CONTEMPORARY ADVANCES IN PHYSICS 



681 



Going towards lower speeds from the highest here represented, one 

 sees that the curves are ascending. This is to be expected; they are 

 the prolongations of the curves for fast electrons exemplified in Fig. 1 ; 

 the cross-section for interception is steadily increasing as the corpuscle- 

 speed diminishes, the atoms (these are monatomic gases) are bigger 

 obstacles for slow electrons than for fast. But instead of always as- 

 cending, they pass through maxima and sink as the electron-speed is fur- 

 ther lowered. This is the surprise. The atoms of argon, krypton and 

 xenon are smaller obstacles for very slow electrons than for moder- 



60 



50 



40 



to 



30 



20 



to 



o 

 o 





Fig. 5 — Cross-section of xenon atom, for very low values of electron-energy. (C) 

 Ramsauer, R. Kollath, Annalen d. Physik.) 



ately slow ones. Below the maxima, there are minima and renewed 

 ascents; these are very late discoveries of Ramsauer and Kollath, just 

 confirmed by Normand and Erode; that of xenon is illustrated by 

 Fig. 5. 



The ordinates of the horizontal dashes in Fig. 4 are the values (multi- 

 plied by iVi) of (To, the gas-kinetic cross-sections of these three kinds of 

 atoms. One sees that c is several times greater than o-q at the maxi- 

 mum of each curve, several times smaller than o-o at the minimum. It 

 was thought formerly that co should be the limit which a approaches 

 as the electron-speed approaches zero, and Lenard's early results with 

 fast electrons seemed to sustain the notion ; it perished at the advent of 

 these data. 



