August 1 1, 1923] 



NA TURE 



209 



as the active deposit — which again went through 

 further changes,. The rays resulted from these succes- 

 sive changes, a-rays in the first and a-, (3-, and y-rays 

 in the last changes. Below is the first part of the 

 thorium disintegration series as it appeared to Sir 

 Krnest Rutherford and myself in 1903 : 



THORIUM- 



-THORIUM X- 



^THORON >., etc. 



I 



In 1905 Sir William Ramsay and 0. Hahn were 

 engaged in extracting radium from thorianite, a new 

 Ceylon mineral containing both uranium and thorium 

 in important quantity. The radium was separated 

 with the barium, and the chlorides fractionated in the 

 usual way. They found a new radio-element to be 

 present and to be separated from the radium with the 

 barium. It proved to be the direct parent of thorium 

 X, and intermediate in the series between the latter 

 and thorium, and they called it radiothorium. In 

 spite of this easy and apparently straightforward 

 separation, the experience of a number of chemists 

 showed that something remained to be explained, for 

 it was found to be difficult to the verge of impossibility 

 to separate radiothorium from thorium. Ramsay and 

 Hahn had in fact " separated " isotopes in 1905, for 

 radiothorium and thorium are isotopes. Yet further 

 work has shown the two to be so alike that no separa- 

 tion by chemical means is possible ! 



Then in 1907, along with the radium which had been 

 separated from thorianite, Hahn discovered another 

 new radio-element, mesothorium, the direct parent of 

 radiothorium and intermediate between it and thorium. 

 In the next year he showed that mesothorium consists 

 of two successive products — the first, the direct product 

 of thorium, mesothorium i, being practically rayless 

 and generating a short-lived product, mesothorium 2, 

 giving powerful /?- and y-rays. 



This resolved the mystery, and one cannot do better 

 than to quote the words of McCoy and Ross (J. Amer. 

 Chem. Soc, 1907, 29, 1709). 



" Our experiments strongly indicate that radio- 

 thorium is entirely inseparable from thorium by 

 chemical processes. . . . The isolation of radio- 

 thorium from thorianite and from pure thorium 

 nitrate . . . may have been accomplished by the 

 separation of mesothorium which in time changed 

 spontaneously into radiothorium." 



Thus the radiothorium separated from the mineral 

 thorianite by Ramsay and Hahn was not the radio- 

 thorium in the mineral, but that subsequently produced 

 from the easily separated mesothorium, after it had 

 been removed from the thorium. If they had frac- 

 tionated the radium-mesothorium-barium mixture at 

 once they would not have discovered radiothorium. 

 The lapse of time after the separation of the meso- 

 thorium is essential. Nowadays many non-separable 

 radio-elements are, like radiothorium, " grown " from 

 their separable parents. Thus radium D, an isotope of 

 lead, is grown from the radium emanation (radon), 

 although it cannot be separated from the mineral, 

 which always contains lead in quantity. 



The first part of the thorium series now runs 2 



• The periods shown in the second line are the periods of .average life of 

 the successive products. These are 1-443 times the period required for one- 

 half of the element to change. 



NO. 2806, VOL. I 12] 



THORIUM- 

 2-10"' years 



►ME.SOTHORIUM i- 

 9-67 years 



-RADIOTHORIUM- 

 2-91 years 



-THORIUM X- 

 5-25 days 



-MESOTHORIUM 2 

 8-9 hours 



-THORON- 

 78 seconds 



In this series thorium and radiothorium and meso- 

 thorium and thorium X are two pairs of isotopes. If 

 we represent the successive products by balls of different 

 colours to indicate their chemical character, isotopes 

 being of the same colour, chemical analysis will sort 

 the balls into their different colours, and the lapse of 

 time will cause some of the colours to change. The 

 ball representing mesothorium will in time turn into 

 that representing radiothorium, so that the latter, 

 before indistinguishable from thorium, becomes known 

 as a separate individual. 



The Isotopes of Uranium. 



It will be noted that the method of separating iso- 

 topes depends upon their being alternate rather than 

 successive in the series. If radiothorium had been the 

 direct product of thorium, the two would to this day 

 never have been separated. The changes of chemical 

 character are, as we shall later see, intimately con- 

 nected with the electric charges on the a- and /?- 

 particles expelled. For successive products to have 

 the same character, no rays, or at least no charged 

 particles, must be expelled. It is always as well, and 

 no subject illustrates the point better than that of 

 isotopes, to reflect not only upon what our methods are 

 able to reveal but also upon what they could not reveal. 



At first it seemed that uranium itself was a case of 

 successive isotopes. Boltwood in 1908 proved from 

 his study of the relative activities of the successive 

 products giving a-rays in minerals, that whereas all of 

 them, except uranium, gave off only one a-particle per 

 atom disintegrating, uranium gave off two. By direct 

 observation with the scintillation method it was proved 

 that the two a-particles from uranium are not simul- 

 taneously expelled, and later it was shown that they 

 possess different velocities. If the slower comes from 

 uranium itself (uranium I), the period of which is 

 known to be 6*io^ years, the swifter must come from 

 the isotope (uranium II), and its period must be some 

 three million years. This is an example of isotopes 

 being revealed by difference of radioactive nature 

 simply, though no other evidence of their separate 

 existences is available. Owing to the long periods of 

 the a-ray- giving members of the early part of the 

 uranium series, it has been much more difficult to 

 unravel than the thorium series. As a result of re- 

 searches too numerous to detail, it has been concluded 

 that the main series is almost entirely analogous to the 

 thorium series and runs 



uranium I- 



6'io' years 



-URANIUM Xi- 

 35-5 days 



-URANIUM X, 

 1-65 minutes 



-URANIUM II 



3-10' years 



-lONIUM- 

 10' years 



>-RADIUM- 



2-440 years 



-RADON y, etc. 



5-55 days^ 



Though two short-lived products probably intervene 

 between the two uraniums, analogous to the two meso- 

 thoriums between thorium and radiothorium, the 

 relation of their period to that of their product, uranium 

 II, is so hopelessly unfavourable that there is no hope 

 of ever being able to put the separate existence of 

 uranium II into evidence in the same way as was done 



