422 Mr. T. Graham on the Molecular Mobility of Gases, 



from 762 to 685 millims. (30 inches to 27) with air in 878 



seconds, and with hydrogen in 233 seconds. 



Air 878 



Hydrogen . . . 233 -3 768. 



The volume of gas which produced this effect was found by the 

 calibration of the tube to be 8*85 cub. centims. Hence 1*22 cub. 

 centim. of the hydrogen entered the diffusiometer in 60 seconds, 

 or one minute. But the pressure under which the hydrogen 

 gas entered was the mean of 762 to 685 millims., or 723*5 

 millims. ; while a whole atmosphere (the height of the barometer 

 at the time) was 765 millims. The volume of the gas has there- 

 fore to be increased as 723*5 to 765 to give the full action of a 

 vacuum. The volume becomes 1*289 cub. centim. in one minute. 

 When the diffusiometer was filled with hydrogen and the gas 

 allowed to diffuse into air, the rise of the mercury was pretty 

 uniform for the first five minutes, being 15*5 millim. divisions 

 in the first two minutes, 7 in the third minute, 7*5 in the fourth 

 minute, and 7 in the fifth minute, making 37 divisions in five 

 minutes. But as in diffusion 1 air may be supposed to enter 

 the tube for 3*8 hydrogen which escape, the hydrogen which 



diffused was more than 37 divisions, by 5-^, that is, by about 



10 divisions. Hence 47 divisions of hydrogen have diffused 

 into air in five minutes. These divisions measured, by the cali- 

 bration of the tube, 6*2 15 cub. centims. One-fifth of this amount, 

 that is, 1*243 cub. centim., diffused in one minute. The result of 

 the whole is that in one minute there passed of hydrogen 

 through the graphite plate, 



1*289 cub. centim. by permeation into a vacuum, 

 1*243 cub. centim. by diffusion into air. 



The numbers indicate a close approach to equality in the 

 velocities of permeation into a vacuum and of diffusion into an- 

 other gas, through the same porous diaphragm. The diffusion 

 appears the slower of the two by a small amount ; but this is as it 

 should be, our estimate of the diffusion-velocity being certainly 

 underrated ; for the initial diffusion, or even the diffusion in 

 the first minute, must obviously be somewhat greater than the 

 average of the first five minutes, which we have taken to repre- 

 sent it — the hydrogen necessarily diffusing out in a diminishing 

 progression, or more slowly in proportion as air has entered the 

 diffusiometer. It is strictly the initial velocity of diffusion (that 

 of the first second if it could be obtained) that ought to be com- 

 pared with the percolation into a vacuum. 



In fine, there can be little doubt left on the mind that the 

 permeation through the graphite plate into a vacuum and the dif- 



