that

these: "By the experiments of single contact, it now appears the adhesive affinity of electricity to lead ore is positive, and to zinc negative." He adds, also, that "gold, silver, copper, brass, regulus of antimony, bismuth, tutenag, mercury, various kinds of wood, and stone, were tried by this method of single contact, and appeared to cause a positive charge. Tin was negative, and a large piece of zinc much more weakly negative than a thin plate of the same metal used in the preceding experiments."

After Mr. Bennet's experiments became known, they were repeated by Cavallo and others, even prior to the experiments of Volta on the electricity produced by the simple contact of metals; but those of the Italian philosopher were made in a very different manner, and gave perfectly unequivocal results. Volta, however, as well as his predecessors in these discoveries, employed his condenser in its simplest form; whilst Bennet and Cavallo had used it in its most complex form of the doubler, whose indications are confessedly not at all times to be depended on. The first set of experiments on record which showed the electric action of metals by simple contact, without the aid of a condenser, were published in Nicholson's Journal for December, 1804. The following is a copy of the author's own account of these experiments, and of the motives which induced him to proceed with them.

On the Electricity exhibited by Metals, without the help of any Condensing Instrument. By Mr. Wм. WILSON.

When I set about making the compound electrical condenser described in my last letter to you, I intended to repeat the experiments of Cavallo, relating to the electricity obtained by the contact of metals related in the third volume of the fourth edition of his Treatise of Electricity; but before the instrument was finished, I was induced (by some experiments I had made relative to the cause of excitation of electricity), to suppose that it is not the contact of the metals that is the cause of the appearance of electrical signs, but the separation of the metals from contact. And this supposition was very much strengthened, when, upon examination, I found that all who have made experiments on this subject have separated the metals from contact before they examined them as to their eleccity.

If the contact of the two metals be the cause of the electrical signs, the whole effect that one metal can have on another will be communicated at the time of contact, however few the points are that form the contact, because both of the metals being conductors of electricity, if one possessed a greater proportion of it than the other, a part will be communicated to the other at the time of contact, to form an equilibrium, and this will be done as well by a few points of contact as by a great many. But if it is the separating them from contact, that is the cause of the electrical sign, the more extensive the contact is the more powerful will the signs be when the metals are separated.

To put this to the test of experiment, I pierced a piece of thin sheet copper full of small holes, just big enough to permit to pass through them two or three particles of filings of another metal at a time, so that almost every particle must be in contact with the copper before it can pass through, and consequently the surface of contact be very great with a comparatively small quantity of metal.

I sifted through this copper sieve some filings of zinc into a tin plate laid on the cap of a gold leaf electrometer, and the gold leaves diverged near an inch with positive electricity, when about half an ounce of filings had been sifted into it. Encouraged by this striking result, I procured sieves and filings of different metals. The results of the trials with them are contained in the following table; where P stands for positive electricity, N for negative, and when it was not strong enough to affect the electrometer, that is denoted by O.

In all the above experiments there was a large surface of contact, and the electrometer only was used; but in those made by Cavallo and others who had a very small surface of contact, electrical signs could not be made to appear without the help of doublers, multipliers, &c., of electricity. I therefore think there can be no doubt about the separating the metals from contact being the cause of their appearing.

The experiments of Mr. Wilson have been considered of a different character to those of Volta, and are supposed to show the electric action of metals by friction against one another, and not by simple contact only. And we believe that Dr. Hare, of Philadelphia, has the credit of having first shown the electric action of the voltaic discs, independently of a condenser. Dr. Hare was the inventor of

the single gold leaf electrometer, a far more delicate instrument than the double gold leaf electrometer of Bennet; and by this instrument the American philosopher was enabled to show the electric action of a copper and zinc pair of discs, by one single contact. The following is Dr. Hare's description of his electrometer, with experiments, as it appeared in the Philosophical Magazine for April,

1824:

The electrometer consists of a globular glass vessel, which stands on a foot, and having a wide neck at top and a narrower one on one side. The upper neck is furnished with a brass socket, to which is fastened a horizontal disc of zinc, six inches diameter, having its centre directly over the centre of the globe. From the zinc disc hangs a narrow slip of gold leaf, which reaches a little lower than the centre of the globe. Through the side neck of the glass passes a brass wire with a ball at each extremity, the inner one of which can be made to approach the pendant gold leaf to any required distance, at pleasure, by means of a micrometer screw. "The electricity produced by the contact of copper and zinc is rendered sensible in the following manner. Place a disc of copper (of the same dimensions as the zinc, and furnished with a glass handle) on the disc of zinc; take the micrometer screw in one hand, touch the copper disc with the other, and then lift this disc from the zinc. As soon as the separation is effected, the gold leaf will strike the ball, usually, if the one be not more than of an inch apart from the other. I have seen it strike at nearly double that distance. Ten contacts of the same discs, of copper and zinc, will be found necessary to produce a sensible divergency in the leaves of the condensing electrometer.* That the phenomenon arises from the dissimilarity of the metals, is easily shown by repeating the experiment with a disc of zinc in lieu of a disc of copper. The separation of the homogeneous discs will not be found to produce any contact between the leaf and ball. I believe no mode has been heretofore contrived by which the electrical excitement resulting from the contact of heterogeneous metals may be detected by an electroscope without the aid of a condenser. It is probable, that the sensibility of this instrument is dependent on that property of electricity which causes any surcharge of it, which may be created in a conducting surface, to seek an exit at the most projecting termination or point connected with the surface. This disposition is no doubt rendered greater by the proximity of the ball, which increases the capacity of the gold leaf to receive the surcharge, in the same manner as the uninsulated disc of a condenser influences the electrical capacity of the insulated disc in its neighbourhood. It must not be expected, that the phenomena above described can be produced in weather unfavourable to electricity. Under favourable circumstances, I have produced it by means of a smaller electrometer, of which the discs are only 2

• The gold leaves, if narrow, short, and well insulated, will diverge by three contacts; sometimes by single contact, when the condenser is used.-EDIT.

inches in diameter. I think I have seen an effect from a disc only an inch in diameter, or from a zinc disc having a copper socket to its handle."-ROBERT HARE.

About the time of the appearance of the above paper by Professor Hare, I was much engaged in researches of this kind, and had occasion to try various plans for showing the electric action produced by the simple contact of bodies; and shortly after I employed a single strip of gold leaf, suspended in the axis of a phial from a zinc disc of about two inches diameter. A hole was made on each of the two opposite sides of the bottle, and through those holes passed two brass wires with a ball at each end. The gold strip thus hung between the two inner balls, which could be adjusted to any distance by screws, as in Dr. Hare's instrument. To the outer ends of the horizontal wires were attached the poles of a dry electric column,* placed horizontally on two glass pillars which stood in a mahogany base board. By means of this instrument I could ascertain the electric condition of the zinc disc after the contact of another metal by the leaning of the gold leaf towards that ball which was in the opposite electric condition. In many cases the gold leaf will strike the ball, when the metals employed are not so large as sixpence. The zinc is moveable, and can be replaced by any other metal. I never insulate the uppermost disc, it having a metallic wire for its handle.-W. S.

Having thus shown the origin and progress of electrical developments by the simple contact of metallic bodies, we will return to those assemblages of metallic bodies constituting the essential elements in the dry electric columns.

In Nicholson's Journal for 1810, we find some excellent papers on voltaic arrangements by J. A. de Luc, Esq., with some animadversions on the Council of the Royal Society for not publishing them. With these latter, however, we have nothing to do at present; and shall present our readers with those parts only which explain the theoretical views and experimental discoveries of this excellent philosopher.

Analysis of the Galvanic Pile. By J. A. DE LUc, Esq.

In January, 1806, I had the honour of presenting to the Royal Society two works, connected with each other by the common object of meteorology, published at Paris in 1803 and 1804, under the titles of Introduction à la Phyisque terrestre par les Fluides expansibles, and Traité Elémentaire sur le Fluide Electro-galvanique. In the latter of these works I had proved, by direct experiments, that

I was not aware, at the time this instrument was made, that an electric pile had previously been applied to an electrometer. But I afterwards found that Bohnenbergen had applied two electric columns to a double gold leaf elec trometer, about the year 1820. This, however, was fortunately a very different instrument to mine.-W. S.