1863.] 013 



a gas in mass cannot penetrate the plate at all. It seems to be 

 molecules only which can pass ; and these may be supposed to pass 

 wholly unimpeded by friction, for the smallest pores that can be 

 imagined to exist in the graphite must be tunnels in magnitude to the 

 ultimate atoms of a gaseous body. The sole motive agency appears 

 to be that intestine movement of molecules which is now generally 

 recognized as an essential property of the gaseous condition of matter. 

 According to the physical hypothesis now generally received*, a 

 gas is represented as consisting of solid and perfectly elastic spherical 

 particles or atoms, which move in all directions, and are animated 

 with different degrees of velocity in different gases. Confined 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 motion, owing to the perfect elasticity of the par- 

 ticles. Now if the containing vessel be porous, like a diffusion! eter, 

 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 con- 

 tinuing 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 in 

 both directions. 



* D. Bernoulli, J. Herapath, Joule, Kronig, Clausius, Clerk Maxwell, and 

 Cazin. The merit of reviving this hypothesis and first applying it to the facts of 

 gaseous diffusion, is fairly due to Mr. Herapath. See ' Mathematical Physics/ in 

 two volumes, by John Herapath, Esq. (1847). 



VOL. XII. 2 X 



