132 CONCEPTION OF THE CHEMICAL ELEMENT 



identical spectra. For this element, lead, Rutherford 

 and Andrade have shown that the secondary y-radia- 

 tion excited by the impact of /3-rays on a block of 

 ordinary lead gave by crystal reflection two lines 

 identical in wave-length with the two strongest 

 lines in the y-ray spectrum of radium-^, an isotope 

 of lead, as Fleck showed, of atomic weight 214. 

 This is of importance as indicating that .AT-rays 

 and y-rays, although no doubt originating in a 

 deeper region of the atom than the ordinary light 

 spectrum, do not originate in the deepest region 

 of all to which the weight of an atom and its 

 radioactive properties are to be referred. 



DESCRIPTION OF THE FIGURE. 



The generalisation, brought up to date, is set 

 forth in detail in the Tables on p. 134 and is illus- 

 trated by the accompanying figure, which is to be 

 read at an angle of 45, making the lines of atomic 

 weight horizontal and the division between the 

 successive places in the periodic table vertical. 

 Starting from uranium and thorium, the series run 

 in an alternating course across the table and extend 

 over the last twelve places as far as the element 

 thallium. At this point, it is interesting to note 

 that the expulsion of an a- instead of a /3-particle 

 would have resulted in the production of an isotope 

 of gold, and so literally have realised the goal of 

 the alchemist. As it happens, a /3-particle is 

 expelled and lead results, so far as the changes have 

 yet been traced, in all cases as the final product. 



It has been necessary, in order to separate the 

 series from one another, to displace the actinium 

 series to the right and the radium series to the 

 left of the centre of the places, but this displacement 

 within the single place is not intended to express 



