Electrical Discharge in Helium and Neon. 101 
reappear when the potential was reduced below the spark potential 
and then raised; consequently, in practice, the potential was 
raised slightly above the spark potential, causing the glow to 
appear, the voltmeter was then momentarily disconnected, a pro- 
ceeding which largely increased the potential across the tube (as 
may be seen from fig. 1) and started the normal discharge. The 
current was at once cut off and the potential reduced to just 
below the spark potential. On again switching on the current 
and raising the potential, the tube sometimes lit with the normal 
‘discharge, but very often the glow reappeared and the process had 
to be repeated. 
Unfortunately it has been so far impossible to make any 
“measurements of the currents passing through the tube under 
these varied conditions, and it is not easy to ascribe any reason 
‘for the glow discharge. It has however been remarked by the 
author in conjunction with F. W. Aston (Roy. Soc. Proc. A. 1912, 
86, p. 176), that helium appears to conduct the discharge in two 
different ways, and the occurrences under consideration appear to 
be a manifestation of the same phenomenon. It seems that an 
explanation might be afforded by the rather crude assumption 
that if the gas be considered analogous to a metallic conductor, it 
can have two (or perhaps more) different resistances. In this case 
the normal discharge would be the one corresponding to the low 
‘resistance, and the glow discharge to the high resistance, for the 
latter is exactly similar to the normal discharge when the external 
resistance is largely increased. The glow resembles the negative 
glow and appears to be on the anode because of the great size of 
the dark space owing to the low current density ; moreover four 
or five Aston dark spaces are often seen at high pressures exactly 
as in the normal discharge. It is noteworthy, however, that the 
glow discharge could not be produced from the normal one by 
decreasing the current, although at low pressures the change often 
occurred spontaneously when no alteration was made in the 
external conditions. 
On a few other occasions, yet another type of discharge 
appeared. If the potential was very slowly raised when the glow 
discharge was taking place, the glow, which was approximately a 
plane disk over the anode, became unstable, and bulging out at 
the centre, moved bodily over to the cathode where it assumed 
the form of a paraboloid having its vertex towards the cathode, 
and separated from it by a small dark space. No change in 
intensity occurred during this process until the potential was 
raised sufficiently to produce the normal discharge. I can offer 
no explanation for this. 
An interesting type of discharge could also be produced as 
follows. When the current was passing through the gas, the 
