corpuscles forward with great
velocity and ionize the gas. These corpuscles will behave like those
shot from the cathode and will accumulate again at some distance from
their origin, forming the bright head of the next striation, when the
process will be repeated. On this view the bright heads of the
striations act like electrodes, and the discharge passes from one bright
head to the next as by a number of stepping stones, and not directly
from cathode to anode. The luminosity at the head of the striations is
due to the recombination of the ions. These ions have acquired
considerable energy from the electric field, and this energy will be
available for supplying the energy radiated away as light. The
recombination of ions which do not possess considerable amounts of
energy does not seem to give rise to luminosity. Thus, in an ionized gas
not exposed to an electric field, although we have recombination between
the ions, we need not have luminosity. We have at present no exact data
as to the amount of energy which must be given to an ion to make it
luminous on recombination; it also certainly varies with the nature of
the ion; thus even with hot Wehnelt cathodes J. J. Thomson has never
been able to make the discharge through air luminous with a potential
less than from 16 to 17 volts. The mercury lamps, however, in which the
discharge passes through mercury vapour are luminous with a potential
difference of about 12 volts. It follows that if the preceding theory be
right the potential difference between two bright striations must be
great enough to make the corpuscles ionize by collision and also to give
enough energy to the ions to make them luminous when they recombine. The
difference of potential between the bright parts of successive
striations has been measured by Hohn (_Phys. Zeit._ 9, p. 558); it
varies with the pressure and with the gas. The smallest value given by
Hohn is about 15 volts. In some experiments made by J. J. Thomson, when
the pressure of the gas was very low, the difference of potential
between two adjacent dark spaces was as low as 3.75 volts.

_The Arc Discharge._--The discharges we have hitherto considered have
been characterized by large potential differences and small currents. In
the arc discharge we get very large currents with comparatively small
potential differences. We may get the arc discharge by taking a battery
of cells large enough to give a potential difference of 60 to 80 volts,
and