chap, xxiii.] DIFFUSION OF GASES. 289 



Gases are found to differ from one another in the 

 rate with which they diffuse. Experiments made by 

 Graham showed the diffusive power to vary with the 

 density, the less dense gas diffusing more rapidly than 

 the denser gas, the gases diffusing in the inverse 

 proportion to the square roots of their densities. 

 Thus, the ratio of the density of hydrogen to that of 

 oxygen being as 1 : 4, their diffusive rates will be as 

 4:1. Two gases being placed in contact with one 

 another, experiment has shown that the mixture will 

 be more rapid as the difference of density between 

 the two is greater. This is to be expected from what 

 has been already seen to apply between two liquids of 

 different densities in contact. The greater the differ- 

 ence of densities the more rapid is the rate of 

 exchange, and as the two liquids come to approximate 

 more nearly to the same condition the rate of exchange 

 is lowered. 



The physiological application of these laws 

 is apparent in respiration. About thirty cubic 

 inches, of a gas containing O, N, and C0 2 in certain 

 proportions in mechanical mixture are drawn into the 

 trachea and upper air-passage with each inspiration. 

 These air passages, as well as those more deeply 

 situated in the lungs, and the air cells into which they 

 ultimately open, are already occupied by a gaseous 

 mixture containing the same gases in different pro- 

 portions. Owing to the expiration immediately suc- 

 ceeding the inspiration, a certain quantity of the 

 inspired air, calculated at a third, is at once expelled, 

 but the remaining two-thirds have already begun to 

 mix by diffusion with the air already in the lungs. 

 Now, the air already in the lungs contains an amount 

 of O that gradually diminishes towards the air cells, 

 where it is least ; and similarly the quantity of CO 2 

 gradually increases towards the air cells, where it is 

 greatest. Thus, though the two-thirds of the inspired 



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