Part 2
As _light_ may be produced by other methods than have been yet described, as from _rotten wood_, _fish_, _meat_ &c. I shall endeavour to give an account, how, and in what manner, that _light_ is produced. Mr. _Boyle_ (in vol. 2. p. 233. of the epitome by _Boulton_) tells us, that by putting _rotten wood_, _fish_, _flesh_ &c. in the receiver of an air-pump, and then extracting the gross air, they all, in a little time, lost their _shining_ or _luminous_ quality; but, upon a re-admission of air, this _luminous_ quality returned. From which we may learn, that the air is essentially necessary to continue this _light_; and, of consequence, that this _light_ is produced by the _action_ and _re-action_ of the air on the body, which cause an _intestine motion_ of its parts; and thereby the emission of the said light; for when any thing begins to ferment and _putrify_ (which cannot be performed in any place, but where the gross air is present) the _intestine motion_ of its parts is thereby _increased_; which _motion_ acting upon the air pressed into it, the air is thereby _ground_ very small, and by the continual pressure of the atmosphere is sent out so small, as to be in form of _light_.--In the common _fermentation_ of liquids, when it rises to any height, those particles of _light_ being _stopped_, and _entangled_ by the _watry parts_ of the fermenting mass, thereby produce _heat_; and some mixtures will produce _light_, _heat_ and _flame_ on fermentation; as _steel_, _water_, and _sulphur_.
By these experiments we have a clear idea of the means and method by which _fermentation_ is performed, which operation has been hitherto unintelligible. From hence we may likewise see the reason why, in an air either _too hot_ or _too cold_, _fermentation_ cannot be performed; for in an air _too cold_, the pressure is _too great_, and also the grains of air _too large_ to _enter_ and _divide_ the thing to be _fermented_; by which means the _motion_ of its parts requisite to _fermentation_ is prevented; but in air that is _too hot_, there must be just the contrary effect, for there the parts of the thing to be _fermented_ are so far _expanded_, and the air so _rarified_, as to pass through the body with little or no obstruction, and of consequence to produce little or no _intestine motion_ of its parts; and that there is in _fermentations an intestine motion_ of the parts of a _fermenting_ liquid, is easily discernible by the naked eye. This also gives us the reason, why the _glow-worm_ shines only in the summer months, when the air is greatly _rarified_, or _divided_, and then requires but a small _motion_ to give it the form of _light_.
The _light_ proceeding from _diamonds_ &c. seems to be easily accounted for from the _motion_ of the _airs_; that there is a _perpetual motion_ of the _airs_, may be proved by the following experiment. In a still place, suppose a close darkened room, illuminated only by one small hole in the window shutter; where, if a person at rest views the enlightened current of air sideways, as it extends along the room, he will perceive a _surprising motion_ of the dusty atoms therein, _perpetually rolling and tossing_ about _with great rapidity_. This _motion_ must be very _considerable_, as it is _perpetual_, and pushed on with the power of _gravity_ in the air equal to a column of water 33 feet high. By this _action_ on _diamonds_, whose _pores_ are very _small_, and the bodies themselves of such a _make_ and _hardness_, as that, when the air is pressed on them with so great a _force_ as that of the _incumbent atmosphere_, the air is _broke exceedingly small_, and, by the continuance of the _pressure_, is sent off from the body in form of _light_. We likewise find from experiments, that where _diamonds_, or other bodies of that kind, have not their outward parts of a proper _make_, or their _substance_ of a _sufficient hardness_ or _closeness of parts_, to emit _light_ with no greater _force_, than the pressure of the _atmosphere_ only, then this deficiency may be supplied by _rubbing_; by which means the air is _ground_ to pieces, and emitted in form of _light_, as is shewn in all electrical experiments.
One great property of _light_, or, as it is commonly called the _electrical effluvia_, is (as Mr. _Hauksbee_ has proved from many experiments,) that it passes _through glass_, as water does through a sieve, or as if no body of matter interposed. This _light_ of itself, when _pure_, or _simple_, hurts or destroys no material bodies; but when _pushed_ on by the following air with _great violence_, and _meeting_ with a _great resistance_ from an impeding body, the _conflict_ is so great, that if the following air _presses_ on with _sufficient violence_, it _dissolves_ or _destroys_ the _solidity_ of any material body. As all bodies become _solid_ according to the _temperature_ of the surrounding air; some retaining their _solidity_ in such a degree of _heat_, as _metals_ of most kinds; whilst others require a particular degree of _cold_ before they become _solid_, as _water_ &c. which when froze is as much a _solid_ as any thing else--this proves that the _solidity_ of bodies depends on the _temperature_ of the air, as will be more fully proved hereafter.
The glass globe, with the electrical experiments, seems to give a very clear idea in what manner the _sun_ is _supported_, how this _terraqueous globe_ and the rest of the _planets_ are made to _move_, and _continued in motion_; and also, what is the cause of the _attraction_ of the _sun_, _earth_, _moon_ and the rest of the _planets_.
That the _sun_ is the chief, material, ruling agent, is very clearly revealed; as are some of its chiefest actions on the planets and the rest of the universe. For, in the sun we find a power of melting, dividing and (with the assistance of the air) expanding and sending out the air, first in form of _fire_, and afterwards in that of _light_; which power seems to be so regulated, as to be able to reach the extremities of the universe, where it is _congealed_, _condensed_, and _returned_ back again to the _sun_, to be again _melted_, _divided_, and sent out as before. This action seems to be represented by water in a _still_, for there the fire, forcing its way through the water contained in the _still_, carries off some of the watry particles with it; and when it has reached the head of the _still_, if it has no passage into the outward air, and the _action_ of the fire is not great enough to break the _still_, it is by the coldness of the surrounding air (or as is commonly the case by the coldness of water placed thereon for that purpose) _condensed_ and _returned_ into the water contained in the body of the _still_, there to be _rarified_, _expanded_, and sent off in _steam_, as before.
We must suppose the streams of _light_ to be always acting on one side of this terraqueous globe, and there _dividing_, _expanding_ and _rarifying_ the air; the consequence of which must be a kind of _vacuum_, which is immediately filled by the _pressing in_ of the _congealed air_ from the extremities; and as the earth is placed in this fluid of airs, we may as easily conceive that the air pressing in as above, must, with it, turn round the earth, in the same manner, and with as much ease, as water does a common mill-wheel, or the air a wind-mill. By these powers also the terraqueous globe is kept _solid_ and _entire_; for, was this force once taken off, it would soon drop to _atoms_, notwithstanding the pretended _attracting_ power of its materials.
To confirm what is here advanced, I shall bring an experiment (which Mr. _Boyle_ has laid down as a paradox) which proves that this terraqueous globe has no _attraction_, nor any _solid_, falling towards it, any _gravitation_; but that all bodies _are forced_ to it by the _incumbent atmosphere_, as much as water is _forced_ up a pump by it (the atmosphere) which, till the time of the great Mr. _Boyle_, was imagined to be performed by, what they called, the _suction_ of the pump. But this is no more than has often happened in philosophy, where when we cannot well account for any thing, we put the _effect for the cause_.
Mr. _Boyle_ found that a _solid body_, as _ponderous_ as any yet known, though near the top of the water it would sink by _its own weight_, yet if it be placed at a greater depth than twenty times its own thickness, it will _not sink_, if its descent be not assisted by the _weight_ of the _incumbent water_. To prove this, he gives us a curious experiment, viz. by keeping off the _pressure_ of the water, from the _top_ of the _sinking body_, and _sinking_ it to a proper depth, he found, that the _most ponderous_ body would be _buoyed up_, and _supported_ by the water only. See the 2d vol. of _Boulton_’s epitome, pag. 305. This experiment shews us beyond all contradiction, that the _earth_ has no power of _attraction_, nor a _descending body_ any power of _gravitation_; for if it had, the farther it was sunk in the water, the nearer it must be to the centre of the earth, and of consequence the _attraction_ must be the greater; but this, we find, is contrary to experience; so that the whole power of descending is impressed upon it by the _air_, or by other bodies forced upon it by the _incumbent air_. This experiment alone is more than sufficient to destroy the fine theories of _attraction_ and _gravitation_; it also shews us how, and by what means, two marble slabs, finely polished, are what they call _attracted_ to each other, and require a great force to separate them; a force in proportion to the breadth of the slabs; but this has been shewn by other experiments to be nothing but the _pressure_ of the _air_, or _atmosphere_; for put them into an air pump, and extract the _gross air_, and they will immediately separate from each other.
One reason, which seems to have led us into the mistake that the _solidity_, or _firmness_ of bodies is not caused by the _air_, has been, that, for the generality, we consider the _air_, or _atmosphere_ as _pressing_ only _downwards_; for if we had considered that it _presseth_ equally _every way_, as well as _downwards_, (as _Boerhaave_ in his chym. by _Shaw_, vol. 1. p. 389. has shew by the following experiment) I believe we should not have overlooked that force, or have thought it insufficient for this operation. “Fill three glass vessels, the one of a cylindrical figure, the other conical, the third bellied with a cylindrical neck; let these be filled to the brim with fair water, and covered with a single piece of paper, so as to touch the surface of the water, and by pressing it down with the hand, prevent the external air insinuating between the paper and the water; if the glasses be now inverted, whilst the paper remains close with the palm of the hand, and the hand be afterwards gently withdrawn, the water will still remain in the glasses. The same holds true, though the glasses be held horizontal, or in any other position.” As the cause of the _solidity_ or _firmness_ of bodies is the _pressure_ of the _air_ or _atmosphere_, so likewise it must depend on the _make_ and _size_ of the _pores_ of such bodies; for bodies whose _pores_ are _smallest_, must be acted upon with a greater power than those whose _pores_ are _largest_, or whose _pores_ are so _large_ as not only to admit _light_, but also _common air_ into them. This Mr. _Hauksbee_ proves by a curious experiment, for having placed two brass hemispheres, of 3-1/2 inches diameter, upon each other, and then extracting the gross air out of them by the air pump, and by these means taking off the _resistance_ of the _common air_ that was _within_ the two brass hemispheres, he says, it required 140 pound weight to separate them; this experiment with that of the two marble slabs before mentioned, is a demonstration of the _power_ that keeps _solid bodies_ from _falling_ to _pieces_. And even, if these slabs are not so _perfectly smooth_, yet the weting them with water, which prevents the gross air from _entering_, will produce the like effect. That solids _expand_ themselves by _heat_ or _fire_, is proved by heating an iron rod in the fire: in which case, it is always found to be _bigger_ and _longer_ when _hot_ than _cold_; and it was the opinion of _Boerhaave_, that cold _consolidates_ all those that are called firm bodies; that is, brings that part, which we call _body_ in them, into a _less compass_ than before, and thus _unites_ the matter thereof more closely together: by which means the _cohesion_ of the whole mass is usually increased; which makes what we call, _strength_ and _firmness_ in bodies.
Having shewn that _solid bodies_ have no power either of _attraction_ or _gravitation_, and that their _firmness_ or _solidity_ depends on the _pressure_ of the _atmosphere_; I shall now endeavour to shew, from electrical experiments, how, and by what means they _descend_ towards the earth.
What is called the _attraction_ of the _earth_ seems to be performed in the same manner as that of the _glass globe_ in electricity; the explaining of which will give us a clear idea, by what means _heavy bodies_ are _forced_ towards the terraqueous globe. In accounting for this electrical _attraction_, Mr. _Hauksbee_ seems to be very clear; for, says he, “if by the heat and rarefaction, consequent upon the attrition, the medium contiguous to the glass be made specifically lighter; then of course, to keep up the ballance, the remoter air, which is denser, must press in towards the tube, and so carry away (in the torrent) the little bodies lying in its way, thither also. The various irregularities in the excitation, or the emission and discharge of the electrical matter or light from the tube (which will be followed with proportional irregularities, in the motion and tendency of the denser air, towards the glass globe, by the hydrostatical laws) may be sufficient to account for the various uncertain motions of the little bodies carried towards the glass globe.” This account of Mr. _Hauksbee_’s being so very clear, it is a little surprising that he should allow the power of _attraction_ to matter, as in some places he does; for this is no more than in other words, telling us, that the air round the ball is _divided_ and _rubbed_ or _ground_ to pieces by the _friction_ between the glass globe and your hand, and thereby made to _expand_ itself; so the air pressing in to make up that deficiency, forces every thing towards the glass globe, that by its number of particles is not able to withstand the _current_ of _air_, pressing towards the globe. So in like manner near the surface of the earth, the sun-beams being _reflected_ by the terraqueous globe, must by these means be in a greater quantity near the surface of the earth, than at a distance from it; and so _divide_, _expand_ and _rarify_ the air near its surface, which _rarified_ or _divided_ air is forced off from the earth on all sides, by the _pressing_ in of the air from above, which must of consequence _drive_ every thing before it, towards the earth. By which we find, that the cause of bodies _descending_ towards the earth, is not from any _property_ either of the _earth_ or of the _descending bodies_; but that these are _forced_ towards that, by the surrounding _air_, in its said _motion_.
To prove that this is the method by which, in electricity, bodies are forced towards the glass globe, I shall bring an experiment or two from Mr. _Hauksbee_; and as these experiments prove to a certainty, that this is the manner of the _attraction_ in _electricity_, it will give us little room to doubt, but that the _attraction_ of the _earth_ is performed in the same manner.
Mr. _Hauksbee_ observed, that the _electrical effluvia_ were not only perceiveable by sight; but also, if the hand was held near the tube, seemed to make such sort of strokes upon the skin, as a number of fine limber hairs pushing against it might be supposed to do: and in order to find whether the electrical _attraction_ was regular and uniform, he made the following curious experiment, shewing that all bodies, not too heavy, are _forced_ (or, as is commonly supposed, _attracted_) to a cylindrical glass, equally all round, if these bodies are, as _they term it_, within the sphere of its activity.
This experiment seemed to affect Mr. _Hauksbee_ so much, that (speaking of electricity) he says, “it affords us a sort of _representation_ of the _great phænomena_ of the _universe_.” Page 53.
“For, says he, having observed (in electricity) that light bodies, placed near any part of the rubbed cylinder, seemed to be equally attracted, I contrived a semicircle of wire, which I could fasten at a constant distance, making it encompass the upper semi-cylindrical surface of the glass, at 4 or 5 inches distance. This wire had several pieces of woollen threads fastened to it at pretty near equal distances. The length of them was such, that being extended in a direction towards the center of that imaginary circle, on the surface of the glass, in the plane of which the wire was placed; they would then reach within an inch of the circumference of that circle: but if left to their own liberty, they hung in that parallel portion represented, fig. 1. The cylinder was placed with its axis parallel to the horizon; and in this posture, it was turned swiftly round; and then by the rapid motion and agitation of the surrounding air, the threads were placed into such portions, as are expressed fig. 2. viz. they were all lifted up and bent upwards from the axis of the cylinder.
“All this while, there was only the swift motion of the cylinder round its axis, without any attrition, but now when I came to apply my hand to the lower part of the glass (so swiftly whirled about) and consequently to add attrition to the former motion; the threads presently began to change their direction, and all harmoniously pointed to the center of the circle, in whose plane the wire was placed, as in fig. 3. neither were they at all disordered or flung out of that position, by the wind occasioned by that violent motion (but as if there had been no such hurry of air about them) they still persisted in their central direction; I found I could by shifting the place of the attrition hither or thither, draw the threads towards this or that end of the cylinder; but yet they all still went uniformly converging towards some center in the axis of it; so that they formed themselves into a sort of conical surface.
“Farther, if the wire with its loose threads was reverted, so as to encompass the lower part of the cylinder (as before it did the upper part) yet the effect still answered with the same exactness. For the threads were all erected into so many strait lines, still directing themselves towards a center in the axis of the glass.
“Hitherto the axis of the cylinder was placed horizontally; in the next place I set it in a vertical position, so that it stood perpendicular to the plane of the horizon; in which case I made use of a wire hoop, which was necessary to be placed parallel to the horizon, that it might encompass the cylinder, in the same manner as the semicircular wire did before: only one small part of this wire was left open, to make way for the touch of the hand, which was to give the attrition. And the wire being thus placed, it was evident that the threads (without some external force to support them) must all flag and hang perpendicularly downwards. Yet, as soon as the motion and attrition were given, the threads presently began to be extended; and as if they were become stiff and hard, formed themselves into an horizontal plane; their loose ends pointing to a center in the axis of the glass, as before.
“And thus (in all sorts of positions whatsoever, both of the wire and of the glass too) were the threads acted upon by a sort of centripetal force; to the laws of which they were always conformable.” See _Hauksbee_’s experiments, page 53 &c.
It may be observed in this experiment, that the _attractive_ power of bodies does not lie in _solids_, as has been falsly imagined, neither have such bodies any _centripetal_ or _centrifugal_ force; but that this _power_ and _force_ are given them from _without_. For, on placing the wire with the threads round the cylinder, they were all _forced_ (or as it is often called, _attracted_) towards the _earth_; but, on giving a _violent motion_ to the cylinder, they were drove from the cylinder, as if forced by a strong wind; but, by applying the hand to the glass cylinder, they were recalled, and all pointed to a center in the axis of the cylinder: and this _central_ direction might be altered at any time, by only moving the hand to different parts of the cylinder; the threads always pointing to the place where the _attrition_ was made: by which we find, that the _central_ force, both of the cylinder and the threads, are caused by the _attrition_ of the _air_ between your hand and the cylinder; whereas, at any other time, they are quite inactive: so we are assured that these _central_ forces which have been imagined to be _within_ the _solid_, are not there, but in the _air without it_.
On putting something between any of the threads and the cylinder; then, those threads would return to their first and _natural position_, viz. point towards the center of the earth. So, as Mr. _Hauksbee_ observes, “in these small orbs of matter we have some little resemblances of the _grand phænomena_ of the _universe_.”
Another thing observable was, “that by putting these threads within a glass, when they became extended, this position of the threads would be altered at any time on the approach of one’s hand, finger, or any other body, to the surface of the glass.” This is sufficient proof that the _light_, or as it is generally called, the _electrical effluvia pass_ through the _glass_, with as much ease as water does through a sieve.
One thing which seemed a little surprising to Mr. _Hauksbee_ was, that upon exhausting the _gross air_ out of the tube or globe made use of in electricity, what he called the power of _attraction_, would cease; but upon suffering the air again to enter, it returned as vigorous as before. This must be the case; for (as was before observed) whenever the gross air is extracted by an air pump, the fluid remaining is nothing but _light_. So, whenever this attrition is performed on an exhausted globe (for want of the resistance of the gross air within, to force off the particles of air ground so small as _light_, and thereby to make an _expansion_ or _rarefaction_ round the glass globe, which has been shewn to be the cause of light bodies being _forced_ towards the globe) these particles of _light_ do immediately enter the globe on one side, and _force_ out the same quantity on the other; in the same manner as water through a sieve, without ever making any _expansion_ or _rarefaction_.
Having thus shewn how and by what means the _fire_ and _light_ in electricity are produced; our next inquiry must be, why some bodies communicate this _light_ to ever so great a distance, and that instantaneously; whilst others will not; and also, why some bodies are _electrical_ and others _non-electrical_.
_Electrical bodies_ are those, whose _pores_ are so _fine_, as to admit nothing through them but _light_, or air ground to a proper fitness; as _metalls_, _glass_, _amber_, _wax_ &c. Bodies _non-electrical_, are all those, which, by the _largeness_ of their _pores_, admit, not only _light_ but also _gross air_.