ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
107 
a flat metallic ring fitting close to the inner surface of the tube immediately beneath 
the tinfoil, and this terminal were connected with a source of electricity and played the 
part of a negative terminal to discharges within the tube. 
It is necessary here to make a remark, the purport of which has to some extent been 
assumed in what has already been stated. Although we have ascribed the sensitive 
effects directly to pulses of electricity which pass to the tinfoil or the parts of the 
conducting system nearest to the tube, we do not mean to imply that the free 
electricity in the tinfoil, or conducting system, affects the discharge by the direct 
action of the free electricity in the one upon that in the other. According to the 
theory of the authors of this paper, the effect of the pulses of electricity passing to 
the outside of the tube is always directly spent in causing a discharge, or its equivalent, 
to proceed from the interior of the tube, and it is the effect of this upon the luminous 
discharge already existing in the tube which produces the appearances observed. In 
the case of positive discharges to the outside of the tube there is no difficulty in 
accepting this, as the discharge is visible ; but in the case of negative pulses the truth 
of the theory is less obvious. Those tubes which only give a slight repulsion require 
no special notice ; the space formed by such repulsion is in all respects analogous to 
Crookes’ space, and the interior of the glass beneath the tinfoil is analogous to a ring- 
shaped negative terminal. But those which commence with very considerable 
repulsion and, as the air-spark is increased, pass suddenly into what we have called the 
discharge-relief-effect, require a separate examination. 
In such tubes, if the air-spark interval be very great, the appearance is substantially 
the same as in other tubes. The space separating the blue luminosity from the interior 
surface of the tube is considerable, and is shaped as we have already described. We 
may assume then that we have here a case of a negative ring-terminal. When 
we decrease the air-spark interval, the luminosity draws closer to the tube, until it is 
separated from it only by a small interval, and the interior of the section of the tube 
surrounded by this blue luminosity is nearly dark, such darkness not being symmetri¬ 
cally disposed, but extending beyond the tinfoil on the side nearest to the positive 
terminal of the tube, thus appearing to drive back the positive luminous column. If 
this be carefully examined, it is impossible to resist the conclusion that we have here 
the analogue of the familiar negative dark-space, so that the ring-shaped terminal 
formed on the interior of the tube has its full complement of Crookes’ space, negative 
halo, and negative dark space. And the correctness of this conclusion is further 
demonstrated by an examination of the appearances produced by placing a small piece 
of tinfoil on the tube (not surrounding it, but forming a small patch upon it) and 
joining it to earth when the air-spark is in the positive. As has been previously 
described, the blue luminosity will appear between two bright tongues which seem to 
issue from the luminous column in the tube and stretch towards the blue haze beneath 
the tinfoil. The space between these two tongues is dark. On closely examining the 
phenomenon it is seen that the dark space extends from beneath the blue haze 
