162 ROYAL SOCIETY OF CANADA 
The range of pressures over which he found the law to apply, 
while considerable, did not extend below 2 cm. of mercury, and his 
results do not in any case indicate that the critical pressure had been 
reached. It is evident then that Paschen’s conclusions are confined 
to pressures higher than the critical pressures. 
Since the statement of this law by Paschen, Peace* alone seems 
to have published results which could throw any additional light on the 
conditions holding for discharge in a gas at very low pressures. Peace 
experimented in air, with parallel plates as electrodes, at various 
distances apart and found that the value of the critical pressure 
increased greatly as the distance between the electrodes was lessened, 
but his results at points below the critical pressure give no evidence 
of the existence of any such law as had been enunciated by Paschen. 
This can be readily seen from the numbers recorded in his paper, 
a few of which, selected from readings taken below the critical pres- 
sure, are given in the following table. These results admit of easy 
comparison since the potential difference in the cases chosen are very 
nearly the same. The product of pressure and spark length should 
be a constant quantity, if Paschen’s law held. 
TABLE OF PEACES RESULTS. 





Applied potential | Pressure in mm. | Distance between | Product of pressure 
difference in volts. of mercury. electrodes in inches.| and spark length. 
649 2°5 ‘082 °205 
660 6 “005 ‘030 
670 5 “O21 “105 
731 25 030 075 

If we compare the first and second of these results where the 
difference in spark potential is only 11 volts, we find the product in 
the first case nearly seven times that in the second. Again, the 
product corresponding to the spark potential, 660 volts, is less than 
one-third that corresponding to 670 volts, a large difference in the 
opposite direction. The same irregularity is exhibited by the product 
corresponding to the spark potential, 731 volts, and it seems difficult 
to understand how experimental errors could be made to explain such 
a wide divergence of results. 
At the critical pressure Peace’s results point to the existence of 
the law, but, as stated above, it would appear that as soon as lower 
pressures were approached the indications were uniformly against 

1 Peace, Proc. Roy. Soc., Vol. 52, p. 99. 
