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XIX. On the Motion of Gases. — Part II. 
By Thomas Graham, Esq., F.R.S., F.C.S., Professor of Chemistry in University 
College, London ; Hon. Fellow of the Royal Society of Edinburgh ; Corresponding 
Member of the Institute of France, of the Royal Academies of Sciences of Berlin 
and Munich, of the National Institute of LFashington, S^c. 
Received June 21, — Read June 21, 1849. 
All experiments on the velocity with which different gases rush into a vacuum, 
or pass under pressure through an aperture in a thin plate, are in strict accordance 
with the physical law that the times of passage for equal volumes are proportional to 
the square roots of the densities of the various gases. Besides being the law of 
“ Effusion,” this is also the law of the Diffusion of one gas into an atmosphere of 
another gas. The result in both cases is simply and exclusively a consequence of 
specific gravity. 
The velocity with which gases of different nature pass through a tube is necessarily 
much influenced by the law of their effusion, when the tube is short and approaches 
in character to an aperture in a thin plate. But if the length of the tube is progres- 
sively increased, its diameter or the aperture remaining constant, then while the re- 
sistance increases and the passage for all gases becomes greatly slower, the velocities 
of the different gases are found rapidly to diverge from those of their effusion. The 
velocities of different gases appear at last however to attain a particular ratio with a 
certain length of tube and resistance ; and preserve the same relation to each other 
for greater lengths and resistances. After attaining this constant ratio, the passage 
of all the gases becomes slower, exactly in proportion to the increased length of the 
tube, that is, in proportion to the resistance. The different gases are now equally 
affected by the resistance, and their relative velocities are therefore undisturbed and 
remain constant. The effect of the law of effusion upon the velocities is no longer 
sensible, and appears to be eliminated. 
As the rates of passage of different gases through a tube appear to depend upon a 
new and peculiar property of gases, I have spoken of it as the Transpiration or Trans- 
pirability of gases. The rates of transpiration appear not to be affected by the ma- 
terial of the tube, as they are found the same for capillary tubes of glass and of 
copper and for a porous mass of stucco. I may add that such experiments exhibit a 
constancy and possess a neatness and precision which is very extraordinary. The 
experiments of M. Poiseuille indicate an equally remarkable constancy and pre- 
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