412 Mr. T. Graham on the Molecular Mobility of Gases. 



height, the rate of flow of 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 relative times of the effusion of gases, as I have spoken of 

 it, are similar to those of molecular diffusion ; but it is important 

 to observe that the phenomena of effusion and diffusion are dis- 

 tinct and essentially different in their nature. The effusion 

 movement affects masses of gas, the diffusion movement affects 

 molecules; and a gas is usually carried by the former kind of 

 impulse with a velocity many thousand times as great as is 

 demonstrable by the latter. 



2. If the aperture of efflux be in a plate of increased thick- 

 ness, and so becomes a tube, the effusion-rates are disturbed. 

 The rates of flow of different gases, however, assume again a 

 constant ratio to each other when the capillary tube is consider- 

 ably elongated, when the length exceeds the diameter by at least 

 4000 times. These new proportions of efflux are the rates of 

 the "Capillary Transpiration" of gasesf. The rates are found 

 to be the same in a capillary tube composed of copper as they 

 are in glass, and appear to be independent of the material of the 

 capillary. A film of gas no doubt adheres to the surface of the 

 tube, and the friction is really that of gas upon gas, and is con- 

 sequently unaffected by the tube-substance. The rates of trans- 

 piration are not governed by specific gravity, and are indeed 

 singularly unlike the rates of effusion. 



The transpiration-velocity of oxygen being 1, that of chlorine 

 is 1-5, that of hydrogen 2*26, of ether vapour the same or nearly 

 the same as that of hydrogen, of nitrogen and carbonic oxide half 

 that of hydrogen, of olefiant gas, ammonia, and cyanogen 2 

 (double or nearly double that of oxygen), of carbonic acid 1*376, 

 and of the gas of marshes 1*815. In the same gas the velocity 

 of transpiration increases with increased density, whether occa- 

 sioned by cold or pressure. 



The transpiration-ratios of gases appear to be in direct rela- 

 tion with no other known property of the same gases, and they 

 form a class of phenomena remarkably isolated from all else at 

 present known of gases. 



There is one property of transpiration immediately bearing 

 upon permeation of the graphite plate by gases. The capillary 

 offers to the passage of gas a resistance analogous to that of 

 friction, proportional to the surface, and consequently increasing 

 as the tube or tubes are multiplied in number and diminished in 

 diameter, with the area of discharge preserved constant. The 



* " On the Motion of Gases," Philosophical Transactions, 1846, p. 573. 

 f Ibidem, p. 591 ; and Philosophical Transactions, 1849, p. 349. 



