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



vessel A filled with liquid air. The distance between the 

 mercury and the cooled glass surface was 1-2 c. cm. As 

 evaporation proceeds rather slowly at room-temperature, 

 the mercury was heated to about 40°-60° by means of 

 the surrounding oil-bath C. After a suitable portion 

 of the mercury was evaporated, we removed the remaining 

 '•heavier" part by evacuating the receiver E and opening 

 the cock D, whereby the mercury by its own weight flowed 

 into the receiver and could be taken out through the 

 cock G. The liquid air was now allowed to evaporate, 

 whereafter the solid mercury melted, dropped into the 

 bottom of H, and was removed from there in just the same 

 way as the "heavier" mercury. The operation was now- 

 repeated with another sample of ordinary mercury, and 

 so on until a total of about 2700 c. cm. was separated 

 in a distilled (D x ) and a residual fraction (Pi). 



In order to obtain heavier mercury, P t and the following 

 P-fractions were subjected to further similar separations. 

 When lighter mercury was desired, J) x and the following 

 D-fractions were treated in the same way. 



For smaller quantities of mercury, as available in the 

 later part of our work, we used two apparatus, as shown 

 in fig. 2, with a capacity of 40 and 8 c. cm. and working 

 as follows : — - 



After having finished the evaporation, the residual 

 mercury is removed by turning the ground- joint at D 

 and thus the whole apparatus about 50°, which permits 

 the mercury to flow into the bulb B. Now the vacuum is 

 destroyed, the point of the capillary tube broken off, and 

 the outflowing mercury caught. The solid mercury, which 

 after evaporation drips off and gathers in the lower space, 

 is then removed in a similar manner. 



By using the arrangement shown in fig. 1 at 45°, about 

 6 c. cm. mercury evaporated per hour, corresponding to an 

 evaporation of 0*35 c. cm. per cm. 2 At this temperature 

 the vapour-pressure of the mercury amounts to about 

 0'01 mm., and the mean free path, in so far as the vapour 

 can be considered as stationary, about 10 mm. Ideal 

 separating conditions were thus nearly fulfilled at this 

 temperature. Yet the evaporating process can be carried 

 out more swiftly without thereby reducing the degree of 

 the separation. For instance, by working at 120° and 

 using the apparatus represented in fig. 2, the large amount 

 of 1*5 c.cm. per hour and per square cm. evaporated ; 

 yet the degree of separation achieved was found to bf> 

 only slightly smaller than the one obtained under " ideal " 

 conditions. At 120° the vapour-pressure of mercury 



