The New Student's Reference Work/Vacuum

Vacuum (văk′ū́-ŭm), the region in a closed vessel from which the air has been largely removed. To completely remove all gases from a closed vessel is impossible. To obtain a complete vacuum appears therefore to be a hopeless task. With an ordinary air-pump, in good working condition, one may produce a vacuum in which the pressure is not more than two or three millimeters of mercury. But by means of a mercury pump one may easily exhaust a vessel until its pressure is only one-millionth that of the atmosphere. If, however, elaborate precautions are taken with a Sprengel pump, a vacuum of only one hundred-millionth that of the atmosphere may be obtained. See Air Pump, and read Kimball's Physical Properties of Gases, chap. v.