406 Mr. T. Graham on the Absorption and 



of the gas by rubber must depend upon a kind of chemical affi- 

 nity 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 eva- 

 poration is the same into a vacuum and into another gas, being 

 equally gas-diffusion in both circumstances. It is not indispen- 

 sable, therefore, to have a vacuum on one side of the rubber mem- 

 brane 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 infla- 

 ting 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 com- 

 position 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 mil- 

 lims. in diameter contracted to 87 millims., and then contained 

 250 cubic centims. gas, of which 53 cubic 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 ex- 

 ternal 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 dif- 

 fusion of hydrogen as a gas. Beginning with 249 volume divi- 

 sions of gas in the tube, the rise of the mercurial column, or 

 reduction of volume, was 1*5 division in the first hour, 1*5 divi- 



