DIFFUSION OF GASES. 75 



Carbonic oxide ... 9' 30" 



Olefiantgas ... 7' 50" 



Coal gas .... 7' 



Hydrogen .... 4' 



Hydrogen, therefore, entered under pressure 2.4 times, or 

 nearly 2 4 times quicker than air, while several gases had the 

 same rate as air. Those gases which percolate very easily, 

 enter the diffusion instrument somewhat in excess, particularly 

 when the plate of stucco is thin. The deviation is perceptible 

 in hydrogen, and olefiant gas, and is also very sensible in coal 

 gas and carburetted hydrogen. 



It can be shown, on the principles of pneumatics, that gases 

 should rush into a vacuum with velocities corresponding to the 

 numbers which have been found to express their diffusion vo- 

 lumes ; that is, with velocities inversely proportional to the 

 square root of the densities of the gases. The law of the diffu- 

 sion of gases has on this account been viewed by my friend, 

 Mr. T. S. Thomson, of Clitheroe, as a confirmation of Dr. Dai- 

 ton's theory, that gases are inelastic towards each other.* 

 It must be admitted that the ultimate result in diffusion is in 

 strict accordance with Dalton's law, but there are certain 

 circumstances which make me hesitate in adopting it as a true 

 representation of the phenomenon, although it affords a conve- 

 nient mode of expressing it. 1. It is supposed, on that law, 

 that when a cubic foot of hydrogen gas is allowed to communi- 

 cate with a cubic foot of air, the hydrogen expands into the 

 space occupied by the air, as it would do into a vacuum, and 

 becomes two cubic feet of hydrogen of half density. The air, 

 on the other hand, expands in the same manner into the space 

 occupied by the hydrogen, so as to become two cubic feet of air 

 of half density. Now if the gases actually expanded through 

 each other in this manner, cold should be produced, and the 

 temperature of the mixed gases should fall 40 or 45 degrees. But 

 not the slightest change of temperature occurs in diffusion, how- 

 ever rapidly the process is conducted. 2. Although the ultimate 

 result of diffusion is always in conformity with Dalton's law, 

 yet the diffusive process takes place in different gases with very 

 different degrees of rapidity. Thus, the external air penetrates 



* Phil. Mag. 3rd series, IV, 321, 



