352 Mr. J. J. Hood on the Rate of 



trace of the second spectrum is to be detected. The reason is 

 to be found in the enormous temperature existing in these 

 bodies. But in the case of stars such as a Orionis, a Herculis, 

 &a, which, judging from their complicated spectra, are in a 

 more advanced stage of cooling, the appearance of the second 

 hydrogen spectrum would not, under these circumstances, be 

 remarkable, since observation shows that the characteristic 

 spectrum is either entirely absent in these stars, or only feebly 

 present. Nevertheless the verification of this expectation by 

 observation must always be nearly or quite impossible on 

 account of feeble intensity of light ; and the same is a fortiori 

 true of the nebute, in which the appearance of the second 

 hydrogen spectrum, together with the characteristic line H^, 

 might have been expected, upon other grounds, with a certain 

 degree of probability. 



XL. On the Rate of Chemical Absorption of Gases, with regard 

 to their Inter diffusion. By John J. Hood, J3.Sc, Assoc 

 R.S.M.* 



[Plate X. tig. 4.] 



WITH the exception of Graham's work on the diffusion 

 of gases, their rates of passage through porous dia- 

 phragms or minute apertures into a vacuum, and Loschmidt's 

 experiments on free diffusion!, no measurements of any note 

 have been made bearing on this important subject. The former 

 chemist has shown that the lighter a gas is the more rapidly 

 does it pass through a porous diaphragm, or, as it is expressed, 

 the rate of diffusion varies as the reciprocal of the square root 

 of the density. The experiments of Loschmidt, which were 

 performed by placing two tubes containing different gases 

 over each other, opening a channel between them, and after a 

 certain time determining the amount of the gases that had 

 exchanged places, gave measurements of the rate of interdif- 

 fnsion of the one gas into the other. The results obtained by 

 this method of studying diffusion, a physical process somewhat 

 different from that of the passage of a gas through a porous 

 diaphragm or minute aperture into a vacuum as worked out 

 by Graham, show a relation to exist between the molecular 

 weights of the gases and their rates of interdiffusion, but 

 which relation is not quite clear J. 



* Communicated "by the Author. 



t Wiener Sitzungsberichte, 1870 ; extended by Wretschko and by Benigar, 

 Bd. lxii. 



X Meyer, Kinetische Theorie der Gase, 1877, p. 164. 



