Electrical Discharge in Heliwm and Neon. 97 
be about 1/20th of that of the lower ones in fig. 2. As far as can 
be judged from his diagram, 20 times the steepness of his flattest 
curve is very slightly steeper than the first and corresponding 
portion of my curve, and in fact, the agreement is as good as can 
be expected. One remarkable effect observed by Strutt, namely 
that the spark potential was lowered by vigorous sparking, or by 
standing, was not noticed at all in the present experiments, and 
in fact the reverse was the case. It has already been mentioned 
that the first discharge passed more easily than the subsequent 
ones, and if the gas was left standing for a few hours the spark 
potential invariably rose slightly. It is quite possible that the 
reverse effect may be produced by mercury vapour, which in my 
experiments was completely excluded. 
With regard to Bouty’s experiments, it is difficult to compare 
the slopes of the curves obtained, as all the conditions in the two 
sets of experiments differ so widely. Considering the closer pair 
of electrodes in my experiments, the rate of change of spark 
potential with pressure at high pressures where the curve has 
assumed its final definite linear form is 30 volts per cm. of 
mercury in the case of helium, and 22 in the case of neon. These 
figures should be proportional to the cohésions diélectriques of 
Bouty which are 18°3 for helium and probably less than 6:0 for 
neon. It will be seen that this is not the case, but on the other 
hand, for pressures not so far removed from the critical pressure, 
the gradient for helium is about 90 volts per cm. while, as far as 
ean be judged, that for neon is not much greater than at higher 
pressures. The ratio for this part of the curve would therefore be 
nearer to the value found by Bouty. 
The Cathode Fall. 
It has recently been shown by Aston (Roy. Soc. Proc. A. 1911, 
84, p. 526), that the supposed anode fall of potential measured by 
many observers is probably an entirely illusory phenomenon 
induced by the methods of measurement, and he concludes that, 
provided there is no positive column, the cathode fall, or difference 
in potential between the cathode and the negative glow, is 
practically equal to the total voltage across the tube provided that 
the current is not large enough for the whole surface of the 
cathode to be covered with the glow. The potential fall in the 
negative glow itself is negligible. In the light of these results, 
the hypothesis put forward by Strutt as a theoretical reason for 
the cathode fall being equal to the minimum spark potential, does 
not seem very plausible, and is in no way consistent with my 
experimental results. Strutt considers that the rise in spark 
VOL, XVII. PT. I. 7 
