458 I)K. W. MANSERGH VARLEY ON THE RHOTO-EEECTRIC DISCHARGE, ETC. 
i){ either the nature or pressure of the gas. No precautions were taken to free the 
gas troni uiercuiy vapour; hut it is hoped at an early date to further investigate the 
currents at these low pressures, taking precautions to absorb all the mercury vapour 
in the leak apparatus. 
Tlie I'esults indicate that the whole of the diiferences observed in the magnitude of 
the currents in tlie various gases depend only upon secondary ionisation occurring at 
the surface of the metal and on ionisation by collision in the gas itself] 
EC. iSOS 
