272 Sir E. Rutherford and Dr. E. N. (La C. Andrade : 



We have seen that the soft 7 rays defined by the absorption 

 coefficient fi — 40 in aluminium correspond to the " L " series 

 of characteristic radiations for an element of atomic 

 number 82. Moseley has examined the spectra of the 

 K series for elements from aluminium to silver and finds 

 them all similar, consisting of two well-marked lines differing- 

 in frequency by about 11 per cent. The frequency of the 

 more intense line (a) is approximately proportional to 

 (N — l) 2 where N is the atomic number of the element. 

 Supposing this relation to hold for all the elements of higher 

 atomic weights, the angle of deflexion for the strong line of 

 the K series for an element of number 82 (radium B) should 

 be 1° 46'. The observed value of the strong line is about 

 1° 40' — a very fair agreement, considering the wide range of 

 extrapolation. 



We may consequently conclude that the penetrating 7 rays 

 from radium B, correspond to the characteristic radiation of 

 the K series of this element. It had been previously supposed 

 that the very penetrating rays from radium C belong to the 

 K series of characteristic radiations for that substance, but if 

 the relation found by Moseley holds even approximately for 

 the heavy elements, this cannot be the case. 



Radium C corresponds to an element of atomic number 83^ 

 and the frequency of its " K" radiation should be only a few 

 per cent, higher than that for radium B. Actually the 

 average frequency of the main radiations from radium C is 

 roughly twice that for the average frequency of the pene- 

 trating rays from radium B. We are thus driven to conclude 

 that in the case of radium C, and probably also thorium B, 

 which emits an even more penetrating 7 radiation than 

 radium C, another type of characteristic radiation is emitted 

 which is of higher mean frequency than for the " K"" series. 

 In other words, it is possible, at any rate in heavy elements, 

 to obtain a line spectrum which is of still higher frequency 

 than the " K " type. This may for convenience be named 

 the " H " series, for no doubt evidence of a similar radiation 

 will be found in other elements when bombarded by high 

 speed cathode rays. 



Connexion of Absorption ivith Frequency. 



Owen has pointed out that the relation between the 

 absorption coefficient fi in aluminium of a characteristic 

 radiation and the atomic weight A can be expressed approxi- 

 mately by A = k/A~* where k is a constant. Taking the 

 numbers given by Moseley for the frequencies of corre- 

 sponding lines of the "K" series for the elements, it can be 



