with the negative side, GH the section of a card between them: when the battery is discharged, a spark will pass from a on that side of the card to s, where it will pierce the card to enter the negative wire.

Exp. The atmospherules of the atoms of the air will be condensed about the point c, as will be understood by observing the position of the short lines at b, fig. 67; and the atmospherules of those at a, fig. 97, will be extended outward, towards s, hence the line of easiest passage the spark will be formed on the positive side a s of the card; so that the place of penetration will be near c, the point of the negative wire.

for

PH. 221. If the experiment, as above, be made with the card in an exhausted receiver, it will be pierced near the middle t, and as the air is admitted, so as to render it more and more dense, the place where it enters the card will be between t and s, and nearer to s as the air is more dense.

Exp. When the air is absent the atmospherules of the atoms of the card are most concerned in preparing, or rather directing, the course of the spark, and these extend towards s on the lower side and towards c on the upper, and most on the lower side from a to t, and on the upper from t to c; hence the aperture will be about t the middle between a and c, but as the air is admitted, and becomes denser, the resistance to the passage at t is augmented by the atmospherules of the air pressing towards c on the upper side, as shewn in the last, hence the effect will be to cause the spark to proceed nearer to s before it pierces the card.

PH. 222. When a powerful electric charge is passed through a slender iron wire, the wire is ignited or dispersed in red hot balls, a greater or less length is burnt, according to the quantity of coated surface, and the height of the charge.

Exp. A very large body of electric fluid, passing rapidly over a very small surface, must partly force its way by displacing a considerable portion of air on every side of its path, by the caloric excited in the air partly, and partly by the caloric contained in the electric fluid, and partly by that excited in the iron by the strong action in so small a space, the effect is produced; and a stronger charge will melt a greater length of wire, because of the greater quantity of fluid, and the greater velocity and compression on the wire.

The various phenomena we have already explained, include the leading facts in common Electricity, those which relate to its chemical actions will be found in the next section of Galvanism; and some relating to atmospheric Electricity in sect. ix. A great many others might have been brought forward, but the most general and difficult have been presented to the Reader, and he will now find it easy to explain all the particular cases on the same principles, referring them to their proper heads in this section.

SECTION VIII.

GALVANISM.

GALVANISM, or voltaic electricity, which has also been called hydro-electricity, is a branch of science of modern date; it originated from some electrical experiments performed by S. Galvani, professor of anatomy at Bologna ; this philosopher published an account of his discoveries in 1791, and they were repeated, varied, and extended by Volta, Valli, Fowler, Hombolt, Monro, Robinson, and others. Signor Volta published an account of his pile constructed of dissimilar metals in contact with each other, and having cloth wetted in salt and water placed between each pair of the metal plates: from a large combination of this kind many important and surprizing facts were discovered. From this time the science was cultivated with great interest, both by the Continental and British philosophers several additional very important facts respect

ing the chemical agency of Volta's pile were very soon discovered by Messrs. Nicholson and Carlisle, Sir H. Davy, Cruikshank, Dr. Wollaston, Dr. Henry, &c. &c. Several variations and alterations were made in the form of the apparatus, and a multitude of new results were published to the world in succession: the principal of these are selected, laid before the Reader, and explained in this section.

Contact of Metals.

PH. 1. If a clean smooth plate of zinc be applied to a similar plate of copper, by means of insulating handles, after the separatiou the zinc is found to be electrified positively, and the copper negatively. The like happens in general when dissimilar metals, or various other dissimilar bodies are employed.

Exp. Particles of air, particularly of oxygen, adhere to the surfaces of metals, and the more so as the metals are the more bright and polished, but, according to the laws of affinity, the atoms of oxygen will be most closely attached to the most oxidable metal, and more loosely to the other; and therefore the atmospherules of ethereal matter on the adhering air will be greatest in the latter case, and hence, when the zinc and copper surfaces are brought together, several atoms of oxygen will leave the copper and adhere to the zinc, bringing with them their atmospherules, partly composed of electric fluid, and from their nearer application to the zinc, than that which they had with respect to the copper, will render the zinc

positive, and leave the copper negative, or if the atoms of gas do not actually leave the copper to adhere to the zinc, they will be so far carried towards the zinc, that a portion of their atmospherules will pass over. The same explanation holds in other cases exhibiting the phenomena in a higher or lower degree according to circumstances.

PH. 2. If a concave plate of copper be pierced with holes, and zinc filings be sifted through, the copper being insulated, the filings will be positive and the sieve negative; if the sieve be zinc and the filings copper, still the zinc will be positive and the copper filings negative.

Exp. This is explained as the preceding phenomenon, but the effect will here be greater, because of the greater surface, and still more on account of the angular points, of the filings.

Excitement of the Electricity.

PH. 3. AB, fig. 68, is an oblong trough divided by partitions into a number of cells (in the figure the front side ABCD is supposed to be removed in order to shew the plates, c, &c. and the interior of the cells, which are here represented wider than necessary); c, c, c, &c. are

plates of copper, and z, z, &c. plates of zinc, in the se

veral cells, the copper plate of one cell being connected with the zinc of the other, by metallic arcs, as a, a, a, &c.

1 2 3

the several pairs being arranged in the same order. Now the cells being filled with water, or water acidulated with sulphuric, muriatic, or nitric acid; on examination it is found, that if the cell, in which is the copper of the first