opposite electric fluids dependent on each other; that they may be uncombined by the power of the machine, and occupy the opposite sides of a jar or any other charged glass; but by means of a conducting communication, they avail themselves of the opportunity of reuniting with a promptitude unparalleled by any other kinds of matter, and thus, whilst travelling in opposite directions, produce all the effects of electric forces, the double protrusion of the card being supposed to be an instance of the effects of their opposite actions.

If we vary this experiment by perforating several plies of writing paper instead of the single card, we find that the protrusion of the paper from the centre of the pile is in opposite directions, and that no individual ply of the paper has the protrusion, or bur, as it is often called, on both sides. Many attempts have been made to explain this phenomenon, though, it must be acknowledged, not very satisfactorily. Those who advocate the existence of two electric fluids think their views triumphantly demonstrated by the double bur experiment, whilst others consider that expansion is the real cause even of both burs, which is by far the most probable view, though it generally happens that the bur or protrusion on the positive side is smaller than that on the negative side.

If, instead of a card or several plies of paper, we were to set up a piece of tin-foil between the balls of the universal discharger, a discharge from 'the jar will perforate the foil. But any of these articles can be much easier penetrated when the expansion of the electric fluid is more limited, than whilst passing through the atmospheric air; that is, by surrounding the interruption with more dense fluids, such as oil, or even water.

If we immerse a pointed wire, bent as in the experiment with the phial of oil, to some depth in a tumbler of water, and opposite to the point place a thin piece of metal, such as thin sheet brass, then by passing a series of sparks from the prime conductor through the pointed wire to the thin metal, the latter will become perforated by the mechanical action of sparks which passed from the point. When the sparks are passed through oil instead of water, they are still more compact by a less lateral expansion, and accomplish the perforation sooner. The lateral forces, however, in all these cases, are still considerable, and mechanical effects are produced by them as decidedly as by the direct action.

It must be here noticed, however, that if a bullet be discharged from a gun through a sheet of copper, effects are produced analagous to those exhibited by the card or paper by an electrical discharge. I have lately received from my friend, Mr. Marsh, of Woolwich, two penny pieces which were shot at whilst flying in the air; and they are both pierced by the leaden shot; one of them by a bullet from a rifle, which has taken a piece out of the side of the coin, to the depth of little more than half a diameter of the ball, leaving a bur on each side of the metal, round the crescented margin of the semi-lunar breach in the edge of the coin. The side which was hit by the

bullet is easily known by the copper being bent by the blow; and the bur on that side is far smaller than that on the other or off side. The other penny has been struck by seven small leaden shotperhaps the size of duck-shot-four of which have pierced the coin, and the other three have made deep indentations, but not passed through. In this, as decidedly as in the former case, the bur occurs on both sides, but less on the near side, or that which the bullet first struck, than on the other.*

In many electrical inquiries experimenters have found it necessary to augment the force beyond that which can be accumulated by a single jar, however large; which has given rise to the formation of the electrical battery, consisting of several jars, so arranged as to have the whole of their inner surfaces united by metallic conductors, and all their outer surfaces also united by similar means. In fig. 2, we have a battery of this kind very accurately represented. The battery consists of twelve jars placed in a mahogany box, whose inside is lined with stout tinfoil; which, of course, comes into intimate contact with the metallic coatings of the

whole series of jars. From the inside of each jar rises a stout brass wire, and the twelve jars are divided into three groups of four each, by three brass rods, each of which unites the interior of four jars; and, by means of two cross wires or rods, the groups of jars can be united with each other; so that, by this contrivance, we have an opportunity of employing either four, eight, or twelve jars, as we please. The horizontal rods are terminated with well polished brass balls, to prevent loss of fluid.

When we employ the battery of jars, we bring one of the horizontal rods into connection with the prime conductor, and it is obvious that, by these means, the whole of the jars will become charged at the same time; and by connecting one of the branches of the discharging rod with a chain in connection with the lining of the box, and approaching any of the rods with the other branch, the discharge of the whole will take place. There are, however, many precautions to be taken in the employment of a formidable battery, some of which it may be necessary to point out before we proceed to experiment with it.

See Mr. Marsh's letter in this number.

OF

ELECTRICITY, MAGNETISM,

AND

CHEMISTRY;

AND

GUARDIAN OF EXPERIMENTAL SCIENCE.

MARCH, 1842.

Historical Sketch of the Production of Electricity by Evaporation, and of Evaporation by Electricity.

The rage for steam electricity having now become greatly abated, we will endeavour to collect such theoretical opinions concerning the cause as we can find in the journals of the day; and also those which were entertained many years ago.

We are not certain that Dr. Franklin was the first philosopher who entertained the idea of the electricity in clouds being taken up from the earth in its aqueous vapours; but such was his opinion, however, as early as the year 1749, as is clearly stated in his fifth letter, which is highly probable to be the first time that such an idea received publicity. From the results of many experiments on atmospherical electricity, Franklin was led to suppose that the generality of thunder clouds are negatively electrical with respect to the earth. Hence he says, in his twelfth letter, "For the most part, in thunder strokes, it is the earth that strikes into the clouds, and not the clouds that strike into the earth." And in order to give an explanation for this negative electric condition of clouds, he proceeds thus:—

"I conceive that this globe of earth and water, with its plants, animals, and buildings, have diffused throughout their substance a quantity of the electric fluid, just as much as they can contain, which I call the natural quantity. That this natural quantity is not the same in all kinds of common matter under the same dimensions, nor in the same kind of common matter in all circumstances; but a solid foot, for instance, of one kind of common matter, may contain more of the electric fluid than a solid foot of some other kind of common matter; and a pound weight of the same kind of common matter may, when in a rarer state, contain more of the electric fluid than when in a dense state. For the electric fluid Ann. of Elec.-Vol. VIII.-No. 45.-March, 1842. N

being attracted by any portion of common matter, the parts of that fluid (which have among themselves a mutual repulsion) are brought so near to each other by the attraction of the common matter that absorbs them, as that their repulsion is equal to the condensing power of attraction in common matter; and then such portion of common matter will absorb no more.

"Bodies of different kinds having thus attracted and absorbed what I call their natural quantity, i. e., just as much of the electric fluid as is suited to their circumstances of density, rarity, and power of attracting, do not, then, shew any signs of electricity among each other; and if more electric fluid be added to one of these bodies, it does not enter, but spreads on the surface, forming an atmosphere; and then such body shews signs of electricity.

"I have, in a former paper, compared common matter to a sponge, and the electric fluid to water; I beg leave once more to make use of the same comparison, to illustrate further my meaning in this particular. When a sponge is somewhat condensed by being squeezed between the fingers, it will not receive and retain so much water as when in its more loose and open state. If more squeezed and condensed, some of the water will come out of its inner parts and flow on the surface. If the pressure of the fingers be entirely removed, the sponge will not only resume what was lately forced out, but attract an additional quantity. As the sponge, in its rarer state, will naturally attract and absorb more water, and in its denser state will naturally attract and absorb less water, we may call the quantity it attracts and absorbs, in either state, its natural quantity, the state being considered.

"Now, what the sponge is to water, the same is water to the electric fluid. When a portion of water is in its common dense state, it can hold no more electric fluid than it has; if any be added, it spreads on the surface. When the same portion of water is rarified into vapour, and forms a cloud, it is then capable of receiving and absorbing a much greater quantity: there is room for each particle to have an electric atmosphere. Thus water, in its rarified state, or in the form of a cloud, will be in a negative state of electricity; it will have less than its natural quantity—that is, less than it is naturally capable of attracting and absorbing in that state. Such a cloud, then, coming so near the earth as to be within the striking distance, will receive from the earth a flash of the electric fluid; which flash, to supply a great extent of cloud, must sometimes contain a great quantity of that fluid."

Again, our philosopher states :-" Thus thunder-clouds are generally in a negative state of electricity compared with the earth, agreeably to most of our experiments; yet, as by one experiment we found a cloud electrized positively, I conjecture that, in that case, such cloud, after having received what was, in its rare state, only its natural quantity, became compressed by the driving winds, or some other means, so that part of what it had absorbed was forced out,

and formed an electric atmosphere around it in its dense state. Hence it was capable of communicating positive electricity to any rod."

The principal object for bringing forward these quotations, is to shew that, in the opinion of this great philosopher, the aqueous vapour rising from the surface of the earth is negatively electrical with respect to the natural electrical condition of the water from which it ascended. There are, however, other topics of high theoretic importance contained in these few paragraphs. One of the great beauties in Franklin's theory is, that, under natural circumstances, each peculiar kind of matter is possessed of a specific quantity of the electric fluid, an idea parallel to that of Dr. Black, respecting the specific heat of bodies; and may eventually, perhaps, become of parallel practical importance. In support of this part of his theory, Dr. Franklin invented a very interesting experiment with a can and chain, now well known amongst electricians. A brass chain, of some few yards long, was coiled in a silver can, and a certain degree of electric action, measured by an electrometer, was given to this apparatus by a spark from a machine. One end of the chain was now lifted up, by a silk thread previously attached to it, until most of the chain was out of the can. With this increased surface the electric tension became much lessened; but when the chain was again let down into the can, the electric tension increased almost to its original degree.

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"Thus," says Franklin, we see that increase of surface makes a body capable of receiving a greater electric atmosphere. But this experiment does not, I own, fully demonstrate my new hypothesis; for the brass and silver still continue in their solid state, and are not rarified into vapour, as the water is in the clouds. Perhaps some future experiment on vapourized water may set this matter in a clearer light. One seemingly material objection arises to the new hypothesis, and it is this. If water in its rarified state, as a cloud, requires, and will absorb, more of the electric fluid than when in its dense state as water, why does it not acquire from the earth all it wants at the instant of its leaving the surface, while it is yet near, and but just rising in vapour

?"

In this doubtful position, we believe, Dr. Frankin left the subject, and it was not till many years after that Volta set the matter somewhat at rest, by producing electric action on the conversion of water into steam. In Franklin's 59th letter, dated Paris, 1767, we find that he has changed his opinion respecting the electricity of the clouds; for he there says "The clouds have often more of this fluid in proportion than the earth; in which case, as soon as they come near enough (that is, within striking distance), or meet with a conductor, the fluid quits them and strikes the earth."

About the year 1752, the Abbé Nollet pursued a series of experiments on the evaporation of water and other liquids by electric agency. The following were the results :

: