Mr. T. Graham on the Molecular Mobility of Gases. 421 



The observation was repeated at the pressure of 100 millims. 

 with barometer at 754 millims. and thermometer at 10° C. 



Air 1920 



Hydrogen , . . . *498 ~ d ***" 



The velocity of hydrogen appears, as usual, to be nearly 3*8 



times that of air; , ^^^ =3*7994. 

 V'06926 



An experiment was made at the same time as the former series 

 upon a mixture of 95 hydrogen and 5 air, which gave an un- 

 looked-for result that led to a great deal of inquiry. It is known 

 that such a mixture is effused through an aperture in a fine plate 

 in a time which is as the square root of the density of the mix- 

 ture, and therefore nearly the arithmetical mean of the two gases 

 effused separately. But in transpiration by a capillary, a mix- 

 ture of 95 hydrogen and 5 air requires a considerably longer 

 time than the gases transpired separately. In fact 5 per cent, 

 of air retards the transpiration of hydrogen nearly as much as 

 20 per. cent, of air would retard the effusion of hydrogen*. Now 

 the mixture in question permeates the graphite plate in 52 7* 5 

 seconds, while the calculated mean of the times of the two gases 

 is 562*1 seconds. 



The mixture has therefore passed neither in the effusion time, 

 nor in a longer time as it would do by capillary transpiration, 

 but, singular to say, in a time considerably shorter than either. 

 The gas that came through was found by analysis to be altered 

 in composition. It contained more hydrogen and less air than 

 the original mixture. Hence it passed through with increased 

 rapidity. On consideration it appeared that such a separation 

 of the mixed gases must follow as a consequence of the movement 

 being molecular. Each gas is impelled by its own peculiar mo- 

 lecular force, which, as has been seen, is capable of causing 

 hydrogen to permeate the graphite plate about 3*8 times as 

 rapidly as air. 



Each gas may permeate a graphite plate into a vacuum with 

 the same relative velocity as it diffuses into another gaseous 

 atmosphere, but it remains a question whether the velocities of 

 permeation and diffusion are absolutely as well as relatively the 

 same. To illustrate this point, hydrogen and air were first 

 allowed to permeate into a vacuum, and then to diffuse into each 

 other, through the same graphite plate, which was 1 millim. in 

 thickness. The plate was a circular disc of 22 millims. in 

 diameter. 



The mercurial column in the barometrical diffusiometer fell 



* Philosophical Transactions, 1846, p. 628. 



