Philosophical transactions, Vol. L. Part I. For the year 1757. · Various
+LETTER II.+
_To the very honourable and learned the_ PRESIDENT _and_ MEMBERS _of the_ Royal Society. [Read May 5, 1757.] Gentlemen, EVERY theoretical inquiry, whose basis does not rest upon experiments, is at once exploded in this well-thinking age; where truth, under your patronage, has at length broke thro’ those clouds, with which superstition, policy, or parade, had overwhelmed her. But experiments themselves, gentlemen, are not exempted from fallacy. A strong inventive faculty, a fine mechanic hand, a clear unbiassed judgment, are at once required for the contrivance, conduct, and application, of experiments; and even where these are joined (such is the condition of humanity!) error too frequently intrudes herself, and spoils the work. My very respectable antagonist, Mr. Eeles, to whose ear, I am convinced, the voice of truth is more agreeable than that of applause, will forgive me the following critique on his performance; as by that means, I am persuaded, the probability of his notions will be intirely destroyed, and the foregoing theory receive additional supports. For this purpose our first endeavour will be to shew the uncertainty of some of the most material principles, that support his arguments; and afterwards, the fallacy of the experiments he has given us. First then, in page 130. Mr. Eeles has asserted, that the greatest possible rarefaction of water is when it boils. I think 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; so 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.
In page 133. 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), I know no experiment to show it. If it be meant, that it is capable of being attracted to a greater distance; I conjecture 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 (page _ib._) “that electric fire will not mix with air:” whence, in the succeeding section, it is argued, “That as each particle of vapour, with its surrounding 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 thro’ 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 thro’ air, and, I am persuaded, 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 cieling 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, thro’ the oil into the air above, high electricity was given. The oil did not appear at all to subside: whence I conclude, 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. This experiment I formerly tried various ways, as I had conceived, if the electric matter would displace air, it might have been applied to answer the end of steam in the steam-engine, and many other great mechanical purposes. But as from the above it appears, that the contrary is true, it is evident, that electric matter surrounding particles of vapour must, in fact, increase their specific gravity, and cannot any-ways be imagined to facilitate their ascent.
I may add further, that if this be true, that it pervades the pores of air, its specific levity cannot, by any means I know, be compared with that of air. Its particular attraction to some bodies, at least to much the greater part of the terraqueous globe, is abundantly greater than that of air to those bodies: and hence its gravitation to the whole globe would appear, at first view, to exceed that of air. But the more I consider this, the more perplexing and amazing it appears to me: and thence must leave it to the investigation of my very ingenious antagonist, or some other able philosopher. I come now to the experiments, that are given us to show all vapour to be electrifed. In these Mr. Eeles seems to have been led into error, by not having observed, that many bodies electrifed will retain that electricity for some time, altho’ in contact with conductors. The Leyden phial may be touched three or four 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 I have observed, the more slow any bodies are to acquire electricity, the more avaritious 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 an horizontal line of the same, about four yards in length, was electrified with a dry wine-glass, according to the method of Mr. Eeles; and, after being touched nine times with my finger, at the intervals of two seconds of time, still manifested signs of electricity, by being attracted at the tenth approach of it.
A cork ball, on the same line and circumstances, after being electrised, was touched at the intervals of ten seconds repeatedly, for seven times, before it was exhausted. The fumes of boiling water were conveyed upon this ball after being electrised; and, after a fumigation for thirty seconds, it shewed signs of electricity, by being attracted to the approaching finger; and, after thirty seconds more, without any fumigation, it again obeyed the finger; and again, after thirty more, but at less and less distances. The same appearances occurred to me from the fumes of resin. From whence I apprehend, that Mr. Eeles, having dipped the electrised 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 had 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 shew this was many times repeated with constant success; _viz._ the cork ball, suspended as above, after being electrised by the wine-glass, and repelled from it, was strongly attracted by a rubbed stick of sealing-wax; and _vice versâ_. In the same manner I 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 me 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 rubbing glass and wax together, the glass accumulated on its surface the identical æther that the wax lost. Nor is this a digression from my design: for if this opposition of the electricity of glass and wax be established, it still contributes to demonstrate the fallacy of Mr. Eeles’s experiments. But what alone would intirely destroy this electric hypothesis, is, that from the experiments of Mr. Franklin and others, the clouds are sometimes found to be electrised _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 upon 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, in page 140. tells us, that himself has passed thro’ 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, I am persuaded, gentlemen, you will be induced to conclude, that tho’ 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. I am, GENTLEMEN, Your very humble Servant, Erasmus Darwin. Litchfield, March 23. 1757. [Illustration: _Philos. Trans. Vol. L._ TAB. VI. _p. 255_. _The grey Coot-footed_ Tringa _shot near Halifax in Yorkshire (January 1757) and Presented to me by Mʳ. Thomas Belton Florist of Worley-clough in Yorkshire. Drawn from nature of the Figures of Life by Geo. Edwards in Feb.⁴ 1757._] XXXI. _An Account of new-discovered Species of the Snipe or Tringa: In a Letter to the Rev._ Tho. Birch, _D.D. F.R.S. from Mr._ George Edwards, _Librarian of the College of Physicians_. [Read May 5, 1757.]
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