A space devoid of matter. The term is ordinarily applied to the results of the exhaustion, of air from a chamber of glass or other substance. The vacuum produced by the air pum p is far from perfect, and various means are in use to produce a more complete exhaustion. A very efficient one is the Sprengel pump, in which mercury flows down a long tube of narrow bore, and carries with it the air from a connected vessel. The best vacuum obtainable by the air pump is 150 times the millionth of an atmosphere ; while the Sprengel pump yields .005 of the millionth of an atmosphere.