160 Prof. A. LI. Hughes on Characteristic 



type and wave-length of those from solid carbon as generated 

 in these experiments. It would be exceedingly interesting, 

 therefore, to apply Millikan's method (which is susceptible 

 of much greater precision than the method of this work) 

 to the measurement of the L emission-lines for solid carbon. 

 That all X-ray phenomena hitherto investigated are inde- 

 pendent of chemical combination and state of aggregation, 

 may well be due to the fact that the phenomena in question 

 have always been associated with internal shells of electrons 

 of the element under consideration, and never with the 

 outermost shell. In fact, we may expect the wave-length 

 of the X-rays associated with the outer shell of electrons to 

 be in the region X 500— X 100, a region which has yet to be 

 explored. 



According to Duane *, there is an accurate relation 

 between the frequencies of the Ka emission-line and the 

 Ka and Lai critical absorption wave-lengths, which, ex- 

 pressedan volts, is as follows : 



"Vlox = Y Ka — "V Ka, • 



Knowing the Ka x emission-line and the Ka critical absorp- 

 tion wave-length, it is possible to predict the La! critical 

 absorption wave-length. This has been done in the last 

 column of Table I. and in fig. 7 (assuming that the relation 

 holds for the light elements as for the heavier). Kurth's 

 experimental values for iron and aluminium are 760 volts 

 and 120 volts, which are considerably in excess of the 

 calculated values 704 volts and 70 volts. (Possibly the 

 relation does not hold accurately when the L shell has but 

 few electrons exterior to it.) If we were to apply the above 

 relation to the calculation of the K«j line of carbon we 

 should get VK a2 = 215 — 34*5 = 180 volts, implying a relative 

 "span" for the K-series of carbon about 50 percent, greater 

 than for the K-series for elements further down in the 

 periodic table. 



Kossel f has discussed from a theoretical standpoint the 

 probable course of the K- and L-series for the light 

 elements. He conjectures that, proceeding from the heavier 

 elements to the lighter, the curve joining the L points should 

 be almost a straight line down to neon (10), and that the 

 L-radiation of neon should simply be that associated with 

 its ionizing potential 16 volts. At neon (now that the 

 L shell is outermost and exposed) the curve takes an abrupt 



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

 f Kossel, Zeits. f. PJnjs. iii. p. 470 (1920). 



