OX THE MOLECULAR MOBILITY OF GASES. 
167 
fined in a vessel, the moving particles are constantly impinging against its sides and 
occasionally against each other, and such collisions take place without any loss of mo¬ 
tion, owing to the perfect elasticity of the particles. Now if the containing vessel be 
porous, like a diffusiometer, then gas is projected through the open channels, by the 
atomic motion described, and escapes. Simultaneously the external air or gas, whatever 
it may be, is carried inwards in the same manner, and takes the place of the gas which 
leaves the vessel. To the same atomic or molecular movement is due the elastic force, 
with the power to resist compression, possessed by gases. The molecular movement is 
accelerated by heat and retarded by cold, the tension of the gas being increased in the 
first instance and diminished in the second. Even when the same gas is present both within 
and without the vessel, and is therefore in contact with both sides of the porous plate, 
the movement is sustained without abatement—molecules continuing to enter and leave 
in equal number, although nothing of the kind is indicated by change of volume or 
otherwise. If the gases in communication be different, but possess sensibly the same 
specific gravity and molecular velocity, as nitrogen and carbonic oxide do, an interchange 
of molecules also takes place without any change in volume. With gases opposed of 
unequal density and molecular velocity, the amount of penetration ceases of course to 
be equal iu both directions. 
These observations are preliminary to the consideration of the passage through a gra¬ 
phite plate, in one direction only, of gas under pressure, or under the influence of its 
own elastic force. It is to be supposed that a vacuum is maintained on one side of the 
porous septum, 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 pas¬ 
sage of different gases is then regulated by their specific gravities, according to a pneu¬ 
matic law which was deduced by Professor John Eobinson from Torricelli’s well-known 
theorem of the velocity of efflux of fluids. A gas rushes into a vacuum with the velo¬ 
city 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. It 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 constant ratio to each other when the capillary tube is considerably 
elongated, when the length exceeds the diameter at least 4000 times. These new pro¬ 
portions of efflux are the rates of the “ Capillary Transpiration of Gases.”f The rates 
were found to be the same in a capillary tube composed of copper as they are in a tube 
of glass, and appear to be independent of the material of the capillary. A film of gas 
no doubt adheres to the inner surface of the tube, and the friction is really that of gas 
upon gas, and is consequently unaffected by the nature of the tube-substance. The 
rates of transpiration are not governed by specific gravity, and are indeed singularly 
unlike the rates of effusion. 
With respect to the different states of gas, liquid, and solid, it may be observed that 
there is no incompatibility with each other in the^e physical conditions. They are often 
found together in the same substance. The liquid and the solid conditions supervene 
* “On the Motion of Gases,” Phil. Trans. 1846, p. 573. 
f Phil. Trans. 184(5, p. 591 and 1849, p. 349. 
