribus Electricitatis in Motu Musculari Commentarius.

that had little of reason or ingenuity to recommend them. We are thus compelled the more to admire the rare sagacity and true genius by which he seized, as if by divination, and varied with so much skill, the extraordinary phenomenon of the seemingly spontaneous convulsions which he had accidently observed.

When these new facts were made known in Italy, they excited general admiration, and all were inclined to favor the views. of Galvani. But the celebrated Volta of Pavia had no sooner repeated the experiments, than he drew from them altogether different conclusions; and it may be said that accident itself, by making known these phenomena subsequently to the sensible effects of artificial electrical influence, had thus sought to indicate their true source. Therefore Volta had no doubts with respect to its nature. Conceiving that the cause of these motions, whatever it was, must be very subtile, since they were produced independently even of the will of the observer, he set himself to determine by exact experiments the precise quantity of electricity necessary to excite convulsions in the organs of frogs, by causing a discharge to pass through them. He thus discovered that this quantity was exceedingly minute, and scarcely sufficient to produce a sensible divergence in the straws of the delicate electroscope which he made use of. This result being obtained, he compared it with the other fact established by the experiments of Galvani himself, that the contact of two or more heterogeneous metals was, or at least thus far appeared to be, necessary to excite the convulsions; and he hence drew this conclusion, that the mere contact of the heterogeneous metals was the unperceived circumstance, which caused the sudden developement of electricity. In following out this truly fundamental idea, Volta collected under one point of view all the experiments hitherto made by Galvani, and he pointed out the means of reproducing the same effects in a certain manner, and with the highest degree of energy. In making use of different metals, he observed that the best was zinc placed in contact with silver or copper, although the convulsions might also be produced by an arc composed of any two metals whatever.

115. From the preceding observations, we infer that the best preparation for repeating the experiments of Galvani is the following. Take a frog and separate the hind legs and a part of

the spine; next remove the flesh and all the parts which cover the lumbar nerves, denoted by NN. Then enclose these nerves in a small strip of copper or zinc; place the frog, thus prepared, upon a nonconducting support, for instance, upon a pane of glass varnished with gum lac; and, taking a piece of any other metal, bent into the form of an arc of a circle, place one of its extremities upon the armature of the nerves, and the other upon the muscle of the thighs; the convulsions will immediately take place, not only in the leg which has been touched, but also in the other. The frog retains its susceptibility of these motions some time after death; and it retains it the longer according as it has been less excited. When beginning to decline, it may be restored by the application of such stimulants as tend to increase animal irritability. The same is to be observed also with respect to the convulsions which are produced in the organs of frogs by the influence at a distance of common electricity; and the only conclusion to be drawn from all we have said, is, that these organs, when fresh, sensibly indicate the smallest discharges of electricity.

116. Guided by the fundamental idea which thus revealed the secret of this kind of action, Volta ascribed to the same cause several phenomena of sensation, which had not as yet been attended to, doubtless because they stood alone, but which, when accurately examined, are found to refer themselves, in the most evident manner, to the action of several metals in mutual contact. For example, he recalled to mind an experiment described in an old work, entitled, Theory of Pleasure, and which is extremely well adapted to show this influence. Take two pieces of different metals, one of silver or copper, and the other of zinc, for instance. Place one of these pieces above, and the other below the tongue, in such a manner that they may project a little beyond the tip of the organ. As long as the pieces are separated from each other by the tongue, no effect is produced. But when they are made to touch each other, a peculiar taste is perceived very much resembling that of the sul phate of iron. Here, according to Volta, electricity is developed by the mutual contact of the two pieces; and the surface of the tongue, which is covered with nervous papillæ of an extraordinary sensibility, serves as a conductor. Sometimes, E. & M.

17

Fig. 59.

also, the excitation is transmitted to other nerves; and if the person is in the dark, he perceives a flash of light in his eyes. All the sensible parts of animals are capable of being affected by such an arrangement. This susceptibility has become in anatomy the certain and delicate means of discovering the most subtile nervous fibres in different parts of the organs of animals.

117. Galvani endeavoured to support his hypothesis of an animal electricity in opposition to the Pavian professor; he urged as an objection to the theory of the latter, the convulsions excited by an arc of a single metal, and endeavoured to vary the circumstances of this experiment. For instance, after a frog is quickly prepared in the manner we have just described, if it be immediately laid upon a bath of very pure mercury in such a way as to form a communication between the nerves and muscle, convulsions are usually exhibited. Volta answered that, even in this case, there might be some heterogeneity in different parts of the conducting arc, either upon the surface of the mercury or by the contact of the metals, used in preparing the animal. Indeed the smallest difference in the substances employed to form the communication is sufficient to cause convulsions, when they do not take place without this difference. For example, if we arm the nerves of a frog with a sheet of impure lead, such as is made use of by glaziers, and then complete the communication by an arc of the same metal, taken from the same leaf, and consequently of an exactly similar nature, effects are rarely produced. But if we complete the communication with purified lead such as assayers use, the armature remaining the same, convulsions will immediately take place; and it is only necessary to rub the arc of a single metal with another metal in order to make it sufficiently heterogeneous, as has been shown by M. Halle. Nevertheless, Galvani did not yield to these arguments; he carried his precautions so far as to prepare the organs of the frog with plates of glass, wrought into the shape of a knife. He still obtained convulsions with an arc of a single metal, but only in the case which we have mentioned, that is, when the animal is very fresh, and in an extremely irritable state. Finally, after having prepared the frog with all this care, he succeeded in producing the contractions by the mere contact of the muscles and nerves of the animal itself, without em