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PROFESSOR GRAHAM ON THE MOTION OF GASES. 
and others, appear wholly inconsistent with that law, such as Mr. Faraday’s curious 
observations of the change of the relative rates of hydrogen and olefiant gases in 
passing through a capillary tube under different pressures ; and my own observation, 
that carbonic acid gas is forced by pressure through a porous mass of stucco as 
quickly or more so than air is, although more than a half heavier; and that other 
gases pass in times which have no obvious relation to their diffusive velocities*. 
In studying this subject, I found that it was necessary to keep entirely apart the 
two cases of the passage of a gas through a small aperture in a thin plate and its 
passage through a tube of sensible length. The phenomena of the first class then 
became well-defined and simple, and quite agreeable to theory. Those of the second 
class also attained a high degree of regularity, where the tubes were of great length, or 
being short were of extremely small diameter. Capillary glass tubes, which varied in 
length from twenty feet to two inches, were found equally available and gave similar 
results, where a sufficient resistance was offered to the passage of the gas. 
The rate of discharge of different gases from capillary tubes appears to be inde- 
pendent of the nature of the material of the tube, in so far as the rates were found to 
be similar for tubes of glass and copper, and even for a porous mass of stucco. But 
while the discharge by apertures in thin plates is found to be dependent in all gases 
upon a constant function of their specific gravity, the discharge of the same gases 
from tubes has no uniform relation to the density of the gases. Both hydrogen and 
carbonic acid, for instance, pass more quickly through a tube than oxygen, although 
the one is lighter and the other heavier than that gas. I shall assume then for the 
present, that in the passage of gases through tubes we have the interference of a new 
and peculiar property of gases ; and on the ground of a radical difference in agency 
speak of the two classes of phenomena under different names. The passage of gases 
into a vacuum through an aperture in a thin plate I shall refer to as the Effusion of 
gases, and to their passage through a tube as the Transpiration of gases. The deter- 
mination of the coefficients of effusion and transpiration of various gases will be the 
principal object of'the following paper. 
Part I.— EFFUSION OF GASES. 
1. Effusion into a Vacuum by a glass jet. 
The glass jet was formed from a short piece of a capillary thermometer tube, of 
which the bore was cylindrical, to which a conical termination was given by draw- 
ing it out when softened by heat and breaking the point. The aperture at the point 
of the jet was cylindrical, in a flat surface, and so small that it could only be seen 
distinctly by means of a magnifying-glass ; its size, compared with other apertures, may 
be expressed by the statement that one cubic inch of air of the usual tension passed 
into a vacuum through this aperture in 2T8 seconds. By means of a perforated 
* Edinburgh Transactions, xii. 238. 
