V. A.
Symbol or abbreviation for voltaic alternatives, q. v.
Vacuum.
A space destitute of any substance. The great pervading substance is in
general sense the atmosphere. It is the gaseous mixture which surrounds
and envelopes the earth and its inhabitants. It consists of a simple
mixture of oxygen, 1 part, nitrogen, 4 parts, with 4 to 6 volumes of
carbonic acid gas in 10,000 volumes of air, or about one cubic inch to
one cubic foot. It presses with a force of about 14.7 lbs. per square
inch under the influence of the force of gravity. The term vacuum in
practise refers to any space from which air has been removed. It may be
produced chemically. Air may be displaced by carbonic acid gas and the
latter may be absorbed by caustic alkali or other chemical. The air may
be expelled and the space may be filled with steam which is condensed to
produce the vacuum. Of course in all cases the space must be included in
an hermetically sealed vessel, such as the bulb of an incandescent lamp.
But the universal method of producing a vacuum is by air pumps. An
absolute vacuum means the entire absence of gas or air, something almost
impossible to produce. A high vacuum is sometimes understood to mean one
in which the path of the molecules is equal in length to the diameter of
the containing vessels, as in Crookes' Radiometer and other apparatus
for illustrating the radiant condition of matter. The air left after
exhaustion is termed residual air or residual atmosphere.
[Transcriber's note: Dry air is about .78 nitrogen, .21 oxygen, .01
argon, .00038 carbon dioxide, and trace amounts of other gases. Argon
was suspected by Henry Cavendish in 1785. It was discovered in 1894 by
Lord Rayleigh and Sir William Ramsay.]
Vacuum, Absolute.
A space free of all material substance. It is doubtful whether an
absolute vacuum has ever been produced.
Vacuum, High.
An approximate vacuum, so nearly perfect that the molecules of the
residual gas in their kinetic motions rarely collide, and beat back and
forth between the walls of the containing vessel, or between any solid
object contained in the vessel and the walls of the vessel. The gas in
such a vacuum is in the radiant or ultra-gaseous state. (See
Ultra-gaseous Matter.)
Vacuum, Low.
A vacuum inferior to a high vacuum; a vacuum in which the molecules
collide with each other and do not move directly from side to side of
the containing vessel.
Vacuum, Partial.
A space partially exhausted of air so as to contain less than an equal
volume of the surrounding atmosphere. It really should come below a low
vacuum, but is often treated as synonymous therewith.
Vacuum, Torricellian.
The vacuum existing above the mercurial column in a barometer tube. The
principle of this vacuum is applied in the Geissler and other air pumps.
(See Pump, Geissler--Pump, Sprengel--Pump, Swinburne.)