433 
ELECTRIC DISCHARGES IN GASES AT LOW PRESSURES. 
2. Ihe values of the spark potentials are not influenced at any pressure by the size 
of the electrodes, provided the discharge takes place in a uniform field. 
3. Plates of iron, zinc, aluminium, and brass were in turn used as electrodes, but 
the material out of which the electrodes were made was not found to affect the 
values of the spark potentials at any pressure. 
4. When the discharge was compelled to pass in a uniform field between parallel 
plates the minimum spark potential in any gas was found to l^e a physical constant 
for that gas, being independent of the pressure and of the distance lietween the 
electrodes. 
^ 5. Evidence has been adduced which indicates that Paschen’s law is applicable to 
discharges in a uniform field between parallel plates as long as the distance between 
the electrodes is greater than the diameter of the sphere of molecular action. 
^ 6. The minimum spark potential has been shown to vary with different gases. 
Ihe results obtained with a large number of elementary and compound gases show 
that the minimum spark potential is a property of the atom rather than the 
molecule, and that for any selected gas it may be calculated by the application of a 
simple additive law. 
In conclusion, I desire to thank President Loudon for the kindly interest he has 
always shown m my work by placing at my disposal every facility the laboratory 
afforded. 
To Dr. J. C. McLennan, also, under whose immediate supervision these experi¬ 
ments were carried out, I am deeply indebted for valued assistance. I cannot 
adequately express how much I owe to Ifis encouragement and advice. 
113 JUL ISOS 
3 K 
4 
VOL. CCI.—A. 
