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228 RADIO-ACTIVE EMANATIONS [CH. 
the capillary tube was noticed, the rapid passage along the wider 
tubes, the delay in passing through a plug of phosphorous pent- 
oxide, and the rapid expansion into the reservoir of the pump. 
When compressed, the luminosity of the emanation increased, and 
became very bright as the small bubble containing the emanation 
was expelled through the fine capillary tube. 
Diffusion of the Emanations. 
151. It has been shown that the emanations of thorium and 
radium behave like radio-active gases, distributed in mimute amount 
in the air or other gas in which they are tested. With the small 
quantities of active material so far investigated, the emanations 
have not yet been collected in sufficient amount to allow the 
examination of their spectrum or to detect them by the balance. 
Although the molecular weight of the emanations cannot yet be 
obtained by direct chemical methods, an indirect estimate of it 
can be made by determining the rate of their inter-diffusion into 
air or other gases. The coefficients of inter-diffusion of various 
gases have long been known, and the results show that the 
coefficient of diffusion of one gas into another is, for the simpler 
gases, approximately inversely proportional to the square root of 
the product of their molecular weights. If, therefore, the coefficient 
of diffusion of the emanation into air is found to have a value, 
lying between that of two known gases A and B, it is probable 
that the molecular weight of the emanation lies between that of 
A and Bb. 
Although the amount of emanation given off from radium is 
too small to be detected by volume’, the electrical conductivity 
produced by the emanation in the gas, with which it is mixed, 
is often very large, and offers a ready means of measuring the 
emanation present. 
Some experiments have been made by Miss Brooks and the 
writer? to determine the rate of the diffusion of the radium emana- 
tion into air, by a method similar to that employed by Loschmidt* 
1 See, however, p. 313 (Feb. 1904). 
2 Rutherford and Miss Brooks, Trans. Roy. Soc. Canada 1901, Chem. News 1902. 
3 Sitzungsber. d. Wiener Akad. 61, 11. p. 367, 1871. 
