Vacuum literally means space empty of matter—i.e. empty of those ordinarily recognised realities whose properties are the objects of our perception. To empty a region once filled with matter is a practical impossibility. The Air-pump (q.v.) enables us to remove from the interior of a vessel a large fraction of the air originally contained therein. By other devices we may to a still greater degree reduce the quantity of gaseous matter filling the region; but even with the most efficient means we find it impossible to get rid of a last residuum. Thus the ideal vacuum is unattainable. The word, however, is used as applicable to the approximate realisation of this absolute emptiness, and the smaller the residuum left the higher is the vacuum said to be. Across such vacua light passes, and magnetic and electrostatic inductions take place with even greater ease than if the region were filled with air at ordinary pressure. Hence we conclude that a vacuum is after all a plenum, not of matter, in the ordinary acceptance of the term, but of some substance capable of transmitting energy. This substance we call the Ether (q.v.).
Besides the ordinary air-pump, there are several forms of apparatus useful for producing vacua. The most efficient of these are the various modifications of the Sprengel pump. In its simplest form the Sprengel pump consists of a long vertical glass tube of narrow bore, down which mercury is allowed to flow. The region to be exhausted is connected by an oblique tube with the vertical tube, at a point some 30 inches (the barometric height) above the lower end of the latter. As the mercury streams down the vertical tube the pressure at the place where the oblique tube enters tends to be less than the atmospheric pressure by an amount equal to the pressure of the mercury column from this place downwards. The air is therefore pressed out of the side tube and connected vessel into the vertical tube, and passes down with the mercury stream and escapes at the lower end. This process goes on until nearly all the air in the connected vessel has been carried away. It is convenient to measure the pressure of high vacua in millionths of atmospheres. With the most improved form of air-pump with valves and cylinders the highest vacuum attainable is 150 times the millionth of an atmosphere, whereas with an improved Sprengel pump it is possible to get a vacuum whose pressure is only 0.005 of the millionth of an atmosphere.