A critical potential of methane and its absorption in the ultra-violet

In a previous paper (1) it was stated that the maximum in the current potential curve found in a three-electrode tube with methane must be due either to a resonance potential of methane or to a large transparency for slow electrons. It was further pointed out that the interpretation of maxima in current-potential curves obtained in four-electrode tubes must also take into account the possibility of abnormal transparency at certain velocities. In the meantime Brode (2) has as a matter of fact found a variable mean free path for electrons in methane using the method of Ramsauer,(3) which presumably accounts for the maximum, since it occurs at about the expected velocity.