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363. Under these circumstances, therefore, any attempt that can be made, in this place, to elucidate the principles of a science in which the efforts of the highest minds have failed to produce a satisfactory explanation, will necessarily appear under great disadvantages, with every probability of suffering the common fate of its predecessors. But as there are some points on which philosophers entertain the same opinion, there is still a possibility, at least, though the attempt may be viewed in the character of a forlorn hope, of simplifying some others, so as to be sufficiently comprehended to gain general assent, and thus advance us one step farther into the apparent intricacies of the theory of voltaic electricity.

364. I believe that there is no difference of opinion, at the present day, respecting the identity of the agency in common electricity, and that which is productive of the phenomena in that branch of physics now under contemplation: hence it is that we are not misunderstood by the term voltaic electricity, when represented as a peculiar branch of electrics. The generality of writers, however, on these matters, make no distinction whatever between voltaic electricity, and galvanic electricity; or, in other words, between voltaism and galvanism, although the sources from which the phenomena emanate are as distinct from each other, as that of the phenomena of ordinary electricity is from either of them. Why this practice is not yet abandoned would not, perhaps, be easily determined; but it must proceed either from an ignorance of their distinction, from an unwarrantable proneness to confound the one with the other, or from a fear of stepping counter to custom, and thus avoiding the imputation of pedantry.

365. I am of opinion, however, that the omission of even a pointed distinction in "voltaic electricity," and "galvanic electricity," by writers of the present day, is not only a dereliction of duty towards their readers, who purchase and read their works under the impression of finding in them expressions of the clearest ideas that the science is capable of admitting, but the practice is fraught with the seeds of misconception and error. Therefore, whatever risk may be run on the score of pedantry, it is quite time that the pratice of confounding the two sources of electric action with one another were laid aside, and that distinction resorted to which alone can lead to clear and unequivocal views, and facilitate the association of ideas with the true character of the source of action.

366. In the preface to Aldini's " Account of the late improvements in Galvanism," published in the year 1803, we find that the distinction between voltaism and galvanism would appear to be very clearly pointed out :-" A just tribute of applause has been bestowed on the celebrated Professor Volta, for his late discovery, and I have no desire to deprive him of any part of that honour to which he is so justly entitled; but I am far from entertaining an idea that we ought on this account, to neglect the first labours of Galvani. Though these two philosophers pursued different routes, they concurred to throw considerable light on the some points of science and the

question now is, to determine which of them deduced the most just consequences from the facts they observed, and then to ascertain whether the facts established by Galvani led to the theory of Voita, or whether those discovered by Volta are connected with the theory of Galvani. For my part, I am of opinion that these two theories may serve in an eminent degree to illustrate each other.

"Last year Professor Volta announced to the public the action of the metallic pile. I here propose to exhibit, according to the principles advanced by Professor Galvani, the action of the animal pile." And again, “In the first part of this work, I shall exhibit the action of galvanisin independently of metals, and explain some of its general properties."

367. It is singular enough, however, that Aldini, who was the nephew of Galvani, and who laboured hard to show the distinction of the sources of galvanic and voltaic electric action, should eventually merge the one in the other, and call a series of "eighty plates of silver and zinc," a "galvanic pile.”*

368. The distinction between the sources of galvanism and voltaism, is very marked, and susceptible of clear and unequivocal definition. The former being either a natural or an artificial association of animal matter, whether alive or dead: whilst the phenomena of voltaism emanate from associations of metals and other inorganic bodies. Galvanism, therefore, comprehends all those phenomena developed by animal electricity, and voltaism comprehends those developed by the simple contact of inorganic bodies, whether solid or fluid.

369. The electrical organs of the Torpedo and Gymnotus Electricus, and the muscular and nervous systems in all animals, are natural associations of animal matter, constituting sources of galvanic electrical action; and are as decidedly galvanic arrangements as are the artificial piles of muscle and brain first arranged by M. La Grave, and shown to the Galvanic Society of Paris: and afterwards operated with by Aldini, and various other philosophers.

370. In most of those electrical arrangements of inorganic matter hitherto formed, one, at least, of the bodies employed, has been a metal, and an association of two metals is invariably employed in every arrangement from which much action is derived. But it must not be considered from the hitherto invariable practice of employing metals in voltaic electrical arrangements, that electric action is not derivable from associations of non-metallic bodies; for it is well known that charcoal will supply the place of a metal, and it may easily be shown that the contact of liquids alone will produce electric action. These and all other associations of inorganic matter are sources of voltaic electricity.

371. With respect to the mode of action in voltaic associations generally, since the whole of that action is acknowledged to be electric, the principles of electricity, properly applied, are conse

* See Aldini's work, page, 218.

quently those to which this modus operandi must be traced, and to which alone it can be attributable. The theoretical views of electricity which I have taken, are clearly described in my first memoir (40-87),* and are those alone which it will be necessary to apply on the present occasion, in forming the basis of the theory of voltaic electricity.

372. In the explanation of the principles of this theory I shall commence with the simplest case, hoping that the reader will bear in mind all that I have said, in my first memoir, respecting the unequable distribution of the electric fluid amongst the bodies composing the surface of the earth, and as far within its body as has hitherto been explored. But, in order to avoid controversy, it may be well to limit the range of my views to those bodies constituting the surface only.

373. Since, then, equal volumes of all the different kinds of matter, whilst surrounded by an equable electric pressure, are charged with different quantities of the electric matter, proportional to their susceptibilities of receiving it, it is obvious that those quantities would vary with every variation of the electric pressure, whether that variation of pressure were general on

Fig. 1.

every side, or partial only, by its limitation to one particular portion of the surface. Let us, for instance, apply this reasoning to the well known experiment with Volta's plates of copper and zinc, than which, I know of no simpler case. See Fig. 1.

374. Prior to bringing the plates into contact with each other, each plate is surrounded by an equable electric pressure; and, consequently, each has its natural share of the electric fluid, due to its susceptibility of receiving it, under such pressure. But when the plates are brought into contact with each other, face to face, it is very obvious that the electric pressure, in the plane of contact, has received a material alteration: it is, in fact, lessened on the face of the copper, but increased on the face of the zinc: and a new distribution of the electric fluid takes place, in consequence of a momentary flow from the copper, through the plane of contact, to the zinc. The zinc having now received more of the fluid than it had whilst under the natural equable pressure, and the copper having lost that quantity, the former becomes positively, and the latter negatively electrical. The distribution, however, is now of a peculiar kind, and very different to the distribution on each individual body prior to contact: which, was equable on every side. The new distribution brings the pair of metals into an electro-polar state; the exterior surfaces of the zinc and copper being, respectively, positively and negatively electrical; not only with respect to each other, but with respect to the plane of contact, and also to exterior bodies

• Annals of Electricity, vol. ii, page 401.

whose natural circumambient electric pressure has not been disturbed375. If the metals be suddenly separated, whilst insulated, the redundant fluid on the zinc has not time to return to the copper: the for. mer is, therefore, left in a positively, and the latter in a negatively electrical condition; but, if the separation be made slowly, the whole, or nearly the whole, of the redundant fluid will return to the copper plate: and the usual equilibriums on the two metals will be restored. 376. Now, since it is an invariable law in electricity, that no flow of the electric fluid can take place from one body to another, unless the former be positive to the latter,* the experiment with Volta's discs furnishes us with a piece of interesting information, which, independently of some such experiment, we had no means of arriving at. It shows us in the most decisive manner, that prior to contact, and whilst the metals were subjected to the same electric pressure, the copper was positive to the zinc; but that, after contact, the zinc was positive to the copper. By proceeding cautiously, with similar experiments, on different bodies, the relative natural electric states of an extensive series of them might easily be obtained: this interesting piece of information, however, remains a desideratum, with the exception of a very few insulated facts.

377. If, whilst the copper and zinc plates were in contact, a copper wire were to connect their outer surfaces, a return of a small portion of the electric fluid, from the zinc to the copper, would take place, and a new distribution, and subsequent equilibrium, would be the results. But, as the transverse sectional area of the conducting arc, which rested on the surface of the zinc, would be extremely small, when compared to the whole area of that surface, the new distribution of the fluid, produced by the application of the arc, would be very little different to that displayed prior to such application: and would not be productive of any change in the general character of the polarization.

378. If, however, the conducting arc were of large transverse dimensions, or if a great number of small ones were employed, so as to cover, by their sectional areas, a large portion of the surface of the zinc plate, the previous distribution, due to the contact of the copper plate, would be very much altered. Nevertheless, the previously displayed polarization, although much lessened in degree, would still retain its original character on every portion of the exterior surface of the zinc that remained unoccupied by the ends of the conducting arcs; and it is not until the whole surface of the zinc is completely covered with copper, that the polarization ceases to be displayed.

I believe that this fact had never been noticed by any writer on electricity until I mentioned it in my Familiar Instructions in the Theory and Practice of Electro-Gilding, &c., published in March last.

+ I am well aware that many persons are of opinion that zinc is invariably positive to copper, but there are very few of them, if any, who have given a reason for entertaining that opinion.

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379. I have made extensive series of experiments on this delicate part of the subject, and from their results I have been led to conclude, that these facts are not limited to the employment of copper and zinc only, but that they are producible, and can be satisfactorily displayed by any two metals whatever and not only by the employment of two distinct kinds of metal, but by two discs of one individual metal, cast from one and the same fused mass.

380. Now, when the polarization first takes place, by the simple contact of Volta's plates, and the new equilibrium has become established, it is obvious that the electric force on the zinc surface has become increased, and that on the copper surface diminished. Hence it is, that the respective electric tendencies of those surfaces, with reference to the circumambient medium, are very different: the zinc having a tendency to dispose of a portion of its fluid, and the copper a corresponding tendency to receive a portion. Under these circumstances we have an opportunity of either increasing or diminishing the sum total, or general stock, of the electric fluid in the two plates. If, for instance, I touch the copper plate with an uninsulated conductor, a new portion of fluid is transmitted to its previously negative surface, and the general stock is increased: but if, on the contrary, I touch the exterior surface of the zinc plate with a similarly situated conductor, a portion of the fluid, previously accumulated on that surface, is delivered over to the conductor, and the general stock becomes diminished. A moment's contact with the conductor, in each case, is sufficient to produce the effects stated: which may be proved by separating the plates slowly afterwards. In the former case both plates are found to be positively electrical; and in the latter case, both are found to be negatively electrical.*

381. The principles on which the display of the above described phenomena depend, may be conveniently taken advantage of in exhibiting the electric conditions of the plates by their simple contact: because the action of each individual plate is enhanced by applying a conductor to the other, whilst they are in contact. If, for instance, I wish to show the positively electric action of the zinc to the greatest advantage, I insulate that plate only, and not the copper: and, on the other hand, if I wish to show the negative electric action of the copper plate to advantage, I insulate it only, and keep my hand in contact with the zinc. By these means, a single contact is sufficient to show the electric character of each plate.

382. Hitherto I have noticed the electro-polarization of the zinc and copper plates, only as a group, or a collective whole; but it will be easily understood that, since the exterior surfaces of the zinc and copper are respectively positive and negative to the plane of contact, each individual plate is also electro-polar: the outer and inner sur

• For the performance of these experiments the most delicate electroscope must be employed. That described at page 427 of this volume, is the one used in my investigations.

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