428 Proceedings of Eoyal Society of Edinburgh. [sess. 
The curves for air, oxygen, and hydrogen given in diagram 11 
were obtained by taking the atmospheric pressure in mms. as 
abscissa, and the leakage per minute for 4 volts as ordinate. 
§ 34. Voltage necessary to produce spark at different atmospheric 
pressures. 
50 100 150 200 250 
Diagram 11. 
We found that, at ordinary atmospheric pressure, sparking took 
-place in air at 4800 volts. At 232 mms. pressure the potential 
necessary to produce a spark fell to between 1500 and 2000 volts. 
At 127 mms. it had fallen to between 1100 and 1300 volts. At 
54 mms. it was 700 volts; at 7 mms. 420 volts ; at 2 mms. about 
400 volts. At about ywoo mm - voltage had risen again to 
2000 volts. 
