[HUGHES] RADIATION FROM BORON 5 



limit of the K series for the helium atom may be identified with the 

 radiating potential 21.2 volts and the ionizing potential 25.4 volts 

 of the normal helium atom, which give X585 (recently identified by 

 Lyman) and X487 for their wave lengths. 



We might have expected the square roots of the potentials 

 corresponding to the K-critical absorption wave lengths for elements 

 of atomic number 3, 4, 5, . . . 11 to lie on a smooth curve (the 

 straight line) joining the values for Mg, Al, etc., on the one hand, 

 and the values for H and He on the other. Our diagram (Fig. 3) 

 shows, however, that the prolongation of the straight line does not 

 pass through the He and H points. Theoretical considerations may 

 indicate a possible cause for the lack of continuity. The K-radiations 

 are associated with the shells of electrons immediately outside the 

 nucleus of the atom. Almost all the elements have one or more 

 shells of electrons external to this one and so these elements are all 

 similar in that the K electrons are shielded, as it were, by one or more 

 external shells. But this is not the case for H and He, for the electrons 

 forming the K shells in these elements (one for H and two for He) 

 are all the electrons they possess. This may possibly account for 

 the experiment values for H and He not lying on the prolongation of 

 the line passing through the experimental values for the heavier 

 elements. 



A closer consideration of the problem of X-rays from elements, 

 such as boron, opens up a number of points for investigation. We 

 know from various investigations on He and other gases, that as w^e 

 increase the potential accelerating the electrons through the gas by 

 small steps, the first line is called out alone by the potential corre- 

 sponding to it, then, the second line is called out in addition to the 

 first when the potential is raised to a second critical value, and so on, 

 until the potential is raised to the ionizing potential, by which time 

 the complete series of lines appears. But with ordinary X-rays the 

 case is strikingly different. The a, (3 and y lines of the K series 

 cannot be called out one by one by potentials corresponding to their 

 frequencies. It is only when the potential accelerating the electrons 

 impinging on the element corresponds to the absorption limit, which 

 is shorter in wave length than any of the emission lines, that the 

 emission lines appear and then they all appear simultaneously. The 

 question then arises as to whether boron and elements near to it 

 behave like the heavier elements or like helium and hydrogen. This 

 problem will possibly be found to be dependent on the situation in 

 the atom of the shell of electrons giving rise to the K series. In most 



