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happen nearly at the same instant with the regular discharge, though really a little posterior, not being themselves discharges, but the effects of a discharge which passed in another channel .
When a strong explosion is directed through a pack of cards or a book, having a piece of tinfoil between several of its leaves, the electrical flash makes an impression in some of those metallic leaves, by which it seems as if the direction of the electric explosion had gone from the outside towards the inside, when, on the other metallic leaves, the impression is' in such a direction, that it indicates the current of electrical fire to have made its way from the inside of the phial towards the outside; so that it appears to some electricians, that, in the time of the explosion of an electrical phial, two streams of electrical fire rush at the same time from both surfaces, and meet or cross one another.
These impressions are not effects of a moving body, striking with force in the direction of its motion; they are made by the burs rising in the neighbouring perforated cards, which rise accidentally, sometimes on one side of a card, and sometimes on the other, in consequence of certain circumstances in the form of their substances or situations. In a single card, supported without touching others, while perforated by the passing fluid, the bur generally rises on both sides, as I once showed to Mr. Symmer at his house. I imagine that the hole is made by a fine thread of electric fluid first passing, and augmented to a bigger thread at the time of the explosion, which, obliging the parts of a card to recede every way, condenses a part within the substance, and forces a part out on each side, because there is least resistance.
When a flash of lightning happens to hit a flat piece of metal, the metal has sometimes been pierced with several holes, whose edges were turned some the one way and some the other; so that it has appeared to some philosophers, that several streams of electrical fire had rushed in one way, and some the opposite way. Such an effect of lightning has been published lately by Father Barletti.
This will be answered in ray remarks on Mr. Barletti's book; which remarks, when finished, I will send you.
Though, from the very charging of the Leyden phial, it seems clear, that the electrical fluid does in reality not pervade the substance of glass, yet it is still difficult to conceive how such a subtile fluid may be forced out from one side of a very thick pane of glass, by a similar quantity of electrical fire thrown upon the other surface, and yet that it does not pass through any substance of glass, however thin, without breaking it. Is there some other fact or illustration besides those to be found in your public writings, by which it may be made more obvious to our understanding, that electrical fire does not enter at all the very substance of glass, and yet may force from the opposite surface an equal quantity.; or that it really enters the pores of the glass without breaking it? Is there any comparative illustration or example in nature, by which it may be made clear, that
Vol. v. 59
a fluid thrown upon one surface of any body, may force out the same fluid from the other surface without passing through the substance?
That the electric fluid, by its repulsive nature, is capable of forcing portions of the same fluid out of bodies without entering them itself, appears from this experiment. Approach an isolated body with a rubbed tube of glass; the side next the tube will then be electrized negatively, the opposite positively. If a pair of cork balls hang from that opposite side, the electrical fluid forced out of the body will appear in those balls, causing them to diverge. Touch that opposite side, and you thereby take away the positive electricity. Then remove the tube, and you leave the body all in a negative state. Hence it appears, that the electric fluid appertaining to the glass tube did not enter the body, but retired with the tube, otherwise it would have supplied the body with the electricity it had lost.
With regard to powder magazines, my idea is, that to prevent the mischief which might be occasioned by the stones of their walls flying about in case of accidental explosion, they should be constructed in the ground; that the walls should be lined with lead, the floor lead, all a quarter of an inch thick, and the joints well soldered; the cover copper, with a little scuttle to enter the whole, in the form of a canister for tea. If the edges of, the cover-scuttle fall into a copper channel containing mercury, not the smallest particle of air or moisture can enter to the powder, even though the walls stood in water, or the whole was under water.
TO JOHN INGENHOUSZ.
An Attempt to explain the Effects of Lightning on the Vane of the Steeple of a Church in Cremona, August, 1777.
1. When the subtile fluid, which we call fire or heat, enters a solid body, it separates the particles of which that body consists .farther from each other, and thus dilates the body, increasing its dimensions.
2. A greater proportion of fire introduced separates the parts so far from each other, that the solid body becomes a fluid, being' melted.
3. A still greater quantity of heat separates the parts so far, that they lose their mutual attraction, and acquire a mutual repulsion, whence they fly from each other, either gradually or suddenly, with great force, as the separating power is introduced gradually or suddenly.
4. Thus ice becomes water, and water vapor, which vapor is said to expand fourteen thousand times the space it occupied in the form of water, and with an explosive force in certain cases capable of producing great and violent effects.
5. Thus metals expand, melt, and explode; the two first effected by the gradual application of the separating power, and all three, in its sudden application, by artificial electricity or lightning.
6. That fluid in passing through a metal rod or wire is generally supposed to occupy the whole dimension of the rod. If the rod is smaller in some places than in others, the quantity of fluid, which is not sufficient to make any change in the larger or thicker part, may be sufficient to expand, melt, or explode the smaller, the quantity of fluid passing being the same, and the quantity of matter less that is acted upon.
7. Thus the links of a brass chain, with a certain quantity of electricity passing through "them, have been melted in the small parts that form their contact, while the rest have not been affected.
8. Thus a piece of tinfoil cut in this form, enclosed in a pack of cards, and having the charge of a large bottle sent through it, has been found unchanged in the broadest part, between a and b, melted only in spots between c and d, and the part between d and e reduced to smoke by explosion.
9. The tinfoil melted in spots between b and c, and that whole space not being melted, seems to indicate that the foil in the melted parts had been thinner than the rest, on which thin parts the passing fluid had therefore a greater effect.
10. Some metals melt more easily than others; tin more easily than copper, copper than iron. It is supposed (perhaps not yet proved), that those which melt with the least of the separating power, whether that be common fire or the electric fluid, do also explode with less of that power. -
11. The explosions of metal, like those of gunpowder, act in all directions. Thus the explosion of gold leaf between plates of glass, breaking the glass to pieces, will throw those pieces into all parts of the room; and the explosion of iron, or even of water, between the joints of stone in a steeple, will scatter the stones in all directions round the neighbourhood. But the direction, given to those stones by the explosion, is to be considered as different from the direction of the light