Natural plumbago or graphite has little or no porosity and cannot be used in these experiments, but the pores of artificial graphite of which pencils are made, appear to be so minute that only isolated molecules of gas are able to pass, without however being at all impeded by friction; for the smallest pores that we can suppose to exist in the graphite must be real tunnels compared with the minuteness of the ultimate atoms or molecules of a gaseous body. The cause of motion appears to reside solely in that internal movement of molecules which is now generally admitted as an essential condition of matter in a gaseous state. The molecules and atoms are assumed to be perfectly elastic and to move in all directions with different velocities according to the nature of the gas. Enclosed in a porous vessel the moving atoms constantly strike against its walls and against one another, but in consequence of their perfect elasticity, no loss of movement results from the collision. When the gases inside and outside of the tube are of the same density and molecular movement, an exchange takes place without any perceptible change of volume; but when the two gases are of different densities and molecular velocities, then the reciprocal penetration ceases to be equal on the two sides. Reciprocal diffusion of gases is accelerated by heat and retarded by cold; the tension of the gases is increased in the first case, and diminished in the second. In Mr. Graham’s experiments relating to effusion, a gas under a constant pressure was on one side of a minute opening in a very thin plate, and a vacuum on the other. The rapidity with which air or gases enter the vacuum depends upon their specific gravity. A gas rushes into a vacuum with the speed acquired by a heavy body in falling from the height of an atmosphere of the gas in question supposed to be everywhere of the same density. The height of this uniform atmosphere will be in an inverse ratio to the density of the gas. An atmosphere of hydrogen, for example, will be 16 times higher than one of oxygen. But the velocity acquired by a heavy body not being in direct proportion to the height, but to the square root of the height, it follows that the rate of flow of different gases into a vacuum will be in an inverse ratio to the square root of their respective densities. The rate of flow of oxygen being represented by 1, that of hydrogen will be represented by 4 the square root of 16. This law has been verified by experiment, and is quite analogous to that which regulates molecular diffusion, but the phenomena are essentially different. It is the gas _en masse_ which partakes of the movements of effusion, whilst only the molecules or atoms of a gas are affected by the movements of diffusion. For that reason the swiftness of the effusion of a gas is many thousand times greater than that of diffusion. The swiftness of the efflux of atmospheric air is as rapid as the velocity of sound.