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XXVIII. On the Motion of Gases. By Thomas Graham, Esq., F.R.S., Professor of 
Chemistry in University College, London ; Hon. Fellow of the Royal Society of 
Edinburgh ; Corresponding Member of the Royal Academies of Sciences of Berlin 
and Munich, of the National Institute of Washington, <§c. 
Received June 18, — Read June 18, 1846, 
The spontaneous intermixture of different gases, and their passage under pressure 
through apertures in thin plates and by tubes, form a class of phenomena of which 
the laws have been only partially established by experiment. The separation of two 
gases by a porous screen, such as a plate of dry stucco, will prevent for a short time 
any sensible intermixture arising from slight inequalities of pressure, but such a 
barrier is readily overcome by the diffusive power of the gases, which is fully equal 
to their whole elastic force. Hence a cylindrical glass jar with a stucco top, filled 
with any gas and standing over water, affords the means of demonstrating the un- 
equal diffusive velocities of air and the gas, by the final contraction or expansion of 
the gaseous contents of the jar, after the escape of the gas is completed. Compared 
with the volume of air which has entered, the volume of gas which has passed simul- 
taneously outwards is found to be in the inverse proportion of the square root of the 
specific gravity of the gas. The diffusive velocities therefore of different gases are 
inversely as the square root of their densities ; or the times of diffusion of equal 
volumes directly as the square root of the densities of the gases*. 
Such is also the theoretical law of the passage of gases into a vacuum, according 
to the well-known theorem that the molecules of a gas rush into a vacuum with the 
velocity they would acquire by falling from the summit of an atmosphere of the gas 
of the same density throughout ; while the height of such an atmosphere, composed 
of different gases, is inversely as their specific gravities. This is a particular case of 
the general law of the movement of fluids, well-established by observation for liquids, 
and extended by analogy to gases. The experiments which have already been made 
upon air and other gases, by M. P. S. Girard and by Mr. Faraday^;, are sufficient 
to show that the discharge of light is more rapid than that of heavy gases ; and are 
interesting as first approximations, although incomplete and lending a very imperfect 
support to the theoretical law. Indeed some results obtained by these experimenters 
* On the Law of the Diffusion of Gases; Transactions of the Royal Society of Edinburgh, vol. xii. p. 222 : 
or Philosophical Magazine, 1834, vol. ii. pp. 175, 269, 351. 
t Annales de Chimie, &c., 2de Ser., t. 16. p. 129. 
% Quarterly Journal of Science, vol. iii. p. 354; and vol. vii. p. 106. 
