VACUUM, or VOID, the name given in physics to the idea of space wholly free of matter, or perfectly empty. In the common phrase, space is called empty when, so far as sir can fill space, it is full of air ; and even in a more scientific form of speech, there is said to be a vacuum when there is only such an approach to a vacuum as the operations of philosophy can procure. Thus in the vacuum of the air pump, however long the attempt at exhaustion may be continued, there is always air left, though In a highly attenuated state; and even in the mercurial vacuum, or in the space which is left over the mercury of the barometer, there is not unfrequently a slight portion of air, and always an atmosphere of the vapour of mercury. Phynieally speaking, it is perhaps Impossible to procure a vacuum : it is most likely that, even if a real vacuum could be procured for an instant, air nr other vapour would at once begin to be disseminated from the aides of the vessel in which the vacuum was made and that the vacuum would thus instantly cease to exist.
lint the question of the existence of vacuum, in its strict and abet). lute sense, and as to whether such a thing were possible or not, was a subject of controversy from before Aristotle to after Newton. It was meant, like other questions of physics, to receive its solution from the exercise of the intellect employing itself upon the apparent pro perties of material bodies. Aristotle and others denied the actual existence of a vacuum, from a want of exact knowledge of the laws of motion. In a vacuum, says Aristotle (' Physic.; 1. iv., c. 8), there would be no reason why motion should be to one part rather than another. He apparently attributes all motion to the pressure of adjacent matter, not only in its commencement, but in its continuance. A modern philosopher would say that, even if the creation of a vacuum destroyed the cause of gravitation, still a body falling downwards into a vacuum would move through it with the velocity which it had at its entrance. Democritue, Epicurus, and others, assert the existence of a vacuum; and most of the different sects among the Greeks seem to admit the possibility of such a thing, though some of them deny its actual existence.
Descartes denied the very possibility of a vacuum, and upon such grounds as will make most persons feel that if Newton had not come, it would have been better to hare kept to Aristotle. There is iu his writings an absolute and palpable confusion between Sparc and mutter, to the extent of an assertion that the destruction of all the matter in a certain space would be the destruction of the space itself. Ile places the essence of matter in the occupation of epace, and thence infers by a wrong conversion that there cannot be space without substance (by which he means matter). As follows : " Vacuum autem philosophic° more sumptum, hoc est, in quo nulla plane sit sulstantia, dari non posse manifest= est ex co quod extensio spatii vd loci externi, non differat ab extension° corporis. Nam cum ex co solo quod corpus sit extensum, mete eoncludamus illud ease substantiam ; quia omnino repugnat ut nihili sit aliqua extensio ; idea etiam de spatio quod vacuum supponitur, est concludendum : quod nempb cram in co sit ex tensio, necessarib etiam in co sit subatantia.V (` Principia Philosophire; part ii., § 16.) " So that," he proceeds (§18), " if God were to destroy all the matter (corpus) in a certain vessel, and to permit no other to come into the piece of it (locum ablati), the sides of the vessel would be contiguous; for when nothing (nihil) comes between two bodies, they must touch each other." Matter and space are both things; but Descartes falls into the extraordinary confusion of ideas which is im plied in first adopting the common sense of the word nothing, as when we say a vacuum is full of nothing, and then arguing from the strict meaning of the word " nothing," and denying that " nothing" can have extension. It is not true, properly speaking, that there is " nothing" in a vacuum, for the very notion of a vacuum is space void of matter.