trified in one of the gold cones, had soda detatched from it, and sustained a loss of weight.

It may now be understood, why, by the agency of galvanism on water, alkali appears at the negative and acid at the positive wire. The fact was, for some time, not a little perplexing to Sir H. Davy; till, at length, he ascertained that all water, however carefully distilled, contains neutral salts in a state of solution. From these impurities, the alkaline and acid elements are separated, agreeably to a law which has already been explained. In the same way, also, the muriatic acid and alkali are accounted for, which some chemists have obtained by galvanizing what was before considered as pure water; a fact which has been urged in proof of the synthetical production of both those bodies. Absolutely pure water, it has been demonstrated by Sir H. Davy, yields nothing but hydrogen and oxygen gases.

All the effects of galvanic arrangements, in producing chemical decompositions, it has been found, may be obtained by ordinary electricity. Its adaptation to the purpose was first successfully attempted by Dr. Wollaston*. The appratus, which he employed, was similar to that already represented (fig. 77, a), excepting that the wires, instead of being exposed to the fluid, contained in the tube, throughout their whole length, were covered with wax, and the points only were laid bare. Or (what was found to answer still better) the wires were inclosed in capillary tubes, which were sealed at their extremities, and then ground away, till the points alone were exposed. The conducting wires, thus arranged, were then introduced into a tube, or other vessel containing the liquid to be operated on, and were connected, the one with the positive, the other with the negative, conductor of an electrical machine, disposed for positive and negative electricityt. When solution of sulphate of copper was thus electrized, the metal was revived round the negative pole. On reversing the apparatus, the copper was re-dissolved, and appeared again at the other wire, now rendered negative.

When gold wires from too of an inch in diameter, thus inclosed, were made to transmit electricity, a succession of sparks afforded a current of gas from water. When a solution of gold in nitro-muriatic acid was passed through the capillary tube; the tube then heated to drive off the acid; and afterwards melted and drawn out, it was found that the mere current of electricity, without sparks, envolved gas from water.

Sir H. Davy has since proved that by a similar apparatus, solution of sulphate is decomposed, potash appearing at the negative, and sulphuric acid at the positive polet.

SECTION IV.

Theory of the Changes produced by Galvanic Electricity.

A FAOT of considerable importance in explaining the phenomena that form the subject of the last section was discovered several years ago by Mr Bennett, and has since been confirmed by the experiments of Volta and Davy. Different bodies, it is found acquire, when brought into contact either by the whole surfaces or by a single point, different states with respect to their quantities of electricity. The best method of performing the experiment is to take two discs or plates, the one of copper, the other of zinc, each about four inches diameter, and furnished with an insulating glass handle; to apply them for an instant to each other by their flat faces; and afterwards, to bring them separately into contact with the insulated plate of the condensing electrometer. The instrument indicates, by the divergence of its gold leaves, the electricity acquired by each of the plates, which in the zinc plate is shown to be positive, in the copper plate negative*.

It had been established, also, by Sir H. Davy, in 1801, that when a galvanic arrangement of the second kind is constructed, by alternating metallic plates with strata of different fluids, alkaline solutions always receive electricity from the metal, and acids on the contrary transmit it to the metal. When an arrangement, for example, is made of water, tin, and solution of potash, the current of electricity is from the tin to the alkali. But, in an arrangement of nitric acid, tin, and water, the circulation of electricity is from the acid to the tin. If then the alkali, after having acquired electricity fom the metal, could be suddenly separated from the combination, there can be no doubt that it would be found in a positive state. For the contrary reason, the acid having given electricity to the metal, must, if it could be detached, be found negative.

Still more satisfactory evidence has been since obtained of the electrical state of the acids and alkalies, by examining what kind of elec tricity they impart to an insulated metallic plate. Various dry acids, being touched on an extensive surface by a plate of copper insulated by a glass handle, the copper was found after contact to have become positively electrified, and the acid negatively. On the contrary, making the experiments with dry earths in a similar manner, the metal became negative. The alkalies gave less distinct results, owing to their attraction for moisture. Bodies, moreover, possessing opposite electrical energies towards one and the same body, are found to possess them with regard to each other. Thus when lime and oxalic acid were brought into contact, the earth was found to be positive, and the acid negative. Sulphur appears to be in the positive

* Volta, in Nicholson's Journal, 8vo. i. 136. Wilkinson, ii. 40, 50, 131. Cuthbertson, 267.

state. Oxygen, judging from those compounds in which it is loosely combined, is negative; and hydrogen, by the same test, positive.

Now, if the common laws of electrical attraction and repulsion operate, as there is every reason to believe they must, among bodies so constituted, it will follow that hydrogen, the alkalies, metals, and oxides, being positively electrified, will be repelled by surfaces which are in the same state of electricity as themselves, and will be attracted by surfaces that are negatively electrified. And, contrariwise, oxygen, and the acids (in consequence of the oxygen they contain), being in a negative state, will be attracted by positive surfaces and repelled by negative ones.

To apply this theory to the simplest possible case, the decomposition of water, the hygrogen of this compound, being itself positively electrified, is repelled by the positive wire and attracted by the negative one; while, on the contrary, oxygen, being negative, is repeiled by the negative wire, and attracted by the positive one. The flame of a candle, which consists chiefly of ignited charcoal, when placed between a positive and negative surface, bends towards the fatter; but the flame of phosphorus, consisting chiefly of acid matter, when similarly placed, takes a direction towards the positive surface. In the case of neutral salts, the negative acid is attracted by the positive wire; and the positively electrified alkali by the negative wire. Thus then a power has been discovered, superior in its energy to chemical affinity, and capable either of counteracting it, or of modifying it according to circumstances. The chemical attraction between two bodies may be destroyed, by giving one of them an electric state opposite to its natural one; or the tendency to union may be increased, by exalting the natural electric energies.

All bodies, indeed, that combine chemically, so far as they have hitherto been examined, have been found to possess opposite states of electricity. Thus copper and zinc are in opposite states to each other; so are gold and mercury; sulphur and metals, acids and alkalies. By bringing two bodies into the same electrical state, which were before capable of union, we destroy their tendency to combination. Thus zinc or iron, when negatively electrified, will not unite with oxygen. Even after combination, it is thought by Sir H. Davy, not improbable, that bodies may still retain their peculiar states of electricity. If oxygen prevail, in any compound, over the combustible or positive base, the compound is negative, as in certain metallic oxides. But the combustible ingredient may be in such proportion, as to predominate, and to give to the compound a positive energy. When precise neutralization is attained, bodies that had before exhibited electrical effects are deprived of this property.

It is an interesting question, but one which can scarcely be deermined in the present state of the science, whether the power of lectrical attraction and repulsion be identical, as Sir H. Davy has ggested, with chemical affinity, or whether it may not rather be nsidered, like caloric, as a distinct force, which only modifies that chemical attraction. On the former hypothesis, two bodies, ich are naturally in opposite electrical states, may have these

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states sufficiently exalted, to give them an attractive force sup to the cohesive affinity opposed to their union: and a combination will take place, which will be more or less energetic, as the opposed forces are more or less equally balanced. Again, when two bodies, repellent of each other, act upon a third with different degrees of the same electrical energy, the combination will be determined by the degree. Or, if bodies, having different degrees of the same eecltrical energy with respect to a third, have likewise different energies with respect to each other, there may be such a balance of attracting and repelling forces as to produce a triple compound.

This hypothesis, it is remarked by Sir H. Davy, agrees extremely well with the influence of mass, which has been so well illustrated by Berthollet; for many particles, acting feebly, may be equal in effect to fewer acting more powerfully. Nor is it at all contradictory to the observed influence of caloric over chemical union; for an increase of temperature, while it gives greater freedom of motion to the particles of bodies, exalts all their electrical energies. This Sir H. Davy ascertained with respect to an insulated plate of copper and another of sulphur, when heated below 212° Fahrenheit; and at a still higher temperature these bodies, as is well known, combine with the extrication of heat and light, and the usual accompaniments of intense chemical action.

On the supposition that electricity is a force which only modifies the action of chemical affinity, we may regard it, when it promotes combination, as producing this effect by counteracting cohesive attraction. When it impedes combinations, or destroys those which are already formed, it probably acts as a force co-operating with elasticity.

SECTION V.

Theory of the Action of the Galvanic Pile.

Two theories have been framed to account for the phenomena of the Galvanic Pile, and of all similar arrangements. The first, originating with Volta, was suggested by the fact, which may be considered, indeed, as fundamental to it-that electricity is excited by the mere contact of different metals. When a plate of copper and another of zinc are inade to touch by their flat surfaces, as was stated in the last section, the zinc, after separation, exhibits positive electricity, and the copper negative. It is natural, therefore, to conclude that a certain quantity of electricity has moved from the copper to the zinc. On trying other metals, Volta found that similar phenomena take place; and by a series of experiments he was led to arrange the powers in the following order, it being understood that the first gives up its electricity to the second; the second to the third; the third to the fourth, and so on.

The metals, then, have been denominated by Volta, from this property, motors of electricity; and the process, which takes place, electro-motion, a term since sanctioned by the adoption of it by Sir H. Davy.

It is on this tranference of electricity from one body to another by simple contact, that Volta explains the action of the instrument discovered by himself, and of all similar arrangements. The iterposed fluids, on his hypothesis, have no effect as chemical agents in producing the phenomena, and act entirely as conductors of electrieity. Without disputing, however, the accuracy or value of the facts which suggested his theory, it is sufficient for its refutation that it is irreconcileable with other phenomena; and especially with the observation, that the chemical agency of the liquids, on the more oxidi zable metal of galvanic arrangements, is essential to their sustained activity. It has been proved, indeed, that the phenomena begin and terminate with the oxidation; and that the energy of the pile bears a pretty accurate proportion to the rapidity of the process. Hence it seems, on first view, an obvious inference, that the oxidation of the metal is the primary cause of the evolution of electricity in galvanic arrangements. It has been proved, however, that it is not necessary to the excitement of electricity, that the amalgam should be oxidated; for the machine continues to act when inclosed in hydrogen gas or carbonic acid; and the electric column of M. de. Luc is composed of dry substances. Even in this instrument, the oxidation of the metals appears to be essential to its activity, for when the column is hermetically confined in a given portion of air, the phenomena cease in time, in consequence of the loss of its oxygen.

But though the chemical agency of the fluids which are employed is now admitted, on all hands, to be essential to the excitement of this kind of electiricty, yet is by no means universally agreed that we are to consider it as the first in the order of phenomena. It has been suggested by Sir A. Davy, as a correction of the theory of V that the electro-motion occasioned by the contact of metals, is primary cause of the chemical changes; and that the changes in no other way efficient, than as they restore the electric equili um. To explain this, let us suppose that in any three pairs of pla of a galvanic trough, the zinc plates z 1, z 2, z 3, (fig. 78), are the state of positive, and the copper plates c1, c2, c 3, in that negative electricity. The liquid, in any cell after the first, will in contact, on the one side, with positively electrified zinc, and on t other with negatively electrified copper. And if the elements coposing the fluid be themselves in different states of electricity, the negatively electrified elements will be attracted by the zinc, and the