DIALTTIC SEPAEATION OF GASES BY COLLOID SEPTA, 403 



Considering the circumstances in which the gases pass through the sheet of rubber 

 into a vacuum, it is not to be expected that any relation will be found among the pre- 

 ceding numbers, as between the coefficients of diffusion in gases. The first absorption 

 of the gas by rubber must depend upon a kind of chemical affinity subsisting between 

 the material of the gas and substance of rubber, analogous to that attraction which is 

 admitted to exist between a soluble body and its solvent, conducing to solution. 

 Carbonic acid being soluble in ether and volatile oils, it is not wonderful that it is also 

 dissolved by the hydrocarbons of rubber. The rubber being wetted through by the 

 liquefied gas, the latter comes to evaporate into the vacuum, and reappears as gas on 

 the other side of the membrane. Now it is known that such evaporation is the same 

 into a vacuum and into another gas, being equally gas-diffusion in both circumstances. 

 It is not indispensable, therefore, to have a vacuum on one side of the rubber membrane 

 as in the experiments detailed above. A foreign gas will answer for the vacuum, as in 

 the experiments of Dr. Mitchell. 



The numbers for the velocity of passage of the different gases in the last Table may 

 be taken also as representing not remotely the relative absorption and liquefaction of 

 the various gases by the substance of rubber. 



The passage of gases through rubber is also illustrated by the rapid collapse of the 

 little balloon when filled with carbonic acid gas, or even with hydrogen, or with marsh-gas, 

 as compared with atmospheric air. The converse fact is observed when the inflating 

 gas is pure nitrogen : then the balloon is found to become further distended after a few 

 hours, in consequence of more oxygen entering from the atmosphere without, than of 

 nitrogen escaping from the balloon during the same time ; while the composition is being 

 equalized on both sides of the membrane, and the gas within the balloon is finally of the 

 same composition as the external air. A rubber balloon filled with nitrogen was found, 

 when roughly gauged, to increase in diameter from 132 to 136 millims. in the course of 

 twenty-four hours. On the other hand, a balloon filled with pure oxygen fell in the 

 same time from 150 to 113 millims. in diameter. 



In forty-eight hours a balloon filled with hydrogen 154 millims. in diameter con- 

 tracted to 87 millims., and then contained 250 cub. centims. gas, of which 53 cub. 

 centims. were absorbed by pyrogallic acid and potash, showing the presence of 21*2 per 

 cent, of oxygen, or sensibly the same proportion as in the external atmosphere. 



If the upper end of a diffusiometer be closed by a thin sheet of rubber, and the instrument 

 standing over mercury be filled with hydrogen gas, a contraction is observed to take place 

 slowly, but to a greater extent ultimately than could be due to the diffusion of hydrogen 

 as a gas. Beginning with 249 volume divisions of gas in the tube, the rise of the mer- 

 curial column, or reduction of volume, was 1-5 division in the first hour, 1-6 division in 

 the second hour, 2-0 in the third hour, 3 in the fourth hour, and 51 divisions in the first 

 twenty-four hours taken together. Then the rise in the following successive days was 

 42, 59, 37, 29, 13, 5, 1, 0-5, 0-5 (in two days), and 0-0, the original volume of 249 

 volumes of hydrogen being finally replaced by 53 volumes of atmospheric air; barom. 



