614 [June 18, 



These observations are preliminary to the consideration of the 

 passage through a graphite plate, in one direction only, of gas under 

 pressure, or under the influence of its own elastic force. It is to he 

 supposed that a vacuum is maintained on one side of the porous sep- 

 tum, and that air or some other gas, under a constant pressure, is in 

 contact with the other side. Now a gas may pass into a vacuum in 

 three different modes, or in two modes besides that immediately 

 before us. 



1 . The gas may enter the vacuum by passing through a minute 

 aperture in a thin plate, such as a puncture in platinum foil made by 

 a fine steel point. The rate of passage of different gases is then re- 

 gulated by their specific gravities, according to a pneumatic law 

 which was deduced by Professor John Eobison from Torricelli's 

 well-known theorem of the velocity of efflux of fluids. A gas rushes 

 into a vacuum with the velocity which a heavy body would acquire 

 by falling from the height of an atmosphere composed of the gas in 

 question, and supposed to be of uniform density throughout. The 

 height of the uniform atmosphere will be inversely as the specific 

 gravity of the gas, the atmosphere of hydrogen, for instance, sixteen 

 times higher than that of oxygen. But as the velocity acquired by a 

 heavy body in falling is not directly as the height, but as the square 

 root of the height, the rate of flow of different gases into a vacuum 

 will be inversely as the square root of their respective densities. 

 The velocity of oxygen being 1, that of hydrogen will be 4, the square 

 root of 16. This law has been experimentally verified*. The times 

 of the effusion of gases, as I have spoken of it, are similar to those 

 of the law of molecular diffusion ; but it is important to observe that 

 the phenomena of effusion and diffusion are distinct and essentially 

 different in their nature. The effusion movement affects masses of 

 gas, the diffusion movement affects molecules ; and a gas is usually 

 carried by the former kind of impulse with a velocity many thousand 

 times greater than by the latter. The effusion velocity of air is the 

 same as the velocity of sound. 



2. If the aperture of efflux be in a plate of increased thickness, 

 and so becomes a tube, the effusion-rates of gases are disturbed. 

 The rates of flow of different gases, however, assume again a con- 

 stant ratio to each other when the capillary tube is considerably 



* " On the Motion of Gases," Phil. Trans. 1846, p. 573 



