VACUUM literally means empty space, or space wholly devoid of matter. From Aristotle to Descartes, metaphysical speculators took the question into their own hands, and, of course, wrote nonsense about it: Thus, Descartes commits the absurdity of say ing that, if a vessel be perfectly empty, its sides must be in contact—confounding the totally distinct ideas of matter and space. The dictum that nature abhors a vacuum, was employed to account for the rise of water in pumps; but it was presently found that nature did not abhor a vacuum through more than an elevation of about 32 feet. See TORRICELIA. When the subject was taken up by its legitimate owners, the experimental philosophers, such absurdities disappeared, but real dirkzilties were detected. So far as experiment has yet guided us, we may assert that vacuum cannot exist. The inter stellar spaces, though probably devoid of ordinary ponderable matter, or at best only occa sionally visited by it, are certainly pervaded by the huniniferons medium. See ETHER, UNDULATORY THEORY, that this is matter (q.v.). is amply proved by the effects of its vibrations on the eye, and by the resistance which it has been discovered to oppose to the motion of Encke's comet. It is not merely for the propagation of light and heat that we are forced to assume that the universe is a plenum; Newton expressly said (see FORCE, where the quotation is given at greater length) " That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by, and through which their action and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it." Nothing could be stronger than this; and we have in addition, the results of modern observation, which show a connection between sun-spots, planetary configurations, and terrestrial magnetism, obviously requiring some material channel to exist between the sun and its secondaries. Faraday's electrical discoveries tend to the same conclusion.
But, in ordinary language, a vacuum is said to be produced (more or less perfect) when ordinary ponderable matter, such as air, is more or less completely removed from the interior of a closed vessel. Till the commencement of the present century, the most perfect vacuum that could be obtained was what is called the Torricellian the space above the mercury in a carefully filled barometer-tube. Such a vacuum; how ever, is almost useless for experimental purposes, and, besides, it contains mercurial vapor.
A suggestion of Davy's, recently re-invented and greatly improved by Andrews, gives the means of procuring a much more perfect vacuum than the Torricellian. An ordinary air-pump removes all hut about the of the gas in the t eceiver—i. e., produces a vacuum of about inch, as it is called. But if the gas employed be carbonic acid, admitted and pumped out several times, so as to get rid, as far as possible, of the last trace of air, the remaining gas will be almost wholly taken up by means of moistened caustic potash pre viously placed in the receiver. Concentrated sulphuric acid should also be present to desiccate the potash when it has done its work. in this way, Andrews easily obtained a vacuum of of an inch, which remained unchanged for a fortnight. Here all but 1 of the air had been removed. Further improvements, devised by Frankland, , Gassiott, and others, have been made in this process, especially for the production of (so-calico) vacuum-tubes for the study of electrical discharges;•and the exhaustion has been sometimes carried so far that the attenuated matter remaining was unable to con duct the discharge of an induction-coil.