July 26, 191 7] 



NATURE 



435 



ment of these results for thorium lead, a series of 

 investigations was published on the atomic weight of 

 lead from uranium minerals by T. W. Richards and 

 collaborators at Har\ard, Maurice Curie in Paris, and 

 Honigschmid and collaborators in Vienna, which 

 show that the atomic weight is lower than that of 

 ordinary lead. The lowest result hitherto ob- 

 tained is 206046, by Honigschmid and Mile. Horovitz, 

 for the lead from' the very pure crystallised pitch- 

 blende from Morogoro (German East Africa), whilst 

 Richards and Wadsworth obtained 206085 for a care- 

 fully selected specimen of Norwegian cleveite. 

 Numerous other results have been obtained, as, for 

 example, 206405 for lead from Joachimsthal pitch- 

 blende, 20682 for lead from Ceylon thorianite, 20708 

 for lead from monazite, the two latter being mixed 

 uranium and thorium minerals. But the essential pro- 

 portion between the two elements has not, unfortunately, 

 been determined. Richards and Wadsworth have also 

 examined the density of their uranium lead, and in 

 ever}' case they have been able to confirm the conclu- 

 sion that the atomic volume of isotopes is constant, 

 the uranium lead being as much lighter as its atomic 

 weight is smaller than common lead. Many careful 

 investigations of the spectra of these varieties of lead 

 show that the spectrum is absolutely the same so 

 far as can be seen. 



Thorium and Ionium. 

 A second quite independent case of a difference in 

 atomic weight between isotopes has been established. 

 It concerns the isotopes thorium and ionium, and it is 

 connected in an important way with the researches 

 of which, on two previous occasions, I have given 

 an account here, the researches on the growth of 

 radium from uranium which have been in progress 

 now for fourteen years. It is the intervention of 

 ionium with its very long period of life which has 

 made the experimental proof of the production of 

 radium from uranium such a long piece of work. 

 Previously only negative results were available. One 

 could only say from the smallness of the expected 

 growth of radium that the period of average life of 

 ionium must be at least 100,000 years, forty times 

 longer than that of radium, and, therefore, that there 

 must be at least forty times as much ionium by 

 weight as radium in uranium minerals, or at least 

 136 grams per 1000 kilos of uranium. Since then 

 further measurements carried out by Miss Hitchins 

 last year have shown definitely for the first time a 

 clear growth of radium from uranium in the largest 

 preparation, containing 3 kilos of uranium, and this 

 growth, as theory requires, is proceeding according to 

 the square of the time. In three years it amounted to 

 2x10-^' grams of radium, and in six years to just four 

 times this quantity. From this result it was con- 

 cluded that the previous estimate of 100,000 years for 

 the period of ionium, though still of the nature of a 

 minimum rather than a maximum, was very near to 

 the actual period. 



Joachimsthal pitchblende, the Austrian source of 

 radium, contains only an infinitesimal proportion of 

 thorium. An ionium preparation, separated by Auer 

 von Welsbach from 30 tons of this mineral, since 'no 

 thorium was added during the process, was an ex- 

 tremely concentrated ionium preparation. The atomic 

 weight of ionium — calculated by adding to the atomic 

 weight of its product, radium, four for the a particle 

 expelled in the change — is 230. whereas that of 

 thorium, its isotope, is sHghtlv above 232. The ques- 

 tion was whether the ionium-thorium preparation 

 would contain enough ionium to show the difference. 

 Honigschmid and Mile. Horovitz have made a special 

 examination of the point, first re-determining as accu- 



NO. 2491, VOL. 99] 



rately as possible the atomic weight of thormm, and 

 then that of the thorium-ionium preparation from 

 pitchblende. They found 232- 12 for the atomic weight 

 of thorium, and by the same method 231-51 for that 

 of the ionium-thorium. A very careful and complete 

 examination of the spectra of the two materials 

 showed for both absolutely the same spectrum and a 

 complete absence of impurities. 



If the atomic weight of ionium is 230, the ionium- 

 thorium preparation must, from its atomic weight, 

 contain 30 per cent, of ionium and 70 per cenf. of 

 thorium by weight. Prof. Meyer has made a com- 

 parison of the number of a particles given per second 

 by this preparation with that given by pure radium, 

 and found it to be in the ratio of i to 200. If 30 per 

 cent, is ionium, the activity of pure ionium would be 

 one-sixtieth of that of pure radium, and its period some 

 sixty times greater, or 150,000 years. This confirms 

 in a very satisfactory manner our direct estimate of 

 100,000 years as a minimum, and incidentally raises 

 rather an interesting question. 



My direct estimate involves directly the period of 

 uranium itself, and if the value accepted for this is 

 too high, that for the ionium will be correspondingly 

 too low. Now, on May 11 Prof. Joly was bringing, 

 before you, I believe, some of his exceedingly in- 

 teresting work on pleochroic halos, from which he has 

 grounds for the conclusion that the accepted period of 

 uranium may be too long. 



But since I obtained, for the period of ionium, a 

 minimum value two-thirds of that estimated by Meyer 

 from the atomic weight, it is difficult to believe that 

 the accepted period of uranium can have been over- 

 estimated by more than 50 per cent, of the real period. 

 The matter could be pushed to a further conclusion if 

 it were found possible to estimate the percentage of 

 thorium in the thorium-ionium preparation, a piece of 

 work that ought not to be beyond the resources of- 

 radio-chemical analysis. This would then constitute 

 a check on the period of uranium as well as on that 

 of ionium. Such a direct check would be of consider- 

 able importance in the determination of geologicaf 

 ages. 



The period of ionium enables us to calcu- 

 late the ratio between the weights of ionium 

 and uranium in pitchblende as 174 to 10', 

 and the doctrine of the non-separability of 

 isotopes leads directly to the ratio between the thorium 

 and uranium in the mineral as 41-7 to 10'. This 

 quantity of thorium is unfortunately too small for 

 direct estimation. Otherwise it would be possible ta 

 devise a very strict test of the degree of non-separ- 

 ability. .As it is, the work is sufficiently convincing. 

 Thirty tons of a mineral containing a majority of the 

 known elements in detectable amount, in the hands of 

 one whose researches in the most difficult field of 

 chemical separation are world-renowned, yield a pre- 

 paration of the order of one-millionth of the weight 

 of the mineral, which cannot be distinguished from 

 pure thorium in its chemical character. Anyone could 

 tell in the dark that it was not pure thorium, for its 

 a activity is 30,000 times greater than that of thorium. 

 This is then submitted to that particular series of 

 purifications designed to give the purest possible 

 thorium for an atomic weight determination, and it 

 emerges without any separation of the ionium, but 

 with a spectrum identical with that of a control speci- 

 men of thorium similarly purified. The complete 

 absence of impurities in the spectrum shows that the 

 chemical work has been very effectively done, and the 

 atomic weight shows that it must contain 30 per cent, 

 by weight of the isotope ionium, a result which agrees 

 with its a activity and the now known period of the 

 latter. 



