DESCRIPTION OF THE FIGURE 133 



any physical significance ; but for the fact that many 

 members would be superimposed, they would all 

 be represented in the centre of the places. The 

 periods of average life, which are always 1-443 

 times the periods of half-change, are shown for 

 each member above or below its symbol, a ? indicat- 

 ing that the period is estimated indirectly from 

 the Geiger-Nuttall relation. 



The figures at the head of each place represent 

 the atomic numbers or number of the place in the 

 periodic table, starting with hydrogen as unity, 

 helium as 2, lithium as 3, and so on. Moseley found 

 that the square-root of the frequency of the charac- 

 teristic X-radiation of an element was, for the 

 A'-series of radiations, proportional to integers less 

 by one than the atomic numbers. Strictly speak- 

 ing, there is no means of determining the absolute 

 value of the atomic number, but the starting point 

 having been fixed for any one element, the others 

 can then be found in terms of it. Moseley assumed 

 the atomic number of aluminium as 13, as it is the 

 thirteenth known element in the list starting with 

 hydrogen as unity. It is unlikely that any new 

 elements will be discovered between hydrogen and 

 aluminium, although if they were it would be 

 necessary to alter the whole of the subsequent 

 atomic numbers to correspond. For ^-radiations of 

 the other series, the square-roots of the frequencies 

 are not proportional to integers even, although the 

 differences are nearly integral for successive elements 

 in the periodic table. The actual numbers in the 

 figure, 92 for uranium, for example, are derived 

 from the assumption that the atomic number of 

 aluminium is 13, but it is well to remember that, 

 although relatively to one another based on experi- 

 mental evidence, the absolute value is to some 



extent arbitrary. 



T 



