WOEK ON KADIUM 161 



on the direct estimation of the density of the radium 

 emanation. 



This research must be regarded as one of the most 

 wonderful ever recorded in the annals of experiment. 

 Remembering the exceedingly small volume of the eman- 

 ation obtainable from a relatively large weight of 

 radium, it is not surprising that other experimenters 

 using methods based on the measurements of the rate 

 of diffusion or of effusion had arrived at widely divergent 

 values for the density of the gas. The majority of the 

 numbers obtained pointed to an atomic weight either 

 176 or 222, and these are the tabular atomic weights of 

 the two next terms following xenon in the periodic table, 

 thus : 



Helium Neon Argon Krypton Xenon I II 



4 20 40 83 130 176 222 



The problem, then, to be attacked was the deter- 

 mination of the weight of emanation evolved in a given 

 time from a known weight of radium. The volume being 

 already known from the experiments of Rutherford in 

 1903, those of Debierne in 1909, and confirmed shortly 

 afterwards by Gray and Ramsay. As shown in the 

 paper communicated to the Royal Society in December 

 1910 (Proc. Roy. Soc. 84 A, p. 536), the total volume of 

 the gas obtainable for weighing scarcely exceeded ^ 

 cubic millimetre, a scarcely visible bubble, and if the 

 atomic weight is assumed to be 222 the weight is less 

 than T ^Vo milligram. It is evident therefore that in 



