172 ROYAL SOCIETY OF CANADA 
A like conclusion must be drawn from the curve shown in Fig. 
IT, which graphically represents the numbers in Table II. In plot- 
ting this curve the products of spark lengths and discharge pressures 
were taken as abscisse and the sparking potentials as ordinates. The 
regularity of the curve which represents the products for the five 
FID. I-Air 






FPofen/ie/ Difference in ho/fs 

Fo Char Dire Poe orsionce tone Lecrocks 
different electrode distances shows clearly that there can be no doubt 
regarding the applicability of Paschen’s law to electric discharges in 
air at pressures at and below the critical point as well as to pressures 
above it. 
IV. EXPERIMENTS IN HYDROGEN. 
In order to demonstrate, if possible, the generality of the law 
which has just been proven to hold for discharges in air, a series of 
measurements were made on the spark potentials in the gases hydrogen 
and carbon dioxide. 
In these experiments exactly the same apparatus was used as in 
the previous experiments in air. 
Preparatory to making the measurements in hydrogen the appara- 
tus was first exhausted of air to a pressure of 1 mm. of mercury or less, 
and then filled with hydrogen to atmospheric pressure. It was then 
exhausted and refilled with hydrogen several times to make certain 
that all air was removed. 
