The smallest particle of matter that can exist alone. It is made up of
atoms, but an atom can never exist alone, but only, with one or two
possible exceptions, combined with one or more other atoms as a
molecule. The molecules under present conditions are not in constant
contact with each other, but are perpetually vibrating through paths, in
solids probably in defined paths, in liquids and gases in perpetually
new paths. The molecules collide with each other and rebound. This
motion is the kinetic motion termed heat. At the absolute zero–minus
273.72° C. (-460.7° F.) the molecules would be in contact and quiescent.
In the gaseous state the molecules of most substances occupy the same
volume; those of a few elements occupy one-half and of others twice the
normal volume. The mean free path of the molecule of hydrogen is about
1/20,000 mm. (1/508,000 inch) (Maxwell) or twice this length (Crookes),
the collisions in hydrogen are about 17,750 millions per second; the
diameter is about 8/10,000,000 mm. (8/254,000,000 inch) ; A particle of
matter 1/4,000 mm. (1/102,000 inch) contains, it is supposed, about
40,000 molecules. The results of different authorities vary so widely as
to deprive the subject of much of its interest. A Sprengel pump, such as
used for exhausting Geissler tubes, or incandescent lamp bulbs, may
leave only one hundred-millionth (1/100,000,000,) of an atmosphere
present, giving the molecules a capability of an average free path of
vibration 33 feet long.