VACUUM, in philosophy, denotes a space empty or devoid of all matter or body.
It has been a matter of much dispute among philosophers, whether there is a possibility of producing in nature a perfect vacuum, or space void of all body whatever. The Cartesians have violently contended against the possibility of this; while the Newtonians have argued as strenuously, not only for the possibility, but even the actual existence of a vacuum. In some instances, however, the arguments on both sides have been very inconclusive. The Cartesians have argued, from the ascent of water in pumps, that nature abhorred a vacuum. The Newtonians have disproved this by showing that the ascent of water in those machines is owing to the pressure of the atmosphere, and cannot be extended beyond a certain space. In like manner the limited ascent of mercury in a barometer, shows that it is occasioned only by the pressure of the atmosphere, and not by any power of suction or abhorrence of a vacuum, which would be equivalent to the pressure of any column of mercury, however high. Another argument in favour of a vacuum is drawn from the motions of the celestial bodies, meeting with no apparent resistance in the heavenly spaces. As no body with which we are acquainted can move through a fluid without sensible resistance, Sir Isaac Newton concludes, for the reason above-mentioned, that "it is necessary the celestial spaces be void of all matter, excepting perhaps some few and much rarefied effluvia of planets and comets, and the passing rays of light." An argument of the same kind is deduced from the motion of pendulums in an exhausted receiver meeting with no sensible resistance from the medium therein contained.—Sir Isaac also contends, that the descent of gravi-
Vacuum. gravitating bodies proves the existence of a vacuum. "If all spaces (says he) were equally full, the specific gravity of that fluid with which the region of the air would in that case be filled, would not be less than the specific gravity of quicksilver or gold, or any other the most dense body; and therefore neither gold, nor any other body, could descend therein.—For bodies do not descend in a fluid, unless that fluid be specifically lighter than the body.—But by the air-pump we can exhaust a vessel, till even a feather shall fall with a velocity equal to that of gold in the open air: the medium, therefore, through which the feather falls must be rarer than that through which the gold falls.
"The quantity of matter, therefore, in a given space, may be diminished by rarefaction: and why may not it be diminished in infinitum? Add, that we conceive the solid particles of all bodies to be of the same density: and that they are only rarefiable by means of their pores; whence a vacuum evidently follows."
The strongest argument, however, for a vacuum, perhaps is, that we cannot have any idea of the possibility of motion without a vacuum. The force of this argument will be increased from the two following considerations. 1. That all motion is either in a straight line, or in a curve which returns into itself, as the circle and ellipse; or in a curve that does not return into itself, as the parabola, &c. And, 2dly, that the moving force must always be greater than the resistance.
For hence it follows, that no force, even though infinite, can produce motion where the resistance is infinite; consequently, there can be no motion either in a straight line or a non-returning curve; because, in either of those cases, the protrusion, and consequently the resistance, would be infinite.—There remains, therefore, only the motion in a revolving curve practicable; which must either be a revolution upon an axis, or an annular motion round a quiescent body; both which are, again, impossible in an elliptic curve: and consequently all motion must be in circles geometrically true; and the revolving bodies must either be spheres, spheroids, cylinders, or portions of them, exactly geometrical; otherwise their revolutions in a plenum would be impossible: but such motions, or such figured bodies, we do not know in nature. Therefore there is a vacuum.
Again, say these philosophers, that there are interspersed vacuities, appears from matter's being actually divided into parts, and from the figures of those parts: for, on supposition of an absolute plenum, we do not conceive how any part of matter could be actually divided from that next adjoining, any more than it is possible to divide actually the parts of absolute space from another: for by the actual division of the parts of a continuum from one another, we conceive nothing else understood, but the placing those parts at a distance from one another, which, in the continuum, were at no distance from one another; but such divisions between the parts of matter must imply vacuities between. As for the figures of the parts of bodies, upon the supposition of a plenum, they must either be all rectilinear, or all concavo-convex, otherwise they would not adequately fill space; which we do not
find to be true in fact. Lastly, The denying a vacuum, supposes what it is impossible for any one to prove to be true, viz. that the material world has no limits.
To every one of these arguments, however, the assertors of a plenum, now, when assailed by the discoveries concerning the electric fluid, are able to give pretty satisfactory answers. In the cases of pumps of all kinds they contend, that the space once occupied by the air is instantly filled up by the electric fluid which flows in through the sides of the receiver, or otherwise during the time the air is extracting; and they are able to give proofs of the existence of this fluid even in the most perfect vacuum which we can make. With regard to the celestial motions it is contended, that the fluids of light and electricity are the same, and that though, by means of this ethereal fluid, a very violent resistance might be occasioned, yet were its strength and agility applied to the purposes of giving motion, there is not the least doubt that every one of the celestial motions might be continued by its means; and that not only might this be the case, but there are strong reasons for believing that it actually is so.
The arguments drawn from the descent of bodies, they also contend, are inconclusive. There can only, they say, be two sources of resistance to a body moving through a fluid; one is the friction of the parts of the fluid upon one another, and the other when the fluid itself has a tendency to move in a direction contrary to the moving body. If we suppose the fluid to be totally devoid both of friction, and of any tendency to move towards a centre or otherwise, it not only can make no resistance to motion, but if once a motion is begun in it, it will continue it for ever. Therefore, in the case of pendulums vibrating in an exhausted receiver, they contend, that the medium in which they move, though equally dense as before, is yet much more devoid of gravity and of friction than the common atmosphere, and therefore the resistance of it is much less; whence the vibrations of the pendulum are much longer continued than in the open air.—The advocates for a plenum also deny that rarefaction implies a diminution of the quantity of matter in any body. They contend, that there are two kinds of matter, one of which is subject to the laws of gravity, and the other not; consequently, if bodies are compounded of those two kinds of matter, they will to us appear to be rare or dense, as the one or the other kind prevails.
As to the arguments taken from the nature of motion, we know not the figure, nor indeed, when we come to examine the subject strictly, can we have any idea of the ultimate component particles of matter; so that we must resolve every thing into the action of infinite wisdom and power in the construction of the universe. If the particles of matter are infinitely divisible, according to one of Sir Isaac Newton's suppositions; then they must yield to motion in every direction as easily as if nothing was present, provided the body moved with force sufficient to break them; and this force we may, indeed, we must suppose to be infinitely small. If, according to another of Sir Isaac Newton's hypotheses, we suppose matter to be composed of solid hard particles which cannot be broken by
Vacuum. by any power whatever, then indeed these particles must have a certain determinate figure, either spherical, concavo-convex, or some other which perhaps has not occurred to human ingenuity. If they are spheres, we are then sure that there must be what is called a vacuum disseminatum or interspersum equal to the whole quantity of matter in the universe. But there is no necessity for supposing this; nor will our inability to produce motion, or to know how it can be produced, ever be an argument for its being done in this or that particular way; nor can we at all argue with regard to the figure of the component parts of matter, or concerning the bounds of the universe; our faculties being limited to those things which are the objects of our senses, either naturally or by some kind of similitude.
It has been observed of late, that in the most perfect vacuum which can be produced, not only in an air-pump, but even in the upper part of a barometer, evaporation both of water and of mercury takes place: it may not be amiss to inquire into the manner in which this process can be supposed to go on. The phenomenon has already been taken notice of under the article EVAPORATION, no 5, and may receive a satisfactory explanation on the principles there laid down. It is there proved, that all kinds of vapour consist of an union of the principle of heat with the evaporating substance. In consequence of this union the vapours are exceeding light; and because fire may well be supposed devoid of gravity altogether, therefore they will rise till they come to a region where the medium is of equal gravity with their own. In rarefied air the vapours will still rise, though the proportion of heat they have in them must then be greater, to make them ascend in a medium of so little specific gravity. It would seem therefore, that even the medium left behind the mercury when sinking down in the barometer is still somewhat heavier than extremely subtle vapours, as they will arise even in that medium, and condense on the upper part of the tube.—It is natural to suppose that the quantity of heat contained in these vapours is very great in proportion to the quantity of terrestrial matter they contain.