Mr. T. Graham on the Molecular Mobility of Gases. 427 



0*9373, which are inversely as the diffusive velocity of the two 



gases. 



Diffusive velocity. 

 Oxygen .... 1 

 Nitrogen .... 1*0669 



The velocity of nitrogen therefore exceeds that of oxygen by 

 about 6*7 per cent. Hence by a simple diffusion of a whole 

 volume of air the oxygen could only be increased 6*7 per cent., 

 according to theory. In experiments such as the preceding, only 

 one-half of the volume of the air is diffused, and consequently 

 only one-half of the stated amount of concentration of oxygen 

 could possibly be produced at each step. About three-fourths 

 of the theoretical separation is actually obtained, although the 

 apparatus works at an obvious disadvantage from the air within 

 the jar being at rest. 



This diffusive method of separation recalls the- original obser- 

 vation of Dobereiner on the escape of hydrogen gas from a fis- 

 sured jar standing over water, which will always hold its place 

 in scientific history as the starting-point of the experimental 

 study of gaseous diffusion. That observation proved to be an 

 instance of double diffusion, air entering the jar by the fissure at 

 the same time that hydrogen escaped by it — although, as Dobe- 

 reiner looked upon the phenomenon, it was more akin to single 

 diffusion or the passage of gas in one direction only*. 



The atmolytic power of other diffusing-plates was tested, besides 

 the artificial graphite. 



The barometrical diffusiometer already described was closed by 

 a plate of red unglazed earthenware, 4 millims. in thickness, 

 which was attached to the glass by resinous cement. 



Dry air was swept over the upper surface, as in operating with 

 the graphite plate. With a mercurial column of 340 millimsc 

 falling to 200 millims., the air which entered was found to con- 

 tain 79*45 per cent, of nitrogen, instead of 79. With a column 

 of mercury, maintained at 508 millims. in the tube, the air enter- 

 ing contained 79*72 nitrogen, and with a column beginning at 

 761 millims., the full barometrical height, and falling to 679 

 millims. in seven minutes, the air entering contained 80*21 

 nitrogen. This is a full degree of separation, exceeding 1 per 

 cent., while the time was greatly shorter than with graphite. 

 Thermometer 19°*5 C. 



With a diffusing-plate of gypsum (stucco) 10 millims. in 

 thickness, the proportion of nitrogen was also increased, although 

 less considerably than with biscuitware. The standard pro- 



* Annates de Chimie, 1825. 

 2F2 



