158 Prof. A. LI. Hughes on Characteristic 



extrapolated K« and Ka lines cross each other between 

 atomic numbers 9 and 10, shows that something has broken 

 down in this region, for it is difficult to conceive of an 

 emission-line being of shorter wave-length than the ■ asso- 

 ciated critical absorption wave-length. The general trend 

 is to regard the critical absorption wave-length as being the 

 fundamental constant of an X-ray series, and for this reason, 

 and also for the one given earlier, we shall make our com- 

 parison with the extrapolated Ka points. The extrapolated 

 Ka critical absorption wave-lengths for boron and carbon 

 are 119 volts and 243 volts. Our experimental values are 

 148 volts and 215 volts. 



Kurth *, in a preliminary note on his work on soft X-rays, 

 states that he finds a definite break in his curves indicating 

 that the K-radiation for carbon occurs at \43*6, which 

 corresponds to 283 volts, a value rather more in excess of 

 the extrapolated value 243 than ours is below. The dis- 

 crepancy between his results for carbon and ours is difficult 

 to understand. No trace of a break near 283 volts (X43'6) 

 was noticed in this investigation. 



As the K-series is supposed to be the series of shortest 

 wave-length emitted by any normal atom, it is customary to 

 regard the Lyman-series for hydrogen as the K-series for 

 hydrogen. The first line, corresponding to the Ka emission- 

 line (and also to the radiating potential 102 volts), has a 

 wave-length A, 12 15 ; while the limit of the series, corre- 

 sponding to the Ka critical absorption wave-length (and also 

 to the ionizing potential 13*5 volts), is at A, 912. For helium 

 the radiating and ionizing potentials are 21*2 volts and 

 25*4 volts, corresponding to X 585 (recently identified by 

 Lyman) and A,4$7. These may be regarded as the Ka 

 emission-line and the Ka critical absorption wave-length 

 respectively. (The square roots of the values for the ionizing 

 potentials for hydrogen and helium are shown in fig. 7.) 

 It will be seen that they are appreciably off the prolongation 

 of the line passing through the K points for magnesium, 

 aluminium, etc. It is a doubtful matter, however, whether 

 we could expect continuity, seeing that the shell of electrons 

 giving rise to the supposed K-series for hydrogen and helium 

 is so differently situated from the K shell of electrons in 

 other elements. In hydrogen and helium the electrons 

 (one and two 4 respectively) which form the K shell are the 

 only electrons these two elements possess, while in all other 

 elements they are surrounded by one or more exterior shells 

 of electrons. (It is also doubtful whether we should try to 

 * Kurth, Pliys. Rev. xvii. p. 528 (1921). 



