ELECTRIC DISCHARGES IN GASES AT LOW PRESSURES. 
429 
The values of the minimum spark potentials obtained in these experiments for the 
different gases are given in the following table :— 
Gas. 
-Minimum spark 
potential iii volts. 
IT, 
278 
()., 
455 
IloS 
414 
c6.> 
419 
NAG 
418 
SO.. 
457 
C.H.. 
4G8 
Owing to the special precautions taken by Strutt"^ to obtain an accurate value 
for the minimum spark potential in nitrogen, measurements were not taken witli 
this gas. Adopting Strutt’s value of 251 volts for nitrogen, it will ])e seen that, 
with tlie exception of oxygen, all the minimum s})ark potentials given above obey 
an additive law ; that is, if H', N', O', &c., represent the si^rk potential constant 
corresponding to an atom of the gases H,, No, Oo, &c., respectively, the minimum 
spark potential for any compound gas whose fnanula is H., . . 0-, &c., will be 
equal to xW -h yN' + 'O' + &c. 
If we assume the truth of this law and calculate H', N', O', &c., from the 
minimum spark potentials for Ho, No, HoS, SOo aiid COo we find : 
H' = 139, N'=12G, C' = 98, S'= 13G, O' = IGl, 
* Strutt, ‘Phil. Trans.,’ A, vol. 193, p. 385, 
