VIBRATIONS OF TUBES OF FORCE 161

gases. The vibrations hitherto considered have been those
of electrical changes, of electrons for visible and ultra-violet
radiations and of positive charges for those in the infra-red.
There is, however, another possible type of vibration which is
not dependent on the motion of electrical charges, but on
the motion of the tubes of force which bind those charges
together.

Suppose, for example, that A and B are two opp6sitely
charged bodies with their charges held rigidly in a fixed
position. When in equilibrium the lines of force would be
distributed in a definite way which can be deduced from the
laws of Electrostatics. Now let this distribution be suddenly
disturbed by the passage through the field of a very rapidly
moving electric charge. The lines of force will be disturbed
from their equilibrium position where the potential energy is
a minimum, and after the moving charge has passed away
they will vibrate about this position. The possible times of
vibration of such a complex system would probably be very
numerous and would be multiples or sub-multiples of D/£
when D is the distance between A and B and c the velocity
of light. This is on the supposition that there are no bodies
in the neighbourhood of AB which when the electric field is
changing can be set in motion and absorb energy. The wave
length of the vibrations would thus be comparable with D,
the distance between the charges, and if these conditions
applied to atoms and molecules the'wave lengths of such
vibrations would be comparable with the diameters of atoms
and molecules, and so would not correspond with visible or
even ultra-violet light, In atoms and molecules, however,
the lines of force do not spread out through an empty field:
the space near the centre of an atom is crowded with electrons
whose free vibrations are exceedingly rapid; these electrons
will be affected by the lines of force in the vibrating electric

M , ,n inn a closed tube and attach the mica strip to the