156 



Prof. A. LI. Hughes on Characteristic 



shall we extrapolate to compare with our experimental values? 

 The principal radiation emitted by the boron and carbon is 

 no doubt the Ka line ; but, if boron ancl carbon behave like 

 the heavier elements, this radiation cannot be excited un- 

 less the quantum of energy in the impinging electron just 

 exceeds that corresponding to the K critical absorption 

 wave-length. As the energy of the impinging electron is' 

 the variable in these experiments, it would seem more 

 correct to compare our values with the extrapolated values 

 of the Ka points rather than the Ka points. 



The data are given in Table I. The wave-lengths of the 



Table I. 



Element. 



~K.a x emission 

 X. Y. 



-line. 



Kg crit. 

 A. 



ibs. wave-length. 

 Y. V'Y. 



v Ka -v Kaj . 



Pe (26)... 



1-932 



6383 



79-90 



1-740 



7087 



84-18 



704 



Cr (24)... 



2-285 



5396 



73-45 



2-067 



5966 



7724 



570 



Y (23)... 



2-498 



4936 



70-26 



2-265 



5444 



73-78 



508 



Ti (22)... 



2-742 



4497 



67-06 



2-494 



4944 



70-32 



. 447 



Ca (20)... 



3-352 



3679 



60-66 



3-063 



4026 



63-45 



347 



K (19)... 



3-734 



3303 



57-47 



3-434 



3590 



59-22 



287 



S (16)... 



5-361 



2300 



47-96 



5-012 



2460 



49-60 



160 



P (15)... 



6142 



2008 



44-81 



5-758 



2142 



46-28 



134 



Si (14)... 



7-109 



1735 



41-65 











Al (13)... 



8-319 



1482 



38-05 



7-947 



1552 



39-40 



70 



Mg(12)... 



Na (11)... 



9-867 

 11-88 



1250 

 1038 



35-35 

 32-21 



9-511 



1296 



36-00 



46 



11-62 

 14-5 



1061 



851 



32-58 

 29-17 



23 

 5 



Ne (10)... 



14-57 



846 



29-08 



F (9)... 



18-32 



673 



2o-95 



18-6 



664 



25-77 



neg. 



(8)... 



23-67 



521 



22-82 



24-7 



500 



22-37 



,, 



N (7)... 



31-78 



388 



19-70 



34-3 



360 



18-98 



>> 



C (6)... 



44-75 



275 



16-59 



50-7 



243 



15-59 



,, 



B (5)... 



67-7 



182 



13-48 



82-8 



149 



12-20 



„ 



Be (4)... 



114-2 



108 



10-37 



159 



77-8 



8-82 



,, 



Li (3)... 



234 



52-8 



7-27 



417 



29-6 



5-44 



" 



He (2)... 



585 



21*2 



4-6 



487 



25-4 



5-05 





H (1)... 



1216 



10-2 



3-2 



916 



135 



3-67 





Ka x emission-lines and the Ka critical absorption wave- 

 lengths are taken from Duane's " Data relating to X-Ray 

 Spectra " *. According to Moseley's law, the square root of 

 the frequency of the X-radiation is almost exactly a linear 

 function of the atomic number of the element. Since, by 



* Duane, ' Bulletin of the National Research Council,' i. p. 383 (1920). 



