been carried on relative to the two hypotheses, as to the origin or cause of the development of the electricity in the galvanic apparatus, viz: whether it is due to the contact of the metals or to the chemical action of the acid on one of them. But it must be evident to those who have paid attention to the history of this branch of science that justice cannot be done to this interesting discussion in a few pages of this report, and that the author has merely given a brief sketch of only one of the hypotheses; but since this is comparatively little known, except in Germany, it will be acceptable to the English reader.]

This discussion has been carried on with no little warmth; but the history of science shows that when a theory is properly established, controversy in reference to it ceases. If any one, at this time, should assert that the earth does not revolve about the sun, astronomers would give themselves little trouble to refute the objection which might be urged against the received theory. Drieberg recently attacked the physical theory of the pressure of the air, but his opinions have not produced the least excitement among physicists. Opposition provokes discussion only when theories have not risen above mere hypothesis, and this is partly the case with reference to the source of the electricity of the galvanic circuit.

The matter in dispute is not fully ripe for decision, and we can only expect a perfect solution of the difficulty when we are better informed of the nature of electricity itself. In Euler's time the theory of the vibrations of light was advocated with much ability, yet this distinguished mathematician was unable to render it generally acceptable, and it was only by the discovery of new facts, particularly those of polarization, that the theory received that form which silenced opposition. The explanation of the origin of the electricity of the pile must rest on the theory of the molecular constitution of matter in relation to the ethereal medium, the existence of which we are obliged to admit in order to generalize the facts of light, heat, and other emanations from the sun. The establishment of a general theory of this kind which will give definite conceptions of the relation of known phenomena, and lead us to infer the existence of facts of which we have as yet no idea, is one of the most important objects of science, and even the attempts which have been. made to arrive at a general view of this kind have been fruitful in new and interesting results.

The materials, however, for the full establishment of such a theory do not at present exist, and consequently we cannot expect more than approximations to a generalization of the character required.

§ 1. Brief sketch of the theories.-Volta found that when a slip of zinc and one of copper were soldered end to end, the one exhibited signs of plus, and the other of negative electricity. He therefore concluded that the electricity was due to the contact of the two metals, and that the acid of the circuit only performed the office of a conductor. This view was at first generally adopted, but as the phenomena came to be more minutely studied, it was found insufficient to explain them, and Wollaston, Davy, and others, adopted the hypothesis that the electricity was due to the chemical action of the acid

on one of the metals. It has been shown that a galvanic current can be produced by the action of two liquids without metallic contact, and therefore the theory of contact requires to be so modified as to extend the idea of contact to that of the liquids as well as the solids of the galvanic combination. On the other hand, it has never been fully proved that the contact of two metals does not in itself produce a disturbance of the electrical equilibrium, though this effect does not appear sufficient to account for the great amount of electricity evolved in the action of the battery. The two theories, properly modified, approximate each other, and each, perhaps, involves elements of truth.

The hypothesis, that the development of electricity is only the consequence of chemical action-that without chemical decomposition of the electrolyte no electricity can appear in the circuit, is that against which the attacks of the advocates of the contact theory were directed; and it is, indeed, opposed to a great number of facts. The chemical theory, in this form, ignores completely the fundamental experiment of Volta; it does not explain how the tension of electricity of the open pile increases with the number of plates. But what is most inconsistent with the maintenance of this theory, is the circumstance that a number of galvanic circuits can be constructed in which, when open, not a trace of chemical decomposition takes place, but which, nevertheless, give rise to currents when they are closed.

Schönbein, in a memoir "On the cause of the hydro-electric current, in his "Beitragen zur Physicalischen Chemie-(Basel, 1844,") has referred to several such circuits. A solution of perfectly neutral sulphate of zinc does not attack zinc; yet a combination of zinc and copper in this solution produces a current.

Another weighty objection to the form of the chemical theory, which attributes the formation of the current to a preceding chemical attack upon one of the metals of the circuit, is, that the electromotive force of a circuit is not at all proportional to the violence of the attack. If the copper of a Daniells' battery be placed in a solution of sulphate of copper, the electro-motive force of the apparatus is almost wholly unchanged, whether the zinc is placed in water, dilute sulphuric acid, or in a neutral solution of sulphate of zinc. This has been proved by Svanberg, among others, by accurate measurements. (Pogg. Ann., LXXIII, 290.) If the current had its origin in chemical action, the electro-motive force should be far greater upon application of dilute acid than of water and sulphate of zinc.

It is a fact, that the current of the water-battery (hydro-kette) cannot circulate without decomposition of the liquid. The decomposition appears essentially connected with the passage of the electricity through the liquid, and the contact theory has fully acknowledged the important part which chemical decomposition in the cells plays in the formation of the current. A dispute as to whether decomposition is the cause of the electrical current, or whether the chemical decomposition in the battery is preceded by a state of electric tension, the source of which we need not at present ask, is the same as though there should be a controversy as to whether the motion of a waterwheel is owing to the fall of water or the weight of water. The weight occasions the fall, and the fall the revolution of the wheel, just

as the electric tension occasions chemical decomposition, in consequence of which the current circulates. Even Faraday, who is prominent in maintaining chemical decomposition as the source of the electrical current, concedes that decomposition is preceded by a state of tension of the liquid; for he says, in the case where he applies his theory of induction to electrolytic decomposition:

"The theory assumes that the particles of the dielectric (now an electrolyte) are, in the first instance, brought, by ordinary inductive action, into a polarized state, and raised to a certain degree of tension or intensity before discharge commences; the inductive state being, in fact, a necessary preliminary to discharge. By taking advantage of these circumstances, which bear upon the point, it is not difficult to increase the tension indicative of this state of induction, and so make the state itself more evident. Thus, if distilled water be employed, and a long, narrow portion of it placed between the electrodes of a powerful voltaic battery, we have at once indications of the intensity which can be sustained at these electrodes, * * * for sparks may be obtained, gold leaves diverged, and Leyden bottles charged.". Twelfth Series of Experimental Researches on Electricity, 1345.

Thus Faraday himself concedes that a polarized state precedes decomposition of the electrolyte in the separate cells of the battery, consequently it precedes the formation of the current. The difference between Faraday's theory of the pile, and the contact theory, is not tɔ be found in the fact of deriving the circulation of the current from chemical decomposition in the cells. The contact theory supposes, with Faraday, that in the water-battery (hydro-kette) the formation of the current is the consequence of chemical decomposition in the cells. It also supposes, with Faraday, that this decomposition must be preceded by a state of tension; and it is only in reference to the cause of this tension, which is nothing else than the electro-motive force, that there can be any difference of opinion.

§ 2. Schönbein's chemical theory.-Schönbein has attempted 30 to modify the propositions of the two theories as to bring them more in harmony. The following are the principal features of his theory, extracted from his own paper:

"Whatever may be the cause or force by which elementary substances are enabled to unite together into an apparently homogeneous body, and to continue in their new combination, this much is certain-that a change must always take place in their condition if a third element is brought into contact with one of the substances, which exercises a perceptible chemical attractive force upon the other components of the compound. To illustrate our idea, let us select water as an example. Oxygen and hydrogen are held together in this compound with a given force; or, to express the same thing in other words, the chemical attractive forces of the elements of water are in a state of equilibrium. An oxidable substance, as zinc, being now brought into contact with water, it will have a chemical attraction of a certain intensity for the oxygen of the water. But in consequence of this attraction, the chemical relation which subsisted between the oxygen and hydrogen before the presence of the zinc must be changed, or the state of the original chemical equilibrium of these

elements is modified in a certain degree or destroyed; or, in other words, under the circumstances mentioned, the oxygen in each particle of water will be attracted in two opposite directions-towards the zinc in contact with the molecule of water, and also towards the particle of hydrogen contained in this molecule.

"Now, since the least mechanical molecular change taking place in a body disturbs its electrical equilibrium, or its particles become electrically polarized, the above described change, caused by the zinc in the original chemical affinity of the oxygen for the hydrogen of the water, is followed by the electrical polarization of the substances in contact with each other. The particle of zinc nearest the water becomes positive; the oxygen side of the molecule of water touching the zinc is negatively polarized; the hydrogen side of the same particle, positively. It is self-evident that the particle of water in contact with the zinc will exert an inductive action on its adjoining molecules, the latter upon the next particles, and so on, until all the molecules of water connected together are in the state of electrical opposition or polarization. Since an inductive action traverses the particles of water from the place where the zinc and water are in immediate contact, all the contiguous particles of zinc become polarized, and in such a manner that the side of each particle turned from the water indicates negative polarity, and the side towards the water positive polarity. By placing in this polarized water a good conductor or a substance easily electrified, which is indifferent towards the oxygen of the water, such as platinum, the sides of the particles of this substance in immediate contact with the water become negatively electrified, and the sides of the same particles turned away from the water positively in consequence of an inductive action, which is exerted by the polarized water upon the platinum.

"All the other particles of the platinum are similarly affected, that is, the side of each molecule turned from the water has positive polarity; that of each towards the water has negative.

"The following diagram gives a clear representation of the electrical condition in which the particles of zinc, water, and platinum are found:

Fig. 1.

"It is very evident that this condition of all the particles of the substance in question will last as long as the cause producing the polarization exists; that is, as long as the chemical attraction of the zinc for the oxygen of the water continues. But if the contact of the zinc and water be broken, the opposite electrical conditions in which the hydrogen and oxygen of each molecule of water exist are neutralized, which is necessarily followed by a like change

"Now, by placing the particle Z' of the arrangement in contact

with P1, the negative side of the former will be in connexion with the positive side of the latter, and the opposite states of the two particles will mutually neutralize each other. But at the same moment in which the equilibrium takes place in these particles, it takes place between each two contiguous particles throughout the whole circuit; consequently between the positive side of a particle of zinc in contact with the water and the negative oxygen particle of a molecule of water in contact with the zinc. Likewise the electro-negative state of a particle of platinum is in equilibrium with the positive state of the oxygen particle of the water molecule with which it is in contact. "The electrical equilibrium which now takes place between each metallic particle and each component of a molecule of water is not possible without a decomposition of the latter, and this very act of equilibrium must be considered as the true and ultimate cause of the electrical decomposition of water.

"Evidently, according to this view, the actual combination of the oxygen with the zinc of the battery is regarded as only a secondary action of the current or the act of electrical equilibrium, and not as the cause or source of the current itself. The chemical combination of the molecules of oxygen and zinc being completed, and a substance being in the water which can remove the oxide of zinc from its place of formation, a new particle of zinc will come in contact with a molecule of water, and the latter, with all the particles of oxygen lying between the zinc and platinum, will be electrically polarized anew. By keeping the circuit closed, a neutralization of the electrical opposition will take place between each two contiguous particles of the voltaic battery, and the decomposition of new molecules of water follows; and thus proceeds polarizing and depolarizing, circulation and electrolysis, until the necessary conditions cease to be fulfilled.

"Suppose now that water is placed between two metals which manifest an exactly equal attraction for oxygen; it is evident that it will be drawn with equal force, under these circumstances, in opposite directions hence the effects upon the particles of water by the metals must be mutually destroyed; the components of these molecules will not be polarized; and in closing such a circuit, neither circulation nor electrolytic action can take place.

But if the water be placed between two metals, one of which has greater affinity for oxygen than the other, the chemical equilibrium existing between the components of each molecule of water will be destroyed, and in proportion to the difference of oxidability of the metals used.

Since the destruction of the chemical equilibrium between the components of the particles of water also involves the destruction of electrical equilibrium, and the latter is as much more considerable as the former is greater, it follows, that the degree of electrical polarization of the molecules of water between metals must be proportional to the difference of oxidability of the said metals; or, to express the same thing differently: the magnitude of the electrical tension which the parts of an open circuit have for each other, is measured by the