34 Prof. J. N. Br^nsted and Prof. G. Hevesy on 



place through a narrow opening, then the lighter isotope 

 (molecular weight m^, which in consequence of its greater 

 molecular velocity hits the opening comparatively more 

 often than the heavier, will have a greater probability 

 for penetrating the opening than the latter (molecular 

 weight =m 2 ). As the molecules which pass through the 

 opening are soon condensed on the other side at low 

 temperature, the probability that they are able to go 

 the reverse way is very slight. U in the initial substance 

 the two isotopes were present in the proportion 1 : 1, and 

 only a small fraction pass through the narrow opening, 

 we must expect the two isotopes on the other- side of the 



opening to be present in the proportion \J —. 



As in the case of the evaporation method, the method 

 here mentioned therefore admits under the most favourable 

 conditions a separation proportional to the square root of 

 the ratio of the two molecular weights. Although not 

 favourable from a practical point of view on account of the 

 slowness of the effusion process, this method is of theoretical 

 importance, and was therefore also used by us in our 

 experiments with mercury. 



3. Separation of the Mercury Isotopes based on the difference 

 in their evaporation velocity. 



A number of reasons have induced us to begin the series 

 of mixed elements, to be separated into pure elements, with 

 mercury. The vapour-pressure of this substance can be 

 varied in the particularly handy temperature-range 0°-100° 

 within wide limits, embracing a practically negligible 

 pressure as well as the already considerable one of about 

 0*3 mm.; besides this, the free mean path . and other 

 magnitudes important for the kinetic theory of gases 

 are well known in the case of mercury. There is hardly 

 another substance which can be produced in a pure state 

 with so little labour, and no substance of which the density 

 can be determined so easily and exactly. A density deter- 

 mination is certainly the most easily approachable and the 

 most exact manner for determining a partial or complete 

 separation of isotopes. The volume of the isotopic atoms 

 is, as is well known, equal*; so that by comparing the 

 density of the " normal " and- " separated ,J mercury, a 

 separation can be easily detected and its degree measured. 



* Compare the agreement found by Th. W. Richards and Ch. Wads- 

 worth (Journ. Amer. Cbem. Soc. xxxviii. p. 221, 1916), F. Soddy 

 ('Nature,' cvii. p. 41, 1921), and especially the conclusion following 

 from Bohr's spectral theory. 



