X-liays from Boron and Carbon. 161 



turn and flattens out, falling but slowly to the point for 

 lithium, which, according to Kossel, is simply the ionizing 

 potential (5*4 volts). Our experimental values for carbon 

 and boron, however, would seem to be considerably above 

 the conjectured line of L points. We also saw that Kurth's 

 values for iron (26) and particularly aluminium (13) were 

 considerably above the calculated line of L points. Further 

 discussion of Kossel's interesting point of view will be post- 

 poned until data as to other elements are available. A glance 

 at fig. 7 makes it very clear that more experimental deter- 

 minations of the K and L points for light elements are 

 much to be desired. Until some method comparable in 

 accuracy with the diffraction grating, or the crystal grating, 

 applicable to the measurement of wave-length between X 200 

 and X12 is developed, we shall no doubt have to infer the 

 wave-length from potential measurements similar in character 

 to those of the investigation. 



Summary. 



The photo-electric effect of the radiation from carbon and 

 boron bombarded by electrons has been measured as a func- 

 tion of the energy of the electrons. Two slight but definite 

 breaks in the curves for both carbon and boron were obtained. 

 These are considered to correspond to the K-and L-absorption 

 wave-lengths. The breaks occur at 215 volts (X 57*5) and 

 34*5 volts (X 358) for carbon, and at 148 volts (X 83'5) 

 and 24'5 volts (X 505) for boron. The L point for carbon 

 is in good agreement with the values measured recently by 

 Millikan for the L emission-lines of carbon vapour. The 

 K points for carbon and boron are approximately where 

 they would be expected on extrapolating the known values 

 of the K« critical absorption wave-lengths for elements 

 heavier than magnesium. 



The author wishes to express his thanks to Mr. R. A. Lyon 

 for his assistance during part of this investigation. 



Queen's University, 

 Kingston, Canada. 

 July 1921. 



Phil. Mac. S. 6. Yol. 43. No. 253. Jan. 1922. M 



