ATOMIC WEIGHT OF IONIUM 141 



examined, but hitherto no differences whatever have 

 been established. 1 



The atomic weight of a mixture of ionium and 

 thorium was found by Honigschmid to be 231-51 as 

 compared with 232-12 for thorium, the spectra being 

 identical and impurities absent in both specimens. 

 The calculated value for the atomic weight of ionium 

 is 230, and the evidence, so far as it yet goes, is in 

 accord with the view that, in the mixture examined, 

 about 30 per cent, was ionium and 70 per cent, 

 thorium. By a simple comparison of the emanating 

 power of the mixture with that of the pure thorium 

 preparation under similar conditions, the proportion 

 of ionium to thorium could be readily determined 

 directly, since ionium does not give an emanation, 

 and this unknown eliminated, but this has still to 

 be done. 



THE DIFFERENT VARIETIES OF ISOTOPES AND 

 HETEROTOPES. 



When isotopes, such as those just considered, 

 possess different atomic weights, it is to be expected, 

 although this has not yet been practically accom- 

 plished, that a separation by physical means, such as 

 prolonged fractional diffusion, ought to be possible. 

 Chlorine and other elements, the atomic weights of 

 which depart largely from an integral value, seem to 

 deserve a further physical analysis by this method. 

 Sir J. J. Thomson's positive-ray method of gas 

 analysis ought to be able to detect such isotopes of 

 different atomic weight without separation, and at 



1 Harkins and Aronberg (Proc. Nat. Acad. Set., 1917, 3, 710), for 

 ordinary lead and uranio-lead of atomic weight 206-34, examining 

 the strongest line, 4058, in the sixth order of spectrum obtained by a 

 lo-inch grating, observed a constant difference of 0-0043 A., but are 

 themselves disposed to await further results before drawing any 

 conclusions. This has now been confirmed (tf. T. R. Merton, Nature, 

 2nd October 1919). 



U 



