V 



O 



MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 



to its influence, and those situated in its centra 



are ready to be influenced by any extraneous pressur^ 



In an experiment made on the passage of ammonia 7 , 

 into atmospheric air through India-rubber, it was found 

 that, though the passage of the gas was resisted by a 

 pressure of seventy-five inches of mercury, or upwards of 

 two atmospheres and a half, it took place apparently as 

 readily as if no such resistance had been opposed. The 

 question at once arises, Whence is this powerful impul- 

 sive force derived ? Clearly not from the action of one 

 gas on the other. To the porous tissue or barrier alone 

 we must refer the seat of this power. 



It is well known that porous solids of all kinds and 

 fluids absorb gaseous matter very readily, in volumes 

 varying according to circumstances. Water, for exam- 

 ple, absorbs its own volume of carbonic acid, and four 

 hundred and eighty times its volume of hydrochloric- 

 acid gas. In the latter case, therefore, an extremely 

 great condensation takes place. So, too, a fragment of 

 porous charcoal absorbs nearly ten times its volume of 

 oxygen, and ninety times its volume of ammonia. These 

 gases, therefore, exist in the absorbing substance in a 

 state of very high compression. And the reasoning 

 which here applies, applies also in the case of two gases 

 separated by a tissue. If, for example, we separate by a 

 medium of this kind a certain volume of ammonia from 

 a like volume of nitrogen gas, though at the outset of 

 the experiment both the gases might be existing under 

 the same pressure, this equality would very rapidly be 

 lost. Ammonia, being absorbed more rapidly than ni- 

 trogen, would be presented to this latter gas not under 

 an equal pressure, but in a state of great condensation. 

 Under such circumstances, the transit of a gas is not 

 analogous to the case in which it flows under common 



O 



pressure into a vacuum, or into another gas ; but the tis- 



