the Separation of the Isotojjes of Mercury. 41 



4. The density determination. 



As has already been mentioned, in determining the partial 

 separation o£ the mercury isotopes, density measurements of 

 the fractions gained through the described separation method 

 were used. The atomic volume of the isotopes being equal, 

 the change in density offers a simple measure of the change 

 in the element weight * as achieved by separation. Fig. 4 

 shows the arrangement used for the density determinations. 

 The mercury is brought into the reservoir 

 -fig- 4- R, which ends below in a capillary tube K 



sufficiently narrow to prevent the mercury 

 from running out when left alone. The 

 capillary is inserted into the 0*3 mm. narrow 

 neck of the little bottle P. When tube V is 

 connected with the Gaede pump, the over- 

 pressure resulting drives the mercury 

 through the capillary, and the pyknometer 

 is slowly filled with mercury. R is now 

 removed, the py knometer placed in a ther- 

 mostat for about an hour and a half, and 

 after removing the drop of mercury, 

 expelled by the heating, weighed. We 

 employed two pyknometers, of which the 

 volumes amounted to about 5 and 1*3 cm. 3 

 respectively. By using the first the density 

 could be determined with an accuracy of 

 2 : 10 6 , with the second the accuracy was 

 correspondingly less. For determining 

 quite small volumes (about 0*2 cm. 3 ) of 

 the heaviest and lightest fractions, a thick- 

 walled glass tube of O'l mm. inner dia- 

 meter, with both ends drawn out very fine, 

 was used as a pyknometer. The mercury 

 was sucked into the pyknometer when held 

 in vertical position, and as it could not run 

 "^=~yi out by itself into the vacuum through the 

 very tine upper opening, the tube could 

 be placed in reversed position in the thermostat. On 

 account of the slight volume of this capillary pykometer, 

 such exact measurements cannot be made with it as with the 

 one of larger capacity. The capillary pyknometer, however, 

 was only used in cases in which the change in density was 

 already very considerable. 



* The term element weight is used for the mean atomic weight of the 

 mix-element: see Harkins, J. Am. Ch. Soc. xliii. p. 1038 (1921). 



P 



