Notices of Boohs, 
276 
[April, 
absorbed by actual liquids or by soft colloids,” and he shows that 
those gases penetrate rubber most readily which are easily lique- 
fied by pressure, the rates of passage of carbonic acid and hy- 
drogen being ii*8i and 4*73 respectively ; that gases undergo 
liquefaction when absorbed by liquids and such colloid substances 
as india-rubber ; and finally, that the complete suspension of the 
gaseous function during the transit through india-rubber cannot 
be kept too much in view. 
Probably the subject with which Mr. Graham’s name will al- 
ways be specially connected is the diffusion of gases. The first 
paper on it was published in the “ Quarterly Journal of Science” 
for 1829. He found that the lighter a gas is the more quickly it 
diffuses away from an open cylinder, and the experiments led 
him to believe that the diffusiveness of gases is inversely as 
some fundtion of their density, apparently the square root of 
their density. In this long series of researches the molecular 
aggregation of the septa through which the gases penetrated 
was always carefully considered, and in dealing with gaseous 
penetration he clearly saw that orifices of excessive minuteness 
might be quite impassable by gases of low diffusive power ; that 
in other cases pores of graphite could only be permeable by 
molecules ; and lastly, as will be subsequently shown, that there 
might be an inter-molecular porosity due entirely to dilatation at 
a high temperature. 
In a paper published in 1831 he established the following law 
of the diffusion of gases : — The diffusion or spontaneous inter- 
mixture of two gases in contact is effedled by an interchange in 
position of indefinitely minute volumes of the gases, which 
volumes are not necessarily of equal magnitude, being in the 
case of each gas inversely proportional to the square root of the 
density of that gas. He speaks of diffusion being effedled by a 
force of the highest intensity, and urges that sensible masses 
are not affedled by diffusion, but only molecules. Diffusion 
afforded, in Graham’s hands, the most conclusive proofs of mo- 
lecular movement, and as an instance of the rapidity of such 
molecular mobility it should be remembered that Maxwell has 
estimated that the initial velocity of a molecule of hydrogen is 
nearly i860 metres per second. 
The continuation of the researches led him to study the passage 
of gases through a minute orifice in a thin disc of platinum, 
a mode of passage, termed effusion, which left no doubt of the 
truth of a general law, that different gases pass through minute 
orifices in times which are as the square roots of their respective 
densities, or with velocities which are inversely as the square 
roots of their respective densities. The research afforded an 
experimental verification of the truth of the mechanical law, that 
the velocity with which a gas rushes into a vacuum, through a 
minute aperture, is the same as that which a heavy body would 
acquire in falling from the height of an atmosphere composed of 
