12-4 Professor Frederick Soddy P^aj 18, 



chansfes sufficiently slowly to accumulate in small but ponderable 

 quantity in a uranium mineral, and so it was shown to be a new 

 member of the alkaline-earth family of elements, with atomic weight 

 i2G*0, occupying a vacant place in the Periodic Table. Polonium 

 changes 4500 times more rapidly, and can only exist to the extent of 

 a few hundredths of a milligram in a ton of uranium mineral. 

 Actinium also, though its life period is still unknown, and very 

 possibly is quite long, is scarce for another reason, that it is not in 

 the main line of disintegration, but in a branch series which claims 

 only a few per cent of the uranium atoms disintegrating. In spite 

 of this, polonium and actinium have just as much right to be con- 

 sidered new elements, probably, as radium has. Polonium has great 

 resemblances in chemical character both to bismuth and tellurium, 

 but was separated from the first by Mme. Curie and from the second 

 by Marckwald. In the position it occupies as the last member of the 

 sulphur group, bismuth and tellurium are its neighbours in the 

 Periodic Table. Actinium resembles the rare-earth elements, and 

 most closely lanthanum, but an enrichment of the proportion of 

 actinium from lanthanum has been effected by Giesel. The small- 

 ness of the quantities alone prevents their complete separation in the 

 foi'm of pure compounds as was done for radium. 



The three gaseous members, the emanations of radium, actinium 

 and thorium, were put in their proper place in the Periodic Table 

 almost as soon as radium was, for, being chemically inert gases, their 

 characterisation Avas simple. They are the last members of the argon 

 family, and the fact that there are three of about the same atomic 

 weight was probably the first indication, although not clearly 

 appreciated, that more than one chemical element could occupy the 

 same place in the Periodic Table. 



The extension of the three disintegration series proceeded apace ; 

 new members were being continually added, but no other new radio- 

 elements — new, that is, in possessing a new chemical character — 

 were discovered. The four longest-lived to be added, radio-lead or 

 radium-D, as it is now more precisely termed, and ionium in the 

 uranium series, and mesothorium-I and radiothorium in the thorium 

 series, could not be separated from other constituents always present 

 in the minerals, radium-1) from lead, ionium and radiothorium from 

 thorium, and mesothorium-I from radium. An appreciable pro- 

 portion of the radioactivity of a uranium mineral is due to radium-D 

 and its products, and its separation would have been a valuable 

 technical achievement, but, though many attempts have made, this 

 has never been accomplished, and, we know now, probably never 

 will be. 



Seven years ago it was the general opinion in the then compara- 

 tively undeveloped knowledge of the chemistry of the radio-elements, 

 that there was nothing especially remarkable in this. The chemist 

 is familiar with many pairs or groups of elements, the separation of 



