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pairs the late Mr. Singer was enabled to charge Leyden jars by a moment's contact. By the employment of coated talc, instead of glass, I find that an extensive surface may be charged by a dry pile of 10,000 pairs.*

392. The principal consideration, in this place, is the perpetuity of the action of the dry electric column; which, when properly constructed, and securely guarded against moisture, appears to be possessed of interminable electric powers. At the time when philosophers, especially the chemical part of them, could form no idea of the changes in the electric characters of bodies but such as emanated from o.cidation, the most fruitful of all sources of error, in this branch of physics, the dry electric column shared the same fate as the wet pile of Volta, in having its action placed to the credit of oxidation. And even when it was found that the dry column would retain its action for several years, a slow oxidation of the metals was the only explanation that could be given for the display of its electricity. This idea became so fashionable eventually, that it became the basis of the prevailing hypothesis which, to this day, is strongly contended for by certain philosophers now in controversy on this interesting topic.

393. To those who have paid attention to the succession of steps that I have hitherto taken in tracing the electric action from metal to metal in each individual pair, and also that action which is due to a series of pairs, as associated in the dry electric column, there can appear no reason whatever for calling into the hypothesis an oxidation of the metals; but, on the contrary, since purity of the metallic surfaces is essential to the development of those peculiar arrangements of the electric forces which I have described, and which form the very soul of the dry electric column; and since the introduction of an oxidizing process would soon pollute these metallic surfaces, and ultimately destroy every particle of their metallic character, nothing can appear more evident in physical science, than that a perpetuity of electric action in the dry pile, depends upon a continuance of the purity of the metal.

394. The inference thus drawn from the simple principles of electricity, is that alone which can direct the practical electrician with certainty to the construction of a permanently acting electric column. It is conformable with all experience, and, I believe, is now acted upon by those who make the best apparatus of this kind. Every care is taken to insulate the elements of the column from the atmospheric air, and from every kind of moisture; which, when properly accomplished, the permanency of electric action becomes secured.

395. As the whole action of the column depends upon electric pressures, any change in the external electric pressure of the atmos

The whole process of forming dry electric columns, with a variety of their interesting applications, will be clearly described in my Cyclopædia of Electricity and Magnetism, shortly to be published.

phere will necessarily affect the action of the apparatus. It is on this account that it becomes av indicator of those Auctions of atmospheric electrical pressure which are almost continually going on; and, as these fluctuations are occasioned by change of temperature, humidity, evaporation, winds, barometric pressures, clouds, &c., and frequently by several of these causes at the same time, there can be no astonishment excited at M. de Luc's failures in attempting to apply the electric Fig. 4. column as a meteorological instrument. The frequent changes in the activity of this apparatus, first noticed by that ingenious and indefatigable philosopher, are exceedingly curious and interesting. He attached to the upper pole of a vertical electric column, a bent wire, which reached downwards as low as the lower pole, and terminated in a small brass ball. Between this ball and the lower pole of the column was suspended, by a silken fibre, a light gilt pith ball; the whole being covered by a glass shade, as represented in Fig. 4. A pen

oo dulous ball vibrates between the lower pole of the column and the opposite ball, carrying the electric fluid from one to the other continually. When the atmosphere is highly charged with the electric fluid ihe ball vibrates with great ra

Fig. 5. pidity; it also moves rapidly in a warın room, or any warm unattenuated atmosphere, but languishes very materially in a moist atmosphere.

396. When two columns are arranged as in Fig. 5, having a bell attached to each lower pole, the one positive, and the other negative, the pendulous brass ball plays between them, and rings both bells; thus warning the observer when any material atmospheric electrical change is taking place. Apparatus of this kind have kept in play for upwards of twenty years, and are still as active as at first; and there can be no satisfactory reason shown why they should ever cease to display their electric forces.

397. Having now disposed of the views which I have taken of the theory of the dry electric column, I shall next endeavour to explain the principles upon which the wet pile, or voltaic battery, operates ; and why its action soon languishes, and ultimately becomes annihilated. And in this attempt, as in that of the previous part of the subject, I shall proceed with the simplest case.

398. The simplest case in the action of the wet pile of Volta, is similar to that of the dry pile, already discussed; and the action of such a pile, under certain circumstances, would be as durable as that of the pile of M. de Luc,or of any dry pile whatever, provided the

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electric condition of each individual metallic surface were uniform ; but, in consequence of an ununiformity of electric action on various points of every metallic surface, as already explained in the first section of this memoir, any moisture, in contact with such surface, would suffer partial decomposition, and the metal itself undergo a change, whether employed as an element in a voltaic series or not.

399. When, however, pure water is employed for the intervening medium between the metallic pairs, the pile assumes electro-polarity in a supereminent degree: and a series of about five hundred pairs would charge an extensive coated surface of glass, by one momentary contact of either of its poles. Five hundred square feet of coated glass have been charged, by a pile of this description, to a higher degree of intensity than can possibly be accomplished, in the same time, by the most powerful electric machine: and by increas. ing the extent of the voltaic series, the charge would Fig. 6. increase in proportion. Hence, from a source of this kind, electricity, to almost any required extent, might be obtained. And, although the local electric action, especially on the surface of the zinc, tends continually to lessen the general electric forces of the pile, and eventually annihilates them altogether, the apparatus retains a considerable degree of action for many successive hours. The theory of the general action of this apparatus is precisely that already given for the dry electric column; and it is a matter of do consequence, whether the metals be piled one upon another, with intervening moistened paper, as represented by Fig. 6, or that they be placed edgewise, in a Cruikshank's trough, represented by Fig. 7, whose cells are

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newly filled with pure water. In this latter form, however, the cells must be perfectly water-tight, and the pairs of metal well insulated from each other by resinous cement.

400. When the voltaic pile, or battery, is employed in its most usual capacity, for the production of electric currents, the principles hitherto described form a part only of the theory of its action ; for stable electro-equilibrium is inconsistent with the idea of a continuous flowing current; therefore, other principles must enter the framework of the theory, in order that its explanations may become satisJactorily applicable to every variety of case.

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401. When a pair of copper

Fig. 8. and zinc plates are united by wire, as in Fig. 8, the pair becomes electro-polar as decidedly as in any

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as when the planes of the plates are laid parallel to each other, as in Fig. 1. They, however, still form a voltaic pair, and the zinc receives a portion of the electric Guid, previously belonging to the copper, and consequently, is in a suitable condition to give up that portion to any conducting body capable of receiving it. The copper plate, also, being now in a negative state, is equally prepared to receive a new portion of Auid from any body appropriately situated to communicate it.

402. Let us now suppose that the metals are immersed in water, which is a compound body, and whose particles are susceptible of motion by the application of slight forces, and of separation from one another by the introduction of other matter amongst them : also that the electro-conduction of water is improvable by admixture with acid, and other liquids ; and its constituents held together by certain electric forces, and consequently susceptible of separation by superior electric forces. Under these circumstances, the first immersion of the plates into pure water, would cause a movement of their electric fluid, in such a manner, that the zinc would give up a portion to the water, and the copper would receive a portion from it: and if nothing further went on, there would be a new distribution of the electric fuid, and again a statical polar equilibrium established, as decidedly as under any other circumstances.

403. With copper, zinc, and water, however, the electric forces of the metals are somewhat more powerful than those which unite the oxygen and hydrogen in the shape of water: and as the particles of water themselves are electro-polar, those of them next to the plates become easily arranged in regular polar order, with respect to the electric forces of those plates. The positive zinc surface attracts, and draws towards it the negative surfaces of the particles of water : and the positive surfaces of another stratum of water become placed in juxtaposition with the negative surface of the copper. The electric forces of both the metals and the water being now arranged in the best possible manner to accomplish a separation of the constituents of the latter, and subsequently urge them in opposite directions, accordingly to their relative electric characters, the combined forces, thus arrayed, vanquish those by which the constituents of the water were held together, conveying the hydrogen to the copper and the oxygen to the zinc.

404. Now, since we are unable to discover, by observation, at what

part of the water its decomposition takes place, the theory is necessarily left in some degree of obscurity on this particular point, which has given rise to much inconclusive discussion, and to opinions of many diverse kinds. In philosophical reasoning, analogy, in the absence of phenomena, osten becomes a valuable substitute; and data, thus supplied, have led to inferences as satisfactory as if drawn from facts themselves : in the present case, however, analogy seems to be productive of various conclusions, nearly all of which are equally supported. It is known, for instance, that alkaline and acid matter, though placed in separate vessels, are made to traverse a voltaic circuit, exterior to the battery, in opposite directions, even through each other,—to exchange places, and occupy each others positions in the two vessels. If this fact were to be made the basis of analogical reasoning, it would equally support either of two opinions, in the decomposition of water by voltaic electrical agency of a single pair, as in Fig. 8. It would be as applicable to the supposition of the decomposition taking place at the centre of the mass of water, between the copper and zinc, as to the supposition that decomposition occurs at both plates at the same time, transporting the constituents in opposite directions, in both cases.

405. It has long appeared to me, that, since all the metals, and carbon, which are the best conductors of electricity known, are invariably carried to the negative pole of the battery, or in the direction of the current, through the liquid part of the circuit

, there is something like a general tendency for the electric fluid to take possession of the best conductors in the liquid mass, and carry them to the next solid conductor; as, for instance, to the copper, in a single pair. Should this be the case, it would be an easy matter to explain the reason of alkaline matter being invariably determined at the

negative metal : for the potassium, or other alkaline metallic base, would arrive there as a pure metal; but being reoxidized as fast as liberated, it would reassume the alkaline state, and dissolve in the liquid. The same reasoning also applies to ammonia, strontia, and other compound bodies, which are known to arrive at the negative metal when, in connexion with other matter, they are submitted to the action of an electric current.

406. It is now some years since I attempted to show the correctness of this view (405) in the Phil. Mag. ;* and I have not yet met with any fact that has tended to militate against it. I there showed that when a mixed solution of two metallic salts, the sulphate of copper and sulphate of zinc, is subjected to the action of an electric current, the copper, which is a better conductor than zinc, is carried alone to the negative polar terminal : and that by this means, a considerable portion of copper can be separated from the zinc. From this fact I had every reason to suppose, that, if any of the metals be compound bodies, as some have thought them to be, their decompo

• I cannot lay my hand on the volume at present; but will take occasion to give the precise reference in another place.

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