[From Nature, Vol. xiv.] 



LXXVII. Diffusion of Gases through Absorbing Substances. 



THE importance of the exact study of the motions of gases, not only as 

 a method of distinguishing one gas from another, but as likely to increase our 

 knowledge of the dynamical theory of gases, was pointed out by Thomas Graham. 

 Graham himself studied the most important phenomena, and distinguished from 

 each other those in which the principal effect is due to different properties of 

 gases. 



The motion of large masses of the gas approximates to that of a perfect 

 fluid having the same density and pressure as the gas. This is the case with 

 the motion of a single gas when it flows through a large hole in a thin 

 plate from one vessel into another in which the pressure is less. The result 

 in this case is found to be in accordance with the principles of the dynamics 

 of fluids. This was approximately established by Graham, and the more accurate 

 formula, in which the thermodynamic properties of the gas are taken into 

 account, has been verified by the experiments of Joule and Thomson. (Proc. 

 R. S., May, 1856.) 



When the orifice is exceedingly small, it appears from the molecular theory 

 of gases that the total discharge may be calculated by supposing that there 

 are two currents in opposite directions, the quantity flowing in each current 

 being the same as if it had been discharged into a vacuum. 



For different gases the volume discharged in a given time, reduced to 

 standard pressure and temperature, is proportional to 



where p is the actual pressure, s is the specific gravity, and 6 the temperature 

 reckoned from 274 C. 



