THE CONSTITUTION OF MATTER 135 



steps. Moseley found that for the K radiation the frequency of the 

 radiation was proportional to (N-a) 2 where N was a whole number 

 which varied by unity in passing from one element to the next of 

 higher atomic weight and a constant about unity. From silver to gold, 

 the spectra given by the L radiations of elements were compared. 

 These spectra consist of about five lines, of which two are relatively 

 very strong. It was found again that the spectra were similar in type 

 and that the frequency of a given line diminished by definite steps in 

 passing from one element to another. The frequency of the radiation 

 in this case was proportional to (N-b) 2 where b was a constant and N 

 a whole number. Moseley concluded that the value of N in these ex- 

 pressions was the atomic number, i. e., the number of the element ar- 

 ranged in order of increasing atomic weight. Taking aluminium as 

 the 13th element, he found that succeeding elements were expressed by 

 the value of N 14, 15, 16, 17, etc., up to 77 for gold. 



There appears to be little doubt that the X-ray spectrum of an 

 element arises from the vibrations of the rings of electrons deep in the 

 atomic structure outside the nucleus. Quite apart from the very in- 

 teresting question of the mode of origin of these very high frequency 

 spectra, it is seen that the fundamental modes of vibration of the dis- 

 tribution of electrons are simply connected with the square of a number, 

 which varies by unity in passing from one element to the next. There 

 appears to be no doubt that the atomic number represents the number 

 of units of positive charge carried by the nucleus, which on account of 

 the atomic nature of electricity can only vary by whole numbers and 

 not by fractions. 



It is obvious that the study of X-ray spectra reveals at once whether 

 any atomic number is missing, and also affords a remarkably simple 

 method of settling the number of elements possible in the rare earth 

 group about which there has been so much difference of opinion. 

 Moseley concluded that from aluminium to gold, only three possible 

 elements were missing which should have atomic numbers 43, 61, 75, 

 and only one element of number 61 appears to be missing in the rare 

 earth group. The frequencies -of the X-ray spectra of these missing 

 elements can be calculated with certainty, and these data should prove 

 an invaluable aid in a search for these missing elements. It has long 

 been known that nickel and cobalt occupy an anomalous position in the 

 periodic table when arranged according to atomic weights. This diffi- 

 culty is now removed, for Moseley found that when arranged in order 

 of nucleus charge, both elements fall into the position to be expected 

 from their chemical properties. 



Nucleus Charge and Chemical Properties 

 It is established by the work of Moseley that the elements can be de- 

 fined by their nucleus charge, and that probably elements exist which 



