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The K radiation is supposed to be the series of shortest wave 

 length in the spectrum emitted by an element, the Ka line being the 

 first and strongest line in this series, while the critical absorption 

 wave length, in X-ray terminology, is the "limit " of the series. Now 

 Bohr's theory of the hydrogen atom, which is confirmed by experi- 

 ment, indicates that the series of which X1216 is the first line, and 

 X912 is the limit, is the shortest wave length series of hydrogen and 

 is, therefore, to be regarded as the K series for hydrogen. (The 

 corresponding potentials are 10.2 and 13.5 volts, the radiating and 

 ionizing potentials for the hydrogen atoms.) The first line and the 



