3 8o SCIENCE PROGRESS 



Hence for two types of atoms we have 



{n 1 /n 3 )* = E 1 dl/E i fi (i) 



= N x al/N 2 a\ 



where N x and N 2 are the atomic numbers. By Moseley's law, 

 using assumption (3) above 



•s/ni/ih—Nt/Na (2) 



Hence a 2 /a 1 = N 1 /N 2 (3) 



Moseley discovered that the JL-series of lines for the elements 

 progress by steps just as the K lines, the frequencies being in 

 each case about one-eighth as great. This shows that if there 

 is an inmost electronic orbit, there must be a second orbit in 

 all elements, whose radius is given by equation (1) above to 

 be about ($)** or four times as great as that of the first. 



Further, it is of interest to inquire, where the highest fre- 

 quency of the if -series would fall for the lightest known element, 

 hydrogen, using the ascertained highest frequency for a heavy 

 element like tungsten and Moseley's law — equation (2) above. 

 This is easily done ; for since for hydrogen N is unity, we 

 simply divide the observed highest frequency for tungsten by 

 the square of its atomic number. The atomic number of 

 tungsten is 74 ; the wave length of the highest frequency 

 radiation given out by tungsten close to *i8x io" 8 cm., and 

 hence the shortest wave length which could be emitted by 

 hydrogen is *i8x io~ 8 x (74Y cm., or approximately 100 

 micromillimetres. Now the head of the ultra-violet series of 

 hydrogen lines recently discovered by Lyman is at 91 micro- 

 millimetres. Considering the uncertainties in the measure- 

 ments, and the fact that (2) above is not quite an exact 

 statement of Moseley's law, the agreement between these two 

 results is good. It seems fairly certain, then, that this Lyman 

 series of ultra-violet lines is nothing but the X-rays of hydrogen, 

 K -series ; further, it seems a just inference from similar con- 

 siderations that the ordinary series of hydrogen lines in the 

 visible regions (Balmer's series) with its head at 365 micro- 

 millimetres is just the X-rays of hydrogen, /.-series. Likewise 

 the series in the infra-red recently discovered by Paschen is 

 the M-series of the hydrogen X-rays, corresponding to series 

 of X-rays softer even than the Z,-series, evidence for which 

 has been obtained recently by electronic bombardment of a 



