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



In a series of four experiments made with hydrogen, the 

 mercury fell from 758 to 685 millims. (29*9 inches to 27 inches) 

 in 252, 256, 254, and 256 seconds ; mean 254*5 seconds. 



In three experiments with oxygen the mercury fell through 

 the same space in 10] 9, 1025, and 1024 seconds; mean 1022*7 

 seconds : 



254*5 



The times of these gases appear therefore to be as 1 to 4*018, 

 while the times calculated as being inversely as the square root 

 of the densities of the same gases are as 1 to 4. 



On another day, with a different height of the barometer, four 

 gases were passed through the graphite plate in succession 

 through a somewhat shorter range, namely, from 754 to 685 

 millims. (29*7 to 27 inches). 



The time of permeation of air was 884 and 885 seconds ; 

 mean 884*5 seconds. 



The time of carbonic acid was 1100 and 1106 seconds; mean 

 1103 seconds. 



The time of oxygen was 936, 924, and 930 seconds; mean 

 930 seconds. 



The time of hydrogen was 229, 235, and 235 seconds ; mean 

 233 seconds. 



These times of permeation are in the following proportion : — 



Times of the permeation of equal 

 volumes of gas through graphite. 



Oxygen 1 



Air 0*9501 



Carbonic acid . . . 1*1860 

 Hydrogen 0*2505 



These numbers approach so closely to the square roots of the 

 density, or the theoretical diffusion-times of the same gases, 

 namely, oxygen ], air 0*9507, carbonic acid 1*176, and hy- 

 drogen 0*2502, that they may be held to indicate the prevalence 

 of a common law. They exclude the idea of capillary transpi- 

 ration, which gives to the same gases entirely different numbers. 



The movement of gases through the graphite plate appears 

 to be solely due to their own proper molecular motion, quite 

 unaided by transpiration. It seems to be the simplest possible 

 exhibition of the molecular or diffusive movement of gases. This 

 pure result is to be ascribed to the wonderfully fine (minute) 

 porosity of the graphite. The interstitial spaces appear to be 

 sufficiently small to extinguish capillary transpiration entirely. 

 The graphite plate is a pneumatic sieve which stops all gaseous 

 matter in mass, and permits molecules only to pass. 



