straight line with it. When one of the servants, who was at hand, touched with the point of the dissecting knife the inner lumbar nerve (D D) of the frog, all the muscles of the thighs appeared to contract as if under the influence of powerful cramps. The assistant thought that the phenomenon occurred when a spark passed between the conductors of the electrical machine. Astonished by this new phenomenon, he turned to me, I being occupied in other matters and absorbed in thought. Thereupon I was inflamed by an incredible haste and desire to prove the same, and bring the hidden mystery to light."*

Here, with extreme candidness and generosity, he gives full credit to his assistants for the accidental discovery. He traced the cause of the strange convulsions to the working of the electrical machine, and found that discharges of lightning could also produce the movements of the muscles of the frog's legs. He continues thus:

"After I had investigated the effects of atmospheric electricity my heart burned with desire to test the power of the daily quiet charge of electricity in the atmosphere." He had noticed that the prepared frog's legs, bound up with brass hooks on an iron railing, showed the same contractions not only in the case of thunderstorms, but also under a clear sky. He was thereupon led to study the effect of touching the nerves with different metals and with non-conductors, such as glass and wax. He found that when the circuit between the nerves was made by two different metals powerful contractions ensued.

* De Bononiensi Scientiarum et Artium Instituto atque Academia Commentarii, tomus vii.

In Fig. 6 we see a representation of the various experiments which he tried. The twitching of the frog's legs served him for a galvanometer-a sensitive indicator of the electrical current which was excited in the circuit of the metals and the muscles and nerves of the frog. A new instrument in physical science often opens a great field of discovery. The frog's legs in the hands of Galvani and his co-workers proved to be such an in

strument.

It was not a difficult step to take from the standpoint of Galvani to that of Volta. The instrument was at hand and the phenomenon had been observed. Galvani attributed the action to the vital electricity of the nerves and the muscles of the frog, while Volta attributed the action entirely to the contact action at the junction between the two metals. The controversy as to the cause and seat of the electro-motive force between two metals in an ordinary battery is still a matter of dispute, and we are little wiser than philosophers were in the days of Galvani and Volta. The labours of these two men, however, opened, as we have said, a great field in electricity. Men began to study the manifold phenomena of the electric current produced by batteries, and the next great step was made by Oersted, who discovered the principle of a new instrument, the galvanometer, the indications of which have led the way to the great practical employment of electricity.

Galvani, we have said, attributed the source of the electrical current which convulsed the frog's leg to the animal electricity of the frog. Volta, however, soon showed that an electrical effect could be obtained without the use of the frog's leg by connecting a piece of zinc to a piece of copper, and he was led to the invention of the voltaic pile, which in its first form consisted

merely of alternate discs of copper and zinc separated by pieces of blotting paper moistened with salt and water. Volta attributed the effect observed by Galvani to the contact between the two metals which were employed to touch the frog's legs. In an ordinary battery the plates forming the battery are not in contact, but are separated by a layer of liquid, which acts chemically upon one of the plates. The question of the seat of the electro-motive force in a voltaic cell is as much a mystery to-day as it was in the times of Galvani and Volta, but we are beginning to see that our best hope of solving the mystery consists in studying the transformations of energy in the battery, and in measuring the heat developed under different conditions. The chemical action is an evidence of an increased molecular activity, so to speak, and in general terms we can conclude that whenever in a circuit we have a difference of molecular activity, at two points in the circuit an electrical circuit results. The phenomena which takes place in a battery consisting of two metals with a liquid between the two metals appear to be far more complicated than those which are manifested along the wire connecting the metals or along the outer circuit. The liquid is broken up. If it is acidulated water, oxygen is given off at the positive pole and hydrogen at the negative.

It can be said, in general, that an electrical current is generated whenever two dissimilar metals connected by a wire are immersed in a liquid which is capable of conducting electricity. For instance, if the handles of a silver spoon and an iron spoon are connected by a copper wire, and the bowls of the spoon are immersed in a tumbler of salt and water, an electric current passes from the silver to the iron along the copper wire, and in the water from the iron spoon to the silver spoon.

Such a battery would be sufficient to send a signal under the Atlantic from America to England, but it would not be powerful enough for commercial use. Before the iron spoon is connected with the silver spoon by a wire, very delicate instruments will show a negative charge of electricity upon the iron spoon, and a positive charge upon the silver spoon. By the use of a great number of zinc and copper plates separated by paper moistened with salt and water, Volta was able to show that a body charged with positive electricity was repelled by the terminal connected to the copper plates, and attracted by that connected to the zinc plates. Indeed, by greatly increasing the number of tumblers containing plates of copper and zinc immersed in salt and water, and connecting each copper plate to each zinc plate by a wire, one can obtain electrical sparks when the final copper plate is brought near to the first zinc plate.

Since any two metals immersed in a suitable conducting liquid constitutes a voltaic cell or battery, it will be readily seen that the number of forms of such batteries is very great. The Leclanché cell, which is so commonly employed at present in houses to supply the electric current for bells, consists mainly of a rod of zinc and a rod of carbon, both immersed in a solution of sal ammoniac. When Prof. Tyndall came to America to deliver lectures on physical science, he brought with him a hundred Grove cells, which consist of platinum plates immersed in strong nitric acid which is contained in a porous cup; this cup is placed in another receptacle filled with sulphuric acid and water, in which there is a zinc plate. The electrical current flows from the platinum to the zine outside the cell, and from the zinc to the platinum inside the cell. The cells are joined, as we have said, in series, zincs to platinums, and finally