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



starting, the gas may be uniform in composition on both sides of 

 the graphite plate. The height of the mercurial column, which 

 measures the aspirating force of the diffusiometer, is preserved 

 uniform by gradually raising the tube in the mercurial trough 

 in proportion as gas enters and the mercury falls. The diffusio- 

 meter is suspended from the roof of the apartment by a cord pass- 

 ing over a pulley and properly weighted, as in former expe- 

 riments. 



The mixture to be diffused consisted of nearly equal volumes 

 of oxygen and hydrogen. The effect of different degrees of 

 pressure on the amount of separation produced was first observed. 

 It will be seen that as the pressure or aspirating force is increased 

 the amount of separation becomes greater. Barom. 0*759millim. ; 

 therm. 18°*3 C. 



Diffusion into a partial vacuum. 



Oxygen. Hydrogen. 



Composition of original mixture in 100 parts. 49'3 50*7 



Diffused by pressure of 100 millims. . . . 47*0 53 



Diffused by pressure of 400 millims. . . . 37*5 62*5 



Diffused by pressure of 673 millims. (meanl 0A ^ Q „ 



of 635-710) ........ J 7d '° 



Diffused by pressure of 747 millims. (mean~l _ Q 



of 736-759) J^' b 77 '^ 



In the last observation, or that with the greatest pressure 

 (747 millims.), the oxygen is reduced to 22*8 per cent, and the 

 hydrogen increased to 77'2 per cent, of the diffused mixture, 

 snowing a considerable separation. The mixed gases appear to 

 make their way through the graphite plate independently, each 

 following its own peculiar rate of diffusion. 



But it is only under the aspiration of a complete vacuum that 

 the separation can attain its maximum, and reach the full differ- 

 ence that may exist between the special diffusibilities of the two 

 gases. The reason is that while we have the original mixture 

 on both sides of the plate, and of equal tension, the gases are 

 not at rest, but diffusion is proceeding as actively through the 

 plate, in opposite directions, as if the gases were different or the 

 tension unequal on the two sides. This is a condition of the 

 molecular mobility of gases (p. 411). The tension therefore being 

 supposed to differ by 100 millims. only, as when the gas above 

 the plate was of 759 millims. tension, and below of 659 millims. 

 (in the first experiment of the last series), then 100 volumes 

 only out of 759 of the mixture are subject to separation. But 

 while these 100 volumes press through they are accompanied by 

 659 volumes of unchanged mixture. The latter 659 volumes 



