Mr. T. Graham on the Molecular Mobility of Gases. 425 



are replaced by an equal bulk of unchanged mixture diffused 

 from below, so that the volumes are not disturbed by this por- 

 tion of the molecular interchange. 



The amount of separation, then, attainable by transmitting a 

 mixed gas through a porous diaphragm by pressure will be in 

 proportion to the pressure — that is, to the inequality of tension 

 on different sides of the diaphragm. 



Oxygen and Nitrogen. — The separation of the gases of the 

 atmosphere by transmission through the graphite plate has a 

 peculiar interest. 



In an experiment resembling those last described, atmospheric 

 air was swept over the upper surface of a graphite plate having 

 a thickness of 2 millims. The gas that penetrated into the 

 vacuum contained, as was to be expected, the lighter and more 

 diffusible constituent in excess. It gave by the pyrogallic acid 

 and potash process of Liebig, 



Oxygen 20 



Nitrogen 80 



This was an increase in the nitrogen of quite 1 per cent. ; for 

 air, analysed for comparison at the same time and in the same 

 manner, gave oxygen 21 '03, and nitrogen 78*97. 



It may be legitimately inferred from the last experiment, that 

 if pure hydrogen in a diffusiometer were allowed to diffuse into 

 the atmosphere through a porous plate, the portion of air which 

 then enters the diffusiometer should also have its composition 

 disturbed. A diffusion of hydrogen through a graphite plate 

 was interrupted before completion. The air which had entered 

 was found to consist of 



Oxygen .... 19*77 

 Nitrogen .... 80*23 



100-00 



The increase of nitrogen is 1*23 per cent. 



While the nitrogen is increased and the oxygen diminished in 

 the air which makes its way under pressure through the graphite, 

 the converse effect must be produced on the air left behind. But 

 the latter result of atmolysis cannot be made apparent without a 

 change in the mode of experimenting. 



With the view of effecting an increase in the proportion of 

 oxygen, a volume of air, confined in a jar suspended over mer- 

 cury, was allowed to communicate through a graphite plate of 2 

 millims. in thickness, with a vacuum sustained by means of an 

 air-pump, the gauge being about 1 inch only below the height of 

 the barometer during the whole time of experimenting. 



The jar containing the air to be atmolysed was formed of a 

 Phil. Mag. S. 4. Vol. 26. No. 177. Dec. 1863. 2 F 



