462 Perkins — Molecular Weight of Radium Emanation. 



tubes into the air, gave about the same value for the molecular 

 weight of the emanation as the estimate made by Rutherford 

 and Miss Brooks. 



In 1904, Bu instead and Wheeler* compared the rate of dif- 

 fusion of the emanation through a porous plate with that of 

 carbon dioxide. Their results showed that, providing Graham's 

 Law applies, the molecular weight of the emanation was about 

 4*1 times that of carbon dioxide or near 180. 



Makowerf compared the rates of diffusion of hydrogen, oxy- 

 gen, carbon dioxide and sulphur dioxide with that of the 

 emanation and, for different plugs of different materials, got 

 results for the molecular weight ranging from 85*5 to 97. 



Reasons for using Mercury Vapor. 



Rutherford and Soddyi have shown that the radio-active 

 emanations are incapable of chemical combination. From rhis 

 resemblance to the inert gases, it is highly probable that the 

 emanations are m on atomic gases. It would also appear from 

 the position of radium emanation in the radio-active series that 

 its molecular weight is probably near 225. Graham's Law has 

 been shown to hold very well for gases of similar molecular 

 structure which did not differ greatly in molecular weights ; 

 but to break down entirely when applied to the diffusion of 

 gases or vapors of complex molecular structure and widely 

 different molecular weights. If the emanation is mon atomic 

 and has a molecular weight near 225, Graham's Law cannot be 

 expected to hold when its rate of diffusion is compared with 

 that of any gas or vapor which has been used in any previous 

 investigation. 



In order to satisfy both the conditions required for appli- 

 cation of Graham's Law, mercury vapor was chosen for a series 

 of comparisons. This is monatomic and has a molecular weight 

 of 200. Since it is necessary to work at quite high temperatures 

 to obtain large enough amounts for accurate weighing, most of 

 my work was done near 250° C. and 275° C. 



Apparatus. 



Electric furnace. — For maintaining the required temper- 

 ature, an electric furnace was constructed in the following 

 way : 



A copper tube, 45 cm long and of about 5'5 cm outside 

 diameter, was coated with a mixture of talc and water glass as 

 recommended by ]STernst. On this insulating base, about 110 



*This Journal, xvii, Feb. 1904. 

 fPhil. Mag., ix, p. 56, 1905. 

 {Phil. Mag., Nov. 1902, 



