ON THE MOLECULAR MOBILITY OE GASES. 
1G9 
paring the time required for the passage of equal volumes of different gases under a con¬ 
stant pressure. Of the following three gases, oxygen, hydrogen, and carbonic acid, the 
time required for the passage of an equal volume of each through a capillary glass tube, 
in similar circumstances as to pressure and temperature, was formerly observed to be as 
follows:— 
Time of capillary transpiration. 
Oxygen. 1 
Carbonic acid . 072 
Hydrogen. 044 
Through a plate of graphite, of half a millimetre in thickness, the same gases were 
now observed to pass, under a constant pressure of a column of mercury of 100 milli¬ 
metres in height, in times which are as follows:—- 
Time of molecular Square root of density 
passage. (oxygen 1). 
Oxygen . 1 . 1 
Hydrogen . 0-2472 0-2502 
Carbonic acid. 1-1886 1*1760 
It appears then that the times of passage through the graphite plate have no relation 
to the capillary transpiration-times of the same gases first quoted above. The new times 
in question, however, show a close relation to the square roots of the densities of the re¬ 
spective gases, as is seen in the last table ; and so far they agree with theoretical times of 
diffusion usually ascribed to the same gases. 
The experiments were varied by causing the gases to pass into a Torricellian vacuum, 
and consequently under the full pressure of the atmosphere. The times of penetration 
of equal volumes of gases were now— 
Times. 
V.Density. 
Oxygen. 
. 1 . 
. 1 
Air . 
. O9501 . 
. 0"9t>07 
Carbonic acid . 
. 1-1860 
. 1-1760 
Hydrogen. 
. 0-2502 
This penetration of the graphite plate by gases appears to be entirely due to their 
own proper molecular motion, quite unaided by transpiration. It seems to offer the 
simplest possible exhibition of the molecular or diffusive movement. This pure result is 
to be ascribed to the wonderfully fine porosity of the graphite. The interstitial spaces, 
or channels, appear to be sufficiently small to extinguish transpiration, or the passage 
of masses entirely. The graphite becomes a molecular sieve, allowing molecules only to 
pass through. 
With a plate of stucco, the penetration of gases under pressure is very rapid, and the 
volumes of air and hydrogen passing in equal times are as 1 to 2-801, which is a number 
for hydrogen intermediate between its transpiration-volume 2-04 and diffusion-volume 
3-8, showing that the passage through stucco is a mixed result. 
With a plate of biscuitware, 2-2 millimetres in thickness, the volume of hydrogen rose 
to 3-754 (air= 1), approaching closely to 3-8, the molecular ratio. 
The rate of passage of a gas through graphite appeared also to be closely proportional 
to the pressure. 
Further, hydrogen was found to penetrate through a graphite plate into a vacuum, 
with sensibly the same absolute velocity as it diffused into air, establishing the import¬ 
ant fact that the impelling force is the same in both movements. The molecular mobi¬ 
lity may therefore be spoken of as the diffusive movement of gases; the passage of gas 
through a porous plate into vacuum, as diffusion in one direction or single diffusion ; and 
ordinary diffusion, or the passage of two gases in opposite directions, as double, com¬ 
pound, or reciprocal diffusion. 
A tnxohjsis .—A partial separation of mixed gases and vapours of unequal diffusibility 
can be effected by allowing the mixture to permeate through a graphite plate into a 
vacuum, as was to be expected from the preceding views. As this method of analysis 
has a practical character and admits of wide application, it may be convenient to distin¬ 
guish it by a peculiar name. The amount of the separation is in proportion to the pres¬ 
sure, and attains its maximum when the gases pass into a nearly perfect vacuum. A 
