CHEMICAL SCIENCE. 169 



as nitrogen and carbonic oxide do, an interchange of molecules also 

 takes place without any change in volume. With gases opposed of un- 

 equal density and molecular velocity, the amount of penetration ceases, 

 of course, to' be equal in both directions. These observations are pre- 

 liminary to the consideration of the passage through a graphite plate 

 in one direction only, of gas under pressure, or under the influence of 

 its own elastic force. It is to be supposed that a vacuum is maintained 

 on one side of the porous septum and that air or some other gas, under 

 a constant pressure, is in contact with the other side. Now, a gas may 

 pass into a vacuum in three different modes, or in two modes besides 

 that immediately before us, 1st. The gas may enter the vacuum by 

 passing through a minute aperture in a thin plate such as a puncture 

 in platinum-foil, made by a fine steel point. The rate of passage of 

 different gases is then regulated by their specific gravities, according 

 to a pneumatic law, which was deduced by Prof. Robinson from Torri- 

 celli's well-known theorem of the velocity of efflux of fluids. A gas 

 rushes into a vacuum with the velocity which a heavy body would ac- 

 quire by falling from the height of an atmosphere composed of the gas 

 in question, and supposed to be of uniform density throughout. The 

 height of the uniform atmosphere will be inversely as the specific grav- 

 ity of the gas ; the atmosphere of hydrogen for instance, sixteen times 

 higher than that of oxygen. But as the velocity acquired by a 

 heavy body in falling is not directly as the height, but as the square 

 root of the height, the rate of flow of the different gases into a vacuum 

 will be inversely as the square root of their respective densities. The 

 velocity of oxygen being 1, that of hydrogen will be 4, the square root 

 of 16. This law has been experimentally verified. The times of the 

 effusion of gases, as I have spoken of it, are similar to those of the law 

 of molecular diffusion ; but it is important to observe that the phenom- 

 ena of effusion and diffusion are distinct and essentially different in 

 their nature. The effusion movement affects masses of gas, the diffu- 

 sion movement affects molecules ; and a gas is usually carried by the 

 former kind of impulse with a velocity many thousand times greater 

 than by the latter. The effusion velocity of air is the same as the 

 velocity of sound. 2. If the aperture of efflux be in a plate of in- 

 creased thickness, and so becomes a tube, the effusion rates of gases 

 are disturbed. The rates of floAV of different gases, however, assume 

 again a constant ratio to each other when the capillary tube is consid- 

 erably elongated, when the length exceeds the diameter at least 4,000 

 times. These new proportions of efflux are the rates of the " Capillary 

 Transpiration of Gases." The rates were found to be the same in a 

 capillary tube composed of copper as they are in a tube of glass, and 

 appear to be independent of the material of the capillary. A film of 

 gas, no doubt, adheres to the inner surface of the tube, and the friction 

 is really that of gas upon gas, and is consequently unaffected by the 

 nature of the tube-substance. The rates of transpiration* are not gov- 

 erned by specific gravity, and are, indeed, singularly unlike the rates 

 of effusion. The transpiration velocity of oxygen being 1, that of chlo- 

 rine is 1.5, that of hydrogen 2.26 ; of nitrogen and carbonic oxide half 

 the velocity of hydrogen ; of olefiant gas, ammonia and cyanogen 2, 

 double, or nearly double, that of oxygen ; of carbonic acid 1.376. In 

 the same gas, the transpirability of equal volumes increases with den- 



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