CONTEMPORARY ADVANCES IN PHYSICS 



687 



were performed with 102-volt, with 226-volt and with 386-volt elec- 

 trons. The double-pointed peaks bear witness to two distinct amounts 

 of energy-loss frequently occurring; one maximum lies between 21 and 

 22 equivalent volts, the other is somewhat greater. Now the minimum 

 amounts of energy which the helium atom can absorb, or in other words 

 the energy- values of its lowest excited states, are 19.77 and 20.55 equi- 

 valent volts; and there are many others distributed between the upper 

 of these and the ionizing-energy, which last is 24.5. It seems natural to 



T 1 — ■ r 



60 80 100 180 200 220 240 



VOLTS 



340 360 380 400 



Fig. 10 — Distribution-in-energ}' of electrons scattered at 10° from helium atoms, as 

 determined by Dymond and Watson. {Proc. Roy. Soc.) 



ascribe the less-displaced of the two points of the shifted peak to proc- 

 esses of excitation, the other to ionization; but one is tantalized to note 

 that the peaks are not located quite accurately enough to settle this. 

 Dymond and Watson find that as the angle of deflection is increased, 

 the electrons scattered with undiminished energy take more and more 

 the lead over those which contribute to the shifted peak. 



Still other curves for helium can be seen in the article of MacMillen. 

 Analyzing the 50-volt electrons scattered at 10°, he was able to plot 

 a curve displaying no fewer than three distinct maxima, not counting 

 the great one corresponding to the elastically-reflected corpuscles — 

 evidence, therefore, of three distinct and distinctive energy-transfers. 

 The most frequent of these he estimated as amounting to 21.50 

 equivalent volts, with a probable uncertainty of 0.15; the others are 



