470 Perkins — Molecular Weight of Radium Emanation. 



weights of the empty tubes, which were, therefore, weighed 

 after each collection. 



Approximation, for k at Constant Supply. 



Since no experimental data were available on the density of 

 mercury vapor, an attempt was made to find approximate 

 values, at the temperature at which diffusion took place by 

 means of the gas laws. 



Using standard thermometers, the corrected temperatures 



were found to be 252-8° C. and 278-2° C. Vapor pressures 



for mercury vapor have been determined with a good deal of 



accuracy at these temperatures. Person'" found the latent 



heat for mercury vapor at 350° C. to be 62. By using 



dV 

 the equation : L == (V ? — Y x ) T —z, where L is the latent 



heat, (V 2 — TJ the difference in vol. of 1 gr. of mercury after 

 and before vaporization, P the vapor pressure and T the abso- 

 lute temp., and extrapolating by means of Boyle's Law to 

 252-8° C. and 278-2° C, the values for the density came out 

 •000550 grs. / ccm. and -000988 grs. / ccm. respectively. The 

 volume of the diffusion chamber was 237 ccm. Making these 

 substitutions in the formula for constant supply given on p. 5, 

 the diffusion constant at 252'8° C. was -0346 ; and, at 278-2° C, 

 •0431. Applying Graham's Law, this gave for the molecular 

 weight of the emanation 208 and 262 respectively, with a mean 

 of 235, showing that in all probability the molecular weight of 

 the emanation was a little greater than that for mercury 

 vapor. The very discordant results for the two temperatures 

 showed that the method was not available because of the 

 uncertainty in the value of the density. Therefore, the other 

 method, using unsaturated vapor, was tried. 



.Results obtained at 250° C. with a small supply of mercury 

 (less than 1 gr.) in the diffusion chamber, showed that diffu- 

 sion began under saturation conditions. For 20 min. periods 

 of collection, since the rate of diffusion was slightly in excess 

 of that of supply, the amounts diffusing showed a slow gradient 

 until a point was reached where a sudden drop took place, 

 showing that vapor alone remained in the diffusion chamber. 

 This result is well illustrated by the following curve (fig. 2) plot- 

 ted from data obtained from the first experiment made under 

 these conditions. Amounts diffusing during 20 min. collec- 

 tions are plotted as ordinates and 20 min. periods of time as 

 abscissae : 



*Pogg. Annal., vol. lxx, 310, 386, 1847. 



