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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 rarefied into vapor, 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 rarefied 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 very great quantity of that fluid...
Or such a cloud, passing over woods of tall trees,
may, from the points and sharp edges of their moist top leaves, receive silently some supply.
A cloud, being by any means supplied from the earth, may strike into other clouds that have not been supplied, or not so much supplied ; and those to others, till an equilibrium is produced among all the clouds that are within striking distance of each other.
The cloud thus supplied, having parted with much of what it first received, may require and receive a fresh supply from the earth, or from some other cloud, which by the wind is brought into such a situation as to receive it more readily from the earth.
Hence repeated and continual strokes and flashes, till the clouds have all got nearly their natural quantity as clouds, or till they have descended in showers, and are united again with this terraqueous globe, their original.
Thus, thunder-clouds are generally in a negative state of electricity compared with the earth, agreeable 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 denser state. Hence it was capable of communicating positive electricity to my rod.
To show that a body in different circumstances of dilatation and contraction is capable of receiving and retaining more or less of the electric fluid on its surface, I would relate the following experiment. I placed a clean wine-glass on the floor, and on it a small silver can. In the can I put about three yards of brass chain; to one end of which I fastened a silk thread, which went right up to the ceiling, where it passed over a pulley, and came down again to my hand, that I might at pleasure draw the chain up out of the can, extending it till within a foot of the ceiling, and let it gradually sink into the can again. From the ceiling, by another thread of fine raw silk, I suspended a small light lock of cotton, so as that when it hung perpendicularly, it came in contact with the side of the can. . Then, approaching the wire of a charged phial to the can, I gave it a spark, which flowed round in an electric atmosphere; and the lock of cotton was repelled from the side of the can to the distance of about nine or ten inches. The can would not then receive another spark from the wire of the phial; but, as I gradually drew up the chain, the atmosphere of the can diminished by flowing over the rising chain, and the lock of cotton accordingly drew nearer and nearer to the cân; and then, if I again brought the phial wire near the can, it would receive another spark, and the cotton fly off again to its first distance; and thus, as the chain was drawn higher, the can would receive more sparks; because the can and extended chain were capable of supporting a greater atmosphere than the can with the chain gathered up into its belly. And that the atmosphere round the can was diminished by raising the chain, and increased again by lowering, is not only agreeable to reason, since the atmosphere of the chain must be drawn from that of the can, when it rose, and returned to it again when it fell; but was also evident to the eye, the lock of cotton always approaching the can when the chain was drawn up, and receding when it was let down again. .
Thus 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 rarefied into vapor, as the water is in clouds. Perhaps some future experiments on vaporized 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 rarefied 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 its wants at the instant of its leaving the surface, while it is yet near, and but just rising in vapor ? To this difficulty I own I cannot at present give a solution satisfactory to myself. I thought, however, that I ought to state it in its full force, as I have done, and submit the whole to examination.
And I would beg leave to recommend it to the curious in this branch of natural philosophy, to repeat with care and accurate observation the experiments. I have reported in this and former papers relating to positive and negative electricity, with such other relative ones as shall occur to them, that it may be certainly known whether the electricity communicated by a glass globe bè really positive. And also I would request all, who may have an opportunity of observing the recent effects of lightning on buildings, trees, &c., that they would consider them particularly with a view to discover the direction. But in these examinations, this one thing is always to be understood, viz., that, a stream of the electric fluid passing through wood, brick, metal, &c., while such fluid passes in small quantity, the mutually repulsive power of its parts is confined and overcome by the cohesion of the parts of the body it passes through, so as to prevent an explosion; but, when the fluid comes in a quantity too great to be confined by such cohesion, it explodes, and rends or fuses the body that endeavoured to confine it. If it be wood, brick, stone, or the like, the splinters will fly off on that side where there is least resistance. And thus, when a hole is struck through pasteboard by the electrified jar, if the surfaces of the pasteboard are not confined or compressed, there will be a bur raised all round the hole on both sides the pasteboard; but if one side be confined, so that the bur cannot be raised on that side, it will be all raised on the other, which way soever the fluid was directed. For the bur round the outside of the hole is the effect of the explosion every way from the centre of the stream, and not an effect of the direction. In every stroke of lightning, I am of opinion that the stream of the electric fluid, moving to restore the equilibrium between the cloud and the earth, does always previously find its passage, and mark out, as I may say, its own course, taking in its way all the conductors it can find, such as metals, damp walls, moist wood, &c., and will go considerably out of a direct course, for the sake of the assistance of good conductors; and that, in this course, it is actually moving, though silently and imperceptibly, before the explosion, in and among the conductors; which explosion happens only when the conductors cannot discharge it as fast as they receive it, by reason of their being incomplete, disunited, too small, or not of the best materials for conducting. ... Metalline rods, therefore, of sufficient thickness, and extending from the highest part of an edifice to the ground, being of the best materials and complete conductors, will, I think, secure the building from damage, either by restoring the equilibrium so fast as to prevent a stroke, or by conducting it in the substance of the rod as far as the rod goes, so that there shall be no explosion but what is above its point, between that and the clouds.