January 2, igiq" 



NATURE 



;57 



formed from thorium through an intermediate pro- 

 duct, mesothorium, chemicalh different from thorium, 

 but chemically identical with radium. No more 

 1 legant addition, not merely to knowledge, but al 



of winning knowledge, can be imagined. 

 Two separate substances, radio-thorium and thorium, 

 in the original analysis of the thorium disintegration 

 series, taken for one, become individually knowable, 

 because the first is formed from the second throu 

 third substance chemically totally distinct from either. 

 Radio-active change thus furnished a new means irf 

 analysis, for which, outside the radio-elements, there 

 i- as yet no 1 quivalent 



Further work on the ch( mica) character of the 

 various members of the disintegration series, notabh 

 1>\ Fleck, who showed that practically all were 

 chemically identical either with some common element 

 or other radio-element, in 1913 paved the way for the 

 ion independent!] arrived at by Russell, 



thi periodic 

 table, as determined bj Moseley, on the assumption 

 that the atomic 1 uminium, the thirteenth 



element in the 1 From hydrogen, is ij. 



The pi riod member is shown 



above or below its symbol, a "?" indicating that 

 the period is indirectly estimated from the range of 

 the a-ray expelled. The last member to be added, 

 eka-tantalum or proto-actinium, the direct parent of 

 actinium in an a-ray change, was discovered this year 

 independent by Cranston and myself, and by Ilahn 

 and Meitner. For this element, for actinium, and for 

 polonium, but for none of the others, are the criteria 

 of unique spectrum and chemical character, as found 

 for radium, to be expected. Moreover, the period of 

 eka-tantalum, as estimated from the range of its 

 a-rays by Hahn and Meitner, makes it appear that 

 in due course determination not only of the spectrum, 

 but also of the atomic weight and complete chemical 



Fajans, and myself, which is brought up to date and 

 illustrated by the accompanying figure. Each a-ra) 

 change was found to cause .1 shifl of two places in the 

 periodic table in one din 1 tion, tnd each /j'-ray changi a 

 shift of one place in the other, the first change being 

 panied by a reduction of four units of atomic 

 mass, a helium atom being expelled, and the second 

 not involvin. le loss of mass. Thus the suc- 



■ places in the periodii table were first associ 

 with unit variation of atomic charge, for the /3-particle 

 i> the negative electron, and the a-particle a helium 

 atom carrying two positive atomic charges. Thi 



1 lenu nts with identical chemical character were found 



■ upy the same place in the periodic table, and 



termed isotopes. Conversely, thi 

 ised by chemical and spectroscopii 

 analysis may be termed heterotopi - 



In the figure, which is to be read at 45 , the 

 numbers at the head of each place — 92 for uranium, 

 and so on- are the atomic numbers, or number of thi 



XO. 2566. VOL. I02J 



nature, of this element will be possible. It is only 

 in this way that the open question whether the 

 .niiniuni series brandies off as shown at uranium-II 

 or at uranium-1 can be settled. 



As the figure shows, so far as the changes have 

 been followed, they all result in the production of 

 isotopes of lead ranging in atomic weight from 206 

 to -mo, the main products being that of uranium, jot\ 

 and both thorium products in the two branches, 208. 

 The conclusion thai lead was the ultimate product of 

 thorium was new, but the prediction that the ultimate 

 products of both uranium and thorium are different 

 isotopes of lead the one with an atomic weight less, 

 and the other with an atomic weight greater, than 

 that of common lead, 207-2 has been completely con- 

 firmed by experiment, and it has also been shown 

 thai ionium has an atomic weight lower than thorium 

 (compare Nature, July 10 and 26, 1017). 



The older chemical 'analysis of matter distinguished 

 only heterobaric heterotopes. The newer methods 



