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Vacuum \Vac"u*um\, n.; pl. E. {Vacuums}, L. {Vacua}. [L., fr.
vacuus empty. See {Vacuous}.]
1. (Physics) A space entirely devoid of matter (called also,
by way of distinction, absolute vacuum); hence, in a more
general sense, a space, as the interior of a closed
vessel, which has been exhausted to a high or the highest
degree by an air pump or other artificial means; as, water
boils at a reduced temperature in a vacuum.

2. The condition of rarefaction, or reduction of pressure
below that of the atmosphere, in a vessel, as the
condenser of a steam engine, which is nearly exhausted of
air or steam, etc.; as, a vacuum of 26 inches of mercury,
or 13 pounds per square inch.

{Vacuum brake}, a kind of continuous brake operated by
exhausting the air from some appliance under each car, and
so causing the pressure of the atmosphere to apply the
brakes.

{Vacuum pan} (Technol.), a kind of large closed metallic
retort used in sugar making for boiling down sirup. It is
so connected with an exhausting apparatus that a partial
vacuum is formed within. This allows the evaporation and
concentration to take place at a lower atmospheric
pressure and hence also at a lower temperature, which
largely obviates the danger of burning the sugar, and
shortens the process.

{Vacuum pump}. Same as {Pulsometer}, 1.

{Vacuum tube} (Phys.), a glass tube provided with platinum
electrodes and exhausted, for the passage of the
electrical discharge; a Geissler tube.

{Vacuum valve}, a safety valve opening inward to admit air to
a vessel in which the pressure is less than that of the
atmosphere, in order to prevent collapse.

{Torricellian vacuum}. See under {Torricellian}.