What is vacuum?

   Vacuum, a space that contains no matter. In the physical sense of the word a vacuum has never existed on the Earth, and its creation is thus far a practical impossibility. The space between the Earth's atmosphere and other astronomical bodies is believed to be a partial vacuum. The term vacuum is commonly used in describing a region of less than atmospheric pressure such as can be produced by exhausting vessels of air.
   The ancients were familiar with the phenomenon but expressed their observations with the non-explanatory dogma that "nature abhors a vacuum." It was not until the end of the 16th century that the physica of vacua was first explored by Galileo, who in connection with his work ou gravity predicted that a coin and a feather would fall at the same rate in a vacuum. Torricelli, Galileo's successor, produced a vacuum by inverting a covered mercury-filled tube into a dish containing mercury so that the open end of the tube was not uncovered until immersed. The level of the mercury in the tube thus inverted fell until it stood about 30 inches (762 mm.) above the level of the mercury in the dish below. Torricelli further observed the variations in the height of the mercury column with altitude and weather conditions, both related to air pressure; he thus devised the first barometer. The vacuum produced by this method is only a partial vacuum, for air dissolved in the mercury and mercury vapor itself soon find their way into the area of reduced pressure.
   Modern applications of the vacuum are innumerable. Electric light bulbs, radio (vacuum) tubes, and X-ray tubes are outstanding examples related to the transmission of light in vacuo; magnetic and electrostatic induction and electric discharge also are facilitated to a certain extent by the absence of air. All pumps depend upon the creation of a partial vacuum for their action; for example, the vacuum cleaner utilizes the flow of air under atmospheric pressure into a region of less than atmos­pheric pressure created by a motor-driven pump for the removal of dust particles. Vacuum distillation permita rapid distillation at temperatures much lower than the boiling points of liquida under atmospheric pressure, consequently it is extensively used where higher distilling temperatures might destroy the product, as, for example, in the sugar refining industry.