196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



tion is borne out in practice and the difference in wave length has 

 only been detected in the case of the isotopes of lead. Aronberg, 30 

 in 1917, discovered a shift of 0.0044 A between ordinary lead and a 

 radio lead of atomic weight 206.3. This result has been confirmed by 

 the subsequent work of Merton, 31 who has recently measured a shift 

 of 0.011 A in one of the lines of an extremely pure Carnotite lead 

 as compared with the same line in ordinary lead. These shifts, 

 though extremely minute, are, however, hundreds of times, larger than 

 the ones predicted by the simple application of the Bohr theory. 



The artificial separation of the isotopes of nonradioactive ele- 

 ments is an exceedingly difficult operation ; indeed, had it been other- 

 wise they must have been discovered long ago. In the case of neon, 

 already described, which is a particularly favorable one, the extreme 

 difference between the lightest and heaviest fractions amounted to 

 0.13 of a unit of atomic weight. Harkins, 32 using a somewhat similar 

 diffusion method, has successfully separated the isotopic hydrochloric 

 acids and obtained a shift of 0.055 of a unit. A beautiful method 

 applicable to certain liquids has been developed by Bronsted and 

 Hevesy. 33 This consists in allowing the liquid to evaporate at so 

 low a temperature and pressure that none of the molecules escaping 

 from its surface can ever return to it again; a concentration of the 

 heavier constituent in the residue must then result. They first ap- 

 plied it to mercury, and the latest separation achieved with the iso- 

 topes of that element is indicated by the figure 0.99974 and 1.00023 

 for the densities of the lightest and heaviest fraction, respectively, 

 the normal density being taken as unity. In atomic weight this 

 separation corresponds to a shift of 0.1 of a unit. They have also 

 applied the same method to a solution of hydrochloric acid in water 

 and obtained a change of atomic weight of about 0.02 of a unit. 



Several other methods of partial separation have been suggested, 

 but the only ones which have been successful in practice are those 

 mentioned above. Complete separation can be achieved by means of 

 positive ray analysis, but the quantities to be obtained in this way 

 are too minute to be of the slightest practical importance. The fact 

 that many of the most familiar elements prove to be mixtures of 

 isotopes is of fundamental theoretical importance, but when we con- 

 sider the extreme difficulties of their separation it seems very un- 

 likely, unless some entirely new method is discovered, that the 

 numerical constants of chemistry are likely to be affected seriously 

 for some time to come. 



30 Aronberg : Proc. Nat. Acad. Sci., 3,710, 1917, and Astrophys. Jour., 47, 96, 1918. 



81 Merton : Proc. Roy. Soc, 99A, 87, 1921. 



82 Harkins and Hayes : J. Amer. Ctaem. Soc, 43, 1803, 1921. 

 "Bronsted and Hevesy: Phil. Mag., 43, 31, 1922. 



