RECENT ADVANCES IN RADIOLOGY 191 



bearing a single atomic number, and occupying therefore a single place 

 in the periodic table, "isotopes." There has not been time as yet for 

 exhaustive investigation, but the only means which has yet been found 

 adequate to a separation of isotopes is diffusion (neon and metaneon), 

 while Sir J. J. Thomson's new positive ray method of gas analysis and 

 atomic weight determinations make it possible to distinguish isotopes 

 from one another. It is needless to say that other methods are being 



sought. 



Among the radioactive substances, or "radiants," as Mr. Eve^* calls 

 them, 34 elements have been discovered; but the study of their isotopic 

 relations reduces them to a much smaller number of groups, about 10 

 in all.^^ Representatives of five of these groups have long been known 

 (IT, Th, Bi, Pb, Tl), while the remainder have been discovered through 

 radioactive researches. Lead is isotopic with Eadium B, Thorium B. 

 Actinium B, and Radium I), while Radium itself is isotopic with Meso- 

 thorium I, Thorium X, and Actinium X.^^ 



Since the chemical reactions and, in great part at least, the physical 

 properties of isotopic elements are indistinguishable, it is very evideni; 

 that in nature they must be close companions. It is well known to all 

 of us that natural minerals are, as a rule, very impure, or that even great 

 chemical differences do not preclude inclusions in crystals or ])revent the 

 simultaneous crystallization of different substances. Hence it is to })e 

 expected that isotopic elements should l)e associated in radioactive min- 

 erals; for example, mesothorimn I with radium. But the period of 

 mesothorium I is several hundred times shorter than that of radium; 

 and, according to Mr. Soddy, a preparation containing 99 per cent 

 radium, together with 1 per cent of mesothorium I, is no less than four 

 times as radioactive as pure radium.^^ Yet being isotopes, radium and 

 mesothorium I are absolutely identical from a chemical point of view 

 and can not be separated. Hence there is no practicable means of ascer- 

 taining whether or not the helium found, say, in a zircon is derived 

 from mesothorium I or from radium. 



Similarly lead or an isotope of lead may be derived from niem1)ers of 

 the radium series, the actinium series, or the thorium series. Only an 

 atomic weight determination of the lead would indicate its origin, and 



■>» See his very readable, and of course authoritative, at'connt "f recent researches on 

 atomic structure in Science, vol. 40, 1914, p. 115. 



"• F. Soddy, the chemistry of the radio elcnients, part 1, li»ll, :iii<l i>Mrl 'J, 1014. Much 

 of what follows Is taken from these admirable mcniographs. 



"■The atomic number of lead Is 82 and that of uranium Is 92. Elements of the atomic 

 numbers 85, 87 seem not to have been found as yet. Mr. Soddy puts actinium In the 

 place whose atomic number should be 89. 



WF. Soddy, op. clt., part 1, p. G9. 



