222 



SCIENCE 



[Vol. LV, No. 1418 



in separating, by diffusion, a mass of chlorine 

 into two portions with different densities. 

 Mercury, too, has been found by positive ray 

 analysis to consist of a niunber of isotopes, 

 probably six, with integral atomic weights 

 197-200, 202, 204. As a confii-mation of this 

 result Bronsted and Hevesy^* have shown that 

 it is possible by fractional distillation to sep- 

 arate mercury into two parts with different 

 densities. 



The list of the elements in so far as they 

 have been investigated for isotopes is given in 

 Table I. In Table II following there is also 

 assembled the isotopes of the various radio- 

 active elements. 



TABLE I 

 Isotopes 



14 Bronsted and Hevesy, Nature, September 30, 

 1920. 



V. Discussion op Isotopes 



A glance at the results in Table I suggests a 

 few observations. 



(a) Isobares and radioactivity. 



It is interesting to note that while iodine 

 with an atomic weight 126.92 has but one 

 isotope, 127, bromine with an atomic weight 

 79.92 has two, 79 and 81. Had it turned out 

 that bromine consisted of but one isotope with 

 weight 80 we should have had an example of 

 an isobare, that is, an atom of one element 

 with an atomic weight the same as that of an 

 atom of a second element. It will be seen that 

 one of the isotopes of krypton has an atomic 

 weight 80. 



It is also of interest to point out, as Harkins 

 has done, that with magnesium having 3 iso- 

 topes and chlorine 2 it is possible to have nine 

 isotopic forms of MgCL. As mercvu'y has six 

 isotopes there would follow the possibility of 

 having 63 isotopic forms of IIg,Cl.,. Similar 

 considerations would apply in regard to other 

 elements. 



G. P. Thomson has recently found that 

 strontium consists of two isotopes of weight 

 85 and 87. He failed however to find one of 

 weight 88 or any higher number the necessity 

 for which the atomic weight of strontium, 

 87.63, would seem to demand. As rubidium 

 was shown to have isotopes of weight 87 and 

 85 we have in strontium and rubidium an 

 example of isobaric isotopes, i. e., the atoms of 

 these two elements are identical in mass. As 

 the nuclear charge of rubidium is 37e while 

 that of strontium is 38e, it follows that the 

 nuclei of rubidium atoms differ from those of 

 strontium atoms only by the inclusion of one 

 electron. This may possibly afford an explana- 

 tion of the radioactivity which rubidium and 

 its salts are known to exhibit. It has "been 

 shown that rubidium emits a soft radiation of 

 beta particles, and since it is now generally 

 agreed that radioactivity is a property of 

 nuclei, it would follow that by the emission of 

 beta raj's, rubidium atomic nuclei are being 

 transmuted into those of strontium. One 

 should expect to find, then, strontium asso- 

 ciated with the sources of rubidium. 



