filled with the proper liquid, a plate of each metal is introduced, but not so as to touch each other. A communication is then made, by a metallic arc, between the zinc plate of each cell and the copper one of the next, precisely as in the chain. of cups. More lately the troughs themselves have been made of earthen ware, and the partitions of the same material; the apparatus being completed, in other respects, in the manner already described. The plates, also, are now so suspended that when not in use, they may all be lifted out of the cells at once,* And it has been recently ascertained that the power of a battery is increased at least one half by placing in each cell one zinc and two copper plates, so that each surface of zine may be opposed to a surface of copper.†

The size of the plates has been varied from one or two inches to several feet. The large battery, described by Mr. Children in the Philosophical Transactions for 1815, consisted of plates each six feet by two feet eight inches, equal to 32 square feet; and the cells were capable of containing about 945 gallons of liquid. For ordinary purposes, plates of two inches square are sufficient; but for the decomposition of several bodies, not less than 100 pairs of plates, each four inches square, are required. The enlargement of the size of the troughs, so as to contain this number, would be extremely inconvenient; and we may therefore combine the power of several troughs, by uniting the zinc end of the one with the copper end of the other, by the intervention of a metallic wire, or by an arc of silver or zinc.

A convenient and powerful galvanic battery has been constructed by Dr. Hare of Philadelphia, of zinc and copper sheets formed into coils. The zinc sheets were about nine inches by six; the copper fourteen by six; more of the latter metal being necessary, as, in every coil, it was made to commence within the zinc, and completely to surround it without.

* See Pepys in Journal of Science, &c. i. 193.

Phil. Trans. 1815.

Some useful information respecting the number and size of plates, adapted to different purposes, is given by Mr. Singer in Nicholson's Journal, xxiv. 174.

The sheets were coiled so as not to leave between them an interstice wider than a quarter of an inch. Each coil was in diameter about two inches and a half; their number amounted to 80; and by means of a lever they were made all to descend together into 80 glass jars, 2 inches and 3-4ths diameter inside, and eight inches high, duly placed to receive them. The effects of this apparatus, in producing ignition, appear to have been very striking, especially at the moment of immersion, when Dr. Hare found the effect to be by much the most powerful.* An apparatus of 80 coils produced a vivid ignition in charcoal; and when the points of the charcoal were drawn three-fourths of an inch apart from each other, a most brilliant arc of flame extended between them.

It may be sufficient to add, in general terms, that every combination, which is capable of forming a simple galvanic circle, may, by sufficient repetition, be made to compose a battery. The combinations, also, which are most active in simple circles, are observed to be more efficient in compound ones. The foregoing tables of Sir H. Davy express, therefore, the powers of compound as well as of simple arrange

ments.

To construct a battery of the first order, it is essential that a fluid be employed, which exerts a chemical action upon one of the metals. Pure water, entirely deprived of air, appears to be inefficient. In general, indeed, the galvanic effect is, within certain limits, proportional to the rapidity with which the more oxidable metal is acted upon by the intervening fluid. Spring water was found sufficient, in Mr. Children's immense battery, to produce the ignition of platina wire. The fluid generally used is nitric acid, diluted with 20 or 30 times its weight of water. Mr. Children recommends a mixture of three parts fuming nitrous acid, and one sulphuric, diluted with thirty parts of water. Directions, also, respecting the best kind and density of acids, for producing galvanic electricity, are given by Mr. Singer. From his experiments it appears, that acid of different densities is required for different purposes. The best wire melting charge is formed with ten

gallons of water, five pounds of nitric acid, and half a pound of muriatic acid.

The power of the apparatus has been found to be increased, when insulated by non-conductors; and when surrounded by an atmosphere of oxygen gas; not sufficiently, however, to make it necessary to resort to either of these expedients in ordinary cases. Oxygen gas disappears in this process, when carried on under a receiver; and, after all the oxygen is absorbed, the effect ceases, and is renewed by introducing a fresh portion. A battery, also, which has ceased to be efficient, has its activity renewed by emptying the cells of their liquor, and uncovering the plates. When the cells are filled with diluted nitric acid, the apparatus continues active, even under the exhausted receiver of an air-pump, or in an atmosphere of carbonic acid or nitrogen gases. But if the cells be filled with water only, all action is suspended, by placing it under any of these circumstances. Hence it appears that the oxidation of one or both of the metals, composing the trough, is essential to the excitement of galvanic electricity.

The electric column may be classed among galvanic arrangements. It was originally contrived by M. de Luc, who formed it of discs of Dutch gilt paper, alternated with similar discs of laminated zinc. These were piled on each other in a dry state, and the instrument, instead of being soon exhausted, like the pile with humid substances, was found to continue active for some years.* A similar pile may be formed by laying a mixture of very finely powdered zinc with common glue and a little sugar, by means of a brush, on the back of Dutch gilt paper; and, when dry, cutting it into discs, which are to be piled on each othert. Zamboni of Verona has constructed a pile of slips of silver paper, on the unsilvered side of which is spread a layer of black oxide of manganese and honey. These papers are piled on each other to the number of 2000; then covered externally with a coating of shell lac; and enclosed in a hollow brass cylinder. Two of these piles are placed at the distance of four or five inches from each other; and between them is suspended on a pivot a light me

* Nicholson's Journal, vol. xxvi.

† Phil. Mag. xlvii. 265.

tallic needle, which is attracted alternately to the one pile and the other, so that it moves between them like a pendulum. This instrument has been applied to the measurement of time, by causing it to give motion to the pendulum of a clock.*

SECTION II.

On the mutual Relation of Electricity and Galvanism.

Is the influence, it may now be inquired, which is called into action in a way so different from that employed for the excitation of ordinary electricity, identical with it or of a different kind? This question will be decided by examining whether any of those phenomena, which are occasioned by the agency of the electric fluid, are produced also by that of galvanism; and we shall find the following striking resemblances:

1. The sensation, produced by the galvanic shock, is extremely similar to that which is excited by the discharge of a Leyden jar. Both influences, also, are propagated through a number of persons, without any perceptible interval of time.

2. Those bodies, which are conductors of electricity, are also conductors of the galvanic fluid, as the metals, charcoal, and a variety of liquids. Again, it is not transmitted by glass, sulphur, and the whole class of electrics, which do not convey ordinary electricity. Among liquids, those only are conductors of electricity and galvanism, which contain oxygen as one of their elements.+

3. The galvanic fluid passes through air and certain other non-conductors, in the form of sparks; accompanied with a snap or report; and, like the electric fluid, it may be made to inflame gun-powder, phosphorus, and mixtures of hydrogen and oxygen gases. It has been found, also, by Mr. Children, that in the Voltaic apparatus there is, what is called in electricity, a striking distance. With a power of 1250 pairs of four inch plates, he found this distance to be one 50th of

Phil. Mag. xlv. 261. and Ann. de Ch. et de Phys. xi. 190. + Cruickshank, in Nicholson's 4to. Journal, iv. 258.

an inch, the thickness of a plate of air, through which the galvanic discharge is able to pass in the form of a spark. Increasing the number of plates, the striking distance will be greater; and the reverse when it is diminished. It is also increased by rarefying the air, through which the spark is

transmitted.

4. The Voltaic apparatus is capable of communicating a charge to a Leyden jar, or even to a battery. If the zinc end of a pile (whether it be uppermost or the contrary) be made to communicate with the inside of a jar, it is charged positively. If circumstances be reversed, and the copper end be similarly connected, the jar is charged negatively. The shocks do not differ from those of a jar or battery, charged to the same intensity by a common electrical machine.

5. Galvanism, even when excited by a single galvanic circle only (such as a piece of zinc, a similar one of copper, and a piece of cloth moistened with a solution of muriate of ammonia), distinctly affects the gold leaf of the condensing electrometer. If the zinc end be uppermost, and be connected directly with the instrument, the electricity indicated is positive; if the pin of the electrometer touch the copper, the electricity is negative. A pile consisting of sixty combinations produces the effect still more remarkably.†

6. The chemical changes produced by galvanic and common electricity, so far as they have hitherto been examined, are precisely similar. These will form the subject of the following section.

SECTION III.

On the Chemical Agencies of Electricity and Galvanism.

THE effects of the electric or galvanic fluids, in producing chemical decomposition, cannot be described, without intro

Cuthbertson's Practical Electricity and Galvanism, p. 261; Volta, in Nicholson's Journal, 8vo. i. 140; Van Marum, in Philosophical Magazine, xii. 162; Singer, p. 126.

† Nicholson, 8vo. i. 139, and ii. 281; Cuthbertson, p. 264; and Singer p. 317; where the cautions necessary to the success of this delicate experiment are particularly described.