Mr. T. S. Thomson on the Diffusion of Gases. 53 



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

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

 nicate 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 of 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 oc- 

 curs in diffusion, however rapidly the process is conducted. 

 2. Although the ultimate result of diffusion is always in con- 

 formity with Dalton's law, yet the diffusive process takes place 

 in different gases with very different degrees of rapidity. Thus 

 the external air penetrates into a diffusion tube with velocities 

 denoted by the following numbers, 1277, 623, 302, according 

 as the diffusion tube is filled with hydrogen, with carbonic acid, 

 or with chlorine gas. Now, if the air were rushing into a 

 vacuum in all these cases, why should it not always enter it 

 with the same velocity ? Something more, therefore, must be 

 assumed than that gases are vacua to each other, in order to 

 explain the whole phaenomena observed in diffusion." 



I cannot admit the validity of either of the objections. In 

 the first place, as to the production of cold. Dr. Dalton has 

 ascertained and published the facts of the case. If air of press- 

 ure 30 inches of mercury is allowed to rush into a vacuum, 

 cold is produced in the vessel from which the air rushes, and 

 a momentary rise of temperature is observed in the vacuum 

 into which it rushes. This disturbance of the equilibrium of 

 temperature only lasts a few seconds, and it was solely by the 

 rate at which the thermometer descended and ascended that 

 Dr. Dalton inferred the " fall of 40 or 45 degrees." 



If oxygen is exposed to hydrogen, on which side is the re- 

 duction of temperature to take place ? I think on neither ; 

 if they began to heat each other, they would naturally at the 

 same time begin to cool each other. If you let a cubic foot 

 of hydrogen loose into a vacuum it goes in at the rate of sound, 

 1142 feet in a second ; but if the vacuum is pre-occupied with 

 oxygen it requires longer time, and any variation of tempera- 

 ture would be unobservable. As to Mr. Graham's objection, 

 No. 2, I have only to observe, that I never committed myself 

 so far as to say that when air was rushing into hydrogen or 

 carbonic acid, or chlorine, it was "rushing into a vacuum." 



In my explanation of the phaenomena of diffusion on Dr. 

 Dalton's theory, investigating the results of the relative velo- 

 cities of different gases into a vacuum and deducing the pre- 



