1921] on Isotopes and Atomic Weights 307 



consisting of six isotopes, 78, 80, 82, 84, 86. The weights of these 

 could be determined with great accuracy by means of the excellent 

 second and third order lines they gave. The first experiments with 

 xenon lead to the observation of five isotopes, the provisional values 

 of which were given as one unit too low. Owing to the kindness 

 of Prof. Travers and Dr. Masson, I have recently been enabled to 

 repeat the analysis with gas much richer in xenon. With this the 

 second order lines could be observed and measured. The five 

 principal isotopes of xenon are 129, 131, 132, 134, 136 ; there is 

 apparently a faint sixth component at 128 and a doubtful seventh 

 at 130. 



Experiments with boron fluoride indicated that boron has at least 

 two isotopes, 10 and 11, and that fluorine is a simple element of 

 atomic weight 19. 



Silicon is another unmistakably complex element having two 

 isotopes, 28 and 29, with a possible additional one, 30. 



Bromine was of great interest. As it has an atomic weight 

 almost exactly 80, it might reasonably be expected to be simple and 

 an isobare of one of the kryptons ; actually it consists of equal parts 

 of 79 and 81. 



Sulphur, phosphorus and arsenic are all apparently simple elements. 

 Mercury is certainly mixed, though its closer components cannot be 

 resolved with the present apparatus. Its very characteristic groups 

 are seen as high as the fifth order, and appear on nearly all the 

 spectra taken. The group consists of a continuous succession of lines 

 forming a baud, 197 to 200, a strong line at 202, and a weak one 

 at 204. Recently, at Copenhagen, Bronsted and Hevesy have suc- 

 ceeded in partially separating the isotopes of mercury by a fractional 

 distillation at extremely low pressure. They give as their figures for 

 the densities compared to normal mercury as unity — 



Condensed mercury ... ... ... 0*999980 



Residual mercury ... 1*000031 



The error of experiment is claimed to be less than one part in a 

 million. 



Selenium, tellurium, antimony and tin have all been used in the 

 discharge tube with no results of any value. This is unfortunate, 

 for the atomic weight of selenium, 79*2, suggests that one of its 

 isotopes must be an isobare of bromine or krypton ; also the relation 

 between tellurium and iodine is of great interest. 



Iodine fortunately gave a very definite result. It is a simple 

 element of atomic weight 127. This is rather surprising, for all 

 the theoretical papers on the isotopic constitution of elements have 

 predicted a complex iodine. Prophecy in physics becomes a difficult 

 trade when experimental results produce these surprises, and appa- 

 rently the only really reliable prediction is that the^e are plenty more 

 in store for us. 



