170 
ON TIIE MOLECULAR MOBILITY OF GASES. 
variety of experiments were made on this subject, of which perhaps the most interesting 
were those upon the concentration of the oxygen in atmospheric air. When a portion 
of air confined in a jar is allowed to penetrate into a vacuum through graphite or un¬ 
glazed earthenware, the nitrogen should pass more rapidly than the oxygen in the pro¬ 
portion of 1*0668 to 1, and the proportion of oxygen be proportionally increased in the 
air left behind in the jar. The increase in the oxygen actually observed when the air in 
the jar was reduced from 1 volume 
To 0*5 volume, was 0*48 per cent. 
0*25 „ „ 0*98 
0*125 ., „ 1*54 ., 
0*0625 „ „ 2*02 „ 
Or, the oxygen increased from 21 to 28*02 per cent, in the last sixteenth part of air left 
behind in the jar. 
The most remarkable effects of separation are produced by means of the tube atmolyser. 
This is simply a narrow tube of unglazed earthenware, such as a tobacco-pipe stem two 
feet in length, which is placed within a shorter tube of glass and secured in its position 
by corks, so as to appear like a Liebig’s condenser. The glass tube is placed in commu¬ 
nication with an air-pump, and the annular space between the two tubes is maintained 
as nearly vacuous as possible. Air or any other mixed gas is then allowed to flow in a 
stream along the clay tube, and collected as it issues. The gas so atmolysed is of course 
reduced in volume, much gas penetrating through the pores of the clay tube into the 
air-pump vacuum; and the slower the gas is collected the greater the proportional loss. 
In the gas collected, the denser constituent of the mixture is thus concentrated in an 
arithmetical ratio, while the volume of the gas is reduced in a geometrical ratio. In one 
experiment the proportion of oxygen in the air after traversing the atmolyser was in¬ 
creased to 24*5 per cent., or 16*7 upon 100 oxygen originally present in the air. With 
gases differing so much in density and diffusibility as oxygen and hydrogen, the sepa¬ 
ration is of course much more considerable. The explosive mixture of two volumes of 
hydrogen and one volume of oxygen, gave oxygen containing only 9*3 per cent, of hy¬ 
drogen, in which a taper burned without explosion ; and with equal volumes of oxygen 
and hydrogen, the proportion of the latter was easily reduced from 50 to 5 per cent. 
Inter diffusion of Gases—double diffusion .—The diffusiometer was much improved in 
construction by Professor Bunsen, from the application of a lever arrangement to raise and 
depress the tube in the mercurial trough. But the mass of stucco forming the porous 
plate in his instrument was too voluminous, in my opinion, and, from being dried by 
heat, had probably detached itself from the walls of the glass tube. The result obtained 
of 3*4 for hydrogen, air being 1, is, I understand, no longer insisted upon by that illus¬ 
trious physicist. It is indeed curious that my old experiments generally rather exceeded 
than fell short of the theoretical number for hydrogen, / y/ i “ 3 ’7997. With 
stucco as the material, the cavities in the porous plate form about one-fourth of its bulk, 
and affects sensibly the ratio in question, according as they are or are not included in the 
capacity of the instrument. Beginning the diffusion always with these cavities filled 
with hydrogen, the numbers now obtained with a stucco plate of 12 millims. in thickness, 
dried without heat, were 8*783,3*8, and 3*739 when the volume of the cavities of stucco 
is added to the air and hydrogen, and 3*931, 8.949, and 3*883 when such addition is not 
made to these volumes. The graphite plate, on the other hand, being thin, and the 
volume of its pores too minute to require to be taken into account, its action is not at¬ 
tended with the same uncertainty. With a graphite plate of 2 millim. in thickness, the 
number for hydrogen into air was 3 876, and of hydrogen into oxygen 4*124, instead of 
4. With a graphite plate of 1 millim. in thickness, hydrogen gave 3*993 to air 1. With 
a graphite plate of 0*5 millim. in thickness, the proportional number for hydrogen to air 
rose to 3*984, 4*068, and 4*067. A similar departure from the theoretical number was 
observed when hydrogen was diffused into oxygen or carbonic acid, instead of air. All 
these experiments were made over mercury and with dried gases. It appears that the 
numbers are most in accordance with theory when the graphite plate is thick, and the 
diffusion slow in consequence. If the diffusion be very rapid, as it is with the thin 
plates, something like a current is possibly formed in the channels of the graphite, taking 
the direction of the hydrogen and carrying back in mass a little air, or the slower gas, 
