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known, will sufficiently explain the ascent of vapours; and then, that some kinds of vapours are not endued with more or less than their natural share of electric æther. The immense rarefaction of explosive bodies by heat, depends either on the escape of air before condensed in them, or on the expansion of the constituent parts of those bodies. Where air is emitted, it cannot be condensed again into the same bulk by cold; but the expansion of the heated parts of bodies, as soon as that heat is withdrawn, ceases to exist.
Nitre comes under the first of these classes: in detonation it emits great quantities of air, not afterwards condensible to the like space. This may be seen by firing a few grains of gunpowder in an unblown bladder, or in a vessel nearly full of water with its mouth inverted. The same is true of all the solid parts of animals and vegetables, when subjected to fire; as appears from the experiments of Dr. Hales. But of water the contrary is evident. In the steamengine, a jet of cold water instantly condenses that immense rarefaction; which could not be, if it was constituted of escaped elastic air. And though this steam must be acknowledged to have some properties of air, such as ventilating a fire, or that a taper blown out by it is capable of being again lighted immediately, and that without a crackling noise, which occurs when touched with water, this does not in the least invalidate our opinion, though it has certainly conduced very much to propagate the former one: since from this way of reasoning, the whole must be air, and we should have no water at all in vapour.
From considering this power of expansion, which the constituent parts of some bodies acquire by heat; many things before utterly inexplicable, became easily understood. Such as, why when bismuth and zinc are fused together, and set to cool, the zinc, which is specifically heavier, is found above the bismuth: Why the buff covering of inflammatory blood, the scum of heated milk, the sedative salt of borax, which are all specifically heavier than the liquids in which they are formed, are still formed at their surface. How benzoin, sulphur, and even the ponderous body mercury, may be raised into vapour, again to be condensed unaltered? And lastly, how water, whose parts appear from the aolipile to be capable of immeasurable expansion, should by heat alone become specifically lighter than the common atmosphere, without having recourse to a shell inclosing air, or other assistant machinery; and when raised, to support them floating, perhaps many days in the atmosphere.
But before we proceed to this 2nd part of our task, it will be necessary previously to consider, first, how small a degree of heat is required to detach or raise the vapour of water from its parent fluid. In the coldest day, or even the coldest night of winter, when the weather is not frosty or very damp, wet linen or paper will become dry in the course of a few hours. A greater degree of heat must indeed cause a quicker evaporation. But were it not for the pressure of
the superincumbent fluid, greatly less than that of boiling water; would instantly disperse the whole so heated into vapour. Secondly, that in the opinion of Sir Isaac Newton, well illustrated by the late lamented Mr. Melvil, the sun-beams appear only to communicate heat to bodies by which they are refracted, reflected, or obstructed; whence by their impulse, a reaction or vibration is caused in the parts of such impacted bodies.
This is supported by the experiment of approaching some light body, or blowing smoke near the focus of the largest glasses; and from observing that these do not ascend, it is evident that the air is not so much as warmed by the passage of those beams through it, yet they would instantly calcine or vitrify every opaque. body in nature. And from this we may collect, that transparent bodies are only heated at their surfaces, and that perhaps in proportion to their quantity of refraction; which will further give and receive illustration from those very curious experiments, of producing cold by the evaporation of liquors, published by Dr. Cullen, in the late volume of Essays Physical and Literary, at Edinburgh. In these experiments a spirit-thermometer was immersed in spirit of wine, and being suddenly retracted, was again exposed to the air; and as the spirit of wine adhering to the glass evaporated, the spirit contained within the thermometer was observed to subside. Now as the difference of the refraction of spirit of wine and glass is exceedingly minute, compared with the difference of refraction of spirit of wine and air; we may consider in the above experiment, the heat to be communicated to the thermometer only at its surface: but here the adherent fluid escapes as soon as heated; by which means the glass, and its contents, are deprived of that constant addition of heat, which other bodies perpetually enjoy either from the sun-beams immediately, or from the emanations of other contiguous warmer bodies; and must thence, in a few minutes, become colder than before.
The use to be made of this principle, is this: That the little spherules of va'pour will thus, by refracting the solar rays, acquire a constant heat, though 'the surrounding atmosphere remain cold.' And as for the minuteness of their diameters, if they are allowed to be globules, they must do this to a very great degree, he apprehends none of these objections will take place, with which Mr. Eeles has so sensibly confuted the former received theories on this subject. If it be asked, how clouds are supported in the absence of the sun? it must be remembered, that large masses of vapour must for a considerable time retain. much of the heat they have acquired in the day; at the same time reflecting how small a quantity of heat was necessary to raise them; and that doubtless even a less will be sufficient to support them, as from the diminished pressure of the atmosphere at a given height, a less power may be able to continue them in
their present state of rarefaction; and lastly, that clouds of particular shapes will be sustained or elevated by the motion they acquire from winds.
Mr. Eeles has asserted, that the greatest possible rarefaction of water is when it boils. And it might be said, with equal propriety, that the greatest rarefaction of solids, was when they began to melt: and this may indeed be verbally true, if we chuse to alter the names of bodies, when they undergo any alteration by fire: for solids take the name of fluids, when they are in fusion; and water the name of vapour, when it is greatly rarefied in the steam-engine. Whence we find this assertion seems to be founded on a confusion in terms, and the fact far from being existent in nature.
He says also, the sphere of electrical activity is said to be increased by heat. If by electrical activity is here meant an increase of its repulsive power (the thing, which seems to be wanted in Mr. Eeles's hypothesis), there is no experiment to show it. If it be meant, that it is capable of being attracted to a greater distance; probably it may, as the heat will rarefy the ambient air, and we know the electric æther is attracted at very great distances in vacuo; but this cannot properly be called an increased activity of electric fire.
We are afterwards told that electric fire will not mix with air: whence, in the succeeding section, it is argued, That as each particle of vapour, with its sur'rounding electric fluid, will occupy a greater space than the same weight of air, they will ascend.' In answer to this, it must be observed, that there are some bodies, whose parts are fine enough to penetrate the pores of other bodies, without increasing their bulk; or to pass through them, without apparently moving or disturbing them. A certain proportion of alcohol of wine mixed with water, and of copper and tin in fusion, are-instances of the first of these; the existence and passage of light through air, and probably of electric fire, are instances of the second.
To illustrate this, the following experiment was instituted. A glass tube, `open at one end, and with a bulb at the other, had its bulb, and half way from thence to the aperture of the tube coated on the inside with gilt paper. The tube was then inverted in a glass of oil of turpentine, which was placed on a cake of wax, and the tube kept in that perpendicular situation by a silk line. from the ceiling of the room. The bulb was then warmed, so that, when it became cold, the turpentine rose about half way up the tube. A bent wire then being introduced through the oil into the air above, high electricity was given. The oil did not appear at all to subside: whence he concluded, that the electric atmosphere flowing round the wire and coating of the tube, above the oil, did not displace the air, but existed in its pores. Hence it is evident, that electric matter surrounding particles of vapour, must increase their specific gravity, and cannot any-ways be imagined to facilitate their ascent.
With regard to the experiments that are given to show all vapour to be electrized in these Mr. Eeles seems to have been led into error, by not having observed that many bodies electrized will retain that electricity for some time, though in contact with conductors. The Leyden phial may be touched 3 or 4 times by a quick finger before the whole is discharged. Almost all light, dry, animal, or vegetable substances, such as feathers and cork, do this in a much greater degree: and in general, the more slow any bodies are to acquire electricity, the more avaricious they are to keep it. Part of the plume of a feather, hanging to a green line of silk about a foot long, which was suspended from the midst of a horizontal line of the same, about 4 yards in length, was electrized with a dry wine glass according to the method of Mr. Eeles; and after being touched 9 times with a finger, at the intervals of 2 seconds of time, still manifested signs of electricity, by being attracted at the 10th approach of it.
A cork ball, on the same line and circumstances after being electrized, was touched at the intervals of 10 seconds repeatedly, for 7 times before it was exhausted. The fumes of boiling water were conveyed on this ball after being electrized; and after a fumigation for 30 seconds, it showed signs of electricity, by being attracted to the approaching finger; and after 30 seconds more without any fumigation, it again obeyed the finger; and again, after 30 more, but at less and less distances. The same appearances occurred from the fumes of resin.. Hence he apprehends that Mr. Eeles, having dipped the electrized down of the juncus bombycinus in vapour for perhaps half a minute (for no time is mentioned,) and finding it still retained its electric attraction, was not aware that this same would have happened, if he had by intervals touched it with his finger, or any other known conductor of electricity.
As Mr. Eeles had here objected, that there was no real opposition in the electric æther of glass, and that from wax; the common experiment to show this was many times repeated with constant success; viz. the cork ball suspended as above, after being electrized by the wine glass and repelled from it, was strongly attracted by a rubbed stick of sealing-wax; and vice versa. In the same manner Dr. D. observed the electric æther from a black silk stocking (which was held horizontally extended by the top and foot, and being rubbed in the midst with an iron poker, was applied to the cork ball,) to be similar to that of glass, and opposite to that of wax. But the following experiment appears to put this matter out of all doubt, and to demonstrate that this difference. is only a plus and minus of the same specific æther, and not different qualities of it, as Mr. Eeles would suppose. A stick of dry sealing-wax was rubbed on the side of a dry wine glass, and a cork ball, suspended as in the former experiments played for some time between them: but glass rubbed with glass, or wax with wax did not manifest any electric appearance. Whence it would appear, that in S
rubbing glass and wax together, the glass accumulated on its surface the identical æther that the wax lost. And if this opposition of the electricity of glass and wax be established, it contributes to demonstrate the fallacy of Mr. Eeles's experiments.
But what alone would entirely destroy this electric hypothesis, is, that from the experiments of Mr. Franklin and others, the clouds are sometimes found to be electrized plus, sometimes minus, and sometimes manifest no signs of electricity at all. Whence to say an accumulation of electric æther supports these clouds, seems an assertion built on a very unstable foundation, whose whole superstructure may well enough be termed an air-built castle, the baseless fabric of a vision. Add to this, that Mr. Eeles tells us, that he has passed through clouds resting on the sides of mountains.. Ought not those clouds to have immediately discharged their electricity, and fallen? And common experience may remind us, that any cold bodies will condense vapour, whatever be their electric properties. So mirrors, or the glass of windows in damp rooms, are most frequently found covered with dew; which, of all other bodies ought most to be exempted from collecting vapours supported by electricity, as they are the least capable to attract or draw off that æther.
From all which, well examined, Dr. D. is persuaded that though clouds may sometimes possess an accumulation of electricity, yet that this is only an accidental circumstance, and not a constant one; and thence can have no possible influence either in the elevation or support of them.
XXXI. Of a New-discovered Species of the Snipe or Tringa.* By Mr. George Edwards, Librarian of the College of Physicians. p. 255.
This specimen of a new-discovered species of the snipe or tringa kind, was lately shot at Sowerby-bridge in Yorkshire. This bird is like in shape to most others of the tringa or snipe kind. By way of distinction he names it the cootfooted tringa, as it differs from other birds of that genus no otherwise than in having its toes webbed in the same particular manner as the fulica, or our baldcoot. The bill is black, and channelled on both sides of the upper mandible; in which channels the nostrils are placed near the forehead: it is compressed somewhat like a duck's bill, and ridged along its upper part. The eyes are placed farther backward from the bill than in many other sorts of birds, in which the wisdom of Providence is remarkable: for birds of this genus commonly feeding in soft. muddy ground on the banks of rivers or the sea, have occasion to thrust their bills deep into the shores, to extract worms and insects; and their eyes would be in danger, were they placed more forward. The fore part of the head, the neck,
This bird is the tringa lobata of Linneus.