of potential than
are necessary in a uniform field. For to maintain the discharge it is
not necessary that the positive ions should act as ionizers all along
their path; it is sufficient that they should do so in the neighbourhood
of cathode. Thus if we have a strong field close to the cathode we might
still get the discharge though the rest of the field were comparatively
weak. Such a distribution of electric force requires, however, a great
accumulation of charged ions near the cathode; until these ions
accumulate the field will be uniform. If the uniform field existing in
the gas before the discharge begins were strong enough to make the
corpuscles produce ions by collision, but not strong enough to make the
positive ions act as ionizers, there would be some accumulation of ions,
and the amount of this accumulation would depend upon the number of free
corpuscles originally present in the gas, and upon the strength of the
electric field. If the accumulation were sufficient to make the field
near the cathode so strong that the positive ions could produce fresh
ions either by collision with the cathode or with the gas, the discharge
would pass though the gas; if not, there will be no continuous
discharge. As the amount of the accumulation depends on the number of
corpuscles present in the gas, we can understand how it is that after a
spark has passed, leaving for a time a supply of corpuscles behind it,
it is easier to get a discharge to pass through the gas than it was
before.

[Illustration: Fig. 15.]

The inequality of the electric field in the gas when a continuous
discharge is passing through it is very obvious when the pressure of the
gas is low. In this case the discharge presents a highly differentiated
appearance of which a type is represented in fig. 15. Starting from the
cathode we have a thin velvety luminous glow in contact with the
surface; this glow is often called the "first cathode layer." Next this
we have a comparatively dark space whose thickness increases as the
pressure diminishes; this is called the "Crookes's dark space," or the
"second cathode layer." Next this we have a luminous position called the
"negative glow" or the "third cathode layer." The boundary between the
second and third layers is often very sharply defined. Next to the third
layer we have another dark space called the "Faraday dark space." Next
to this and reaching up to the anode is another region of luminosity,
called the "p