Mr. T. Graham on the Molecular Mobility of Gases* 41 1 



animated with different degrees of velocity in different gases. 

 Confined in a vessel the moving particles are constantly impin- 

 ging against its sides and occasionally against each other, and 

 this contact takes place without any loss of motion, owing to the 

 perfect elasticity of the particles. 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 is carried inwards in the same 

 manner, and takes the place of the gas which leaves the vessel. 

 To this 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, or is in contact with both sides of 

 our porous plate, the movement is sustained without abatement — ■ 

 molecules continuing to enter and to leave the vessel in equal 

 number, although nothing of the kind is indicated by a change 

 of volume or otherwise. If the gases in communication be dif- 

 ferent but possess sensibly the same specific gravity and mole- 

 cular 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 permeation ceases of course to be equal in both directions. 



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. 

 We are to suppose a vacuum to be maintained on one side of the 

 porous septum, and air or any other gas, under a constant pres- 

 sure, to be in contact with the other side. Now a gas may pass 

 into a vacuum in three different modes, or in two other 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 regulated by their specific gravities, according to a 

 pneumatic law which was deduced by Professor John Eobison 

 from Torricellr'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 atmo- 

 sphere composed of the gas in question, and supposed to be of 

 uniform density throughout. The height of the uniform atmo- 

 sphere would be inversely as the density of the gas, the atmo- 

 sphere 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 



2E 2 



