1GG 
ON THE MOLE CUE AH MOBILITY OF OASES. 
trict, and whether mixed or otherwise, the success of it is most likely to de¬ 
pend upon its mainspring being carefully attended to, viz. to supply medicines 
and preparations of a genuine kind in preference to those of a doubtful cha¬ 
racter. 
I remain, Gentlemen, yours faithfully, 
Windermere, September , 1863. A Major Associate. 
ON THE MOLECULAR MOBILITY OF GASES. 
BY THOMAS GRAHAM, F.R.S., MASTER OF THE MINT. 
The molecular mobility of gases is here considered in reference chiefly to the passage 
of gases, under pressure, through a thin porous plate or septum, and to the partial sepa¬ 
ration of mixed gases which can be effected, as will be shown, by such means. The 
investigation arose out of a renewed and somewhat protracted inquiry regarding the dif¬ 
fusion of gases (depending upon the same molecular mobility), and has afforded certain 
new results which may prove to be of interest in a theoretical as well as in a practical 
point of view'. 
In the diffusiometer, as first constructed, a plain cylindrical glass tube, rather less than 
an inch in diameter and about ten inches in length, was simply closed at one end by a 
porous plate of plaster of Paris, about one-third of an inch in thickness, and thus con¬ 
verted into a gas receiver.* A superior material for the porous plate is now found in 
the artificially compressed graphite of Mr. Brockedon, of the quality used for making 
writing-penciis. This material is sold in London in small cubic masses about two 
inches square. A cube may easily be cut into slices of a millimetre or two in thickness 
by means of a saw of steel spring. By rubbing the surface of the slice without wetting 
it upon a flat sandstone, the thickness may be further reduced to about one-half of a 
millimetre. A circular disk of this graphite, which is like a wafer in thickness, but 
possesses considerable tenacity, is attached by resinous cement to one end of the 
glass tube above described, so as to close it and form a diffusiometer. The tube is filled 
with hydrogen gas over a mercurial trough, the porosity of the graphite plate being 
counteracted for the time by covering it tightly with a thin sheet of gutta-percha. On 
afterwards removing the latter, gaseous diffusion immediately takes place through the 
pores of the graphite. The whole hydrogen will leave the tube in forty minutes or an hour, 
and is replaced by a much smaller proportion of atmospheric air (about one-fourth), as 
is to be expected from the law 7 of the diffusion of gases. During the process, the mercury 
will rise in the tube, if allowed, forming a column of several inches in height,—a fact 
which illustrates strikingly the intensity of the force with which the interpenetration of 
different gases is effected. The native or mineral graphite is of a lamellar structure, and 
appears to have little or no porosity. It cannot be substituted for the artificial graphite 
as a diffusion-septum. Unglazed earthenware comes next in value to graphite for this 
purpose. 
The pores of artificial graphite appear to be really so minute, that a gas in mass can¬ 
not penetrate the plate at all. It seems to be molecules only which can pass; and these 
may be supposed to pass w’holly unimpeded by friction, for the smallest pores that can 
be imagined to exist in the graphite must be tunnels in magnitude to the ultimate atoms 
of a gaseous body. The sole motive agency appears to be that intestine movement of 
molecules which is now generally recognized as an essential property of the gaseous con¬ 
dition of matter. 
According to the physical hypothesis now generally received,! a gas is represented as 
consisting of solid and perfectly elastic spherical particles or atoms, which move in all 
directions, and are animated with different degrees of velocity in different gases. Con- 
* “ On the Law of the Diffusion of Gases,” Transactions of the Royal Society of Edin¬ 
burgh, vol. xii. p. 222; or, ‘ Philosophical Magazine,’ 1834, vol. ii. pp. 175, 169, 351. 
f D. Bernoulli, J. Herapath, Joule, Kronig, Clausius, Clerk Maxw y eli, and Cazin. The 
merit of reviving this hypothesis and first applying it to the facts of gaseous diffusion, is 
fairly due to Mr. Herapath. See ‘ Mathematical Physics,’ in two volumes, by John Hera¬ 
path, Esq. (1847). 
