46 DALTON'S HYPOTHESIS. 



follow, for it is highly probable that the nature of the barrier itself is very much 

 concerned in the final action. A gas may penetrate into another with a force not 

 greater than one atmosphere, and yet, because of the disturbing agency of the medium 

 through which it must go, it may succeed in lifting a column of mercury equivalent to 

 a pressure of many atmospheres. 



149. The evidence proving that gases do not infiltrate each other with a pressure 

 greater than one atmosphere is very cogent. Much of its weight is derived from the 

 identity of the resulting volumes of commingled gases ; but the most important fact re- 

 lates to the passage of these substances into each other, when the barrier separating 

 them is very porous and has no condensing action, as is the case with a stucco plug, 

 which opposes simply a mechanical impediment to their motion, acting, as will be here- 

 after proved, merely as a temporary valve ; a mode of action totally different to that of 

 closer textures. The final volumes exchanged being inversely proportional to the 

 square root of the densities, and these final volumes representing the true initial veloci- 

 ties, we have a striking illustration of that law of gaseous mechanics, that the velocities 

 of different gases, rushing into a vacuum, are inversely proportional to the square root 

 of their densities. Consequently, we are constrained to infer that one gas acts towards 

 another in the same manner as if it were a vacuum ; and, therefore, that the force im- 

 pelling the particles of one gas into the interstices of another never exceeds the pres- 

 sure of one atmosphere. 



150. In an experiment made on the passage of ammonia into atmospheric air, it was 

 found, that though the passage of the gas was resisted by a pressure of seventy- five inches 

 of mercury, or upward of two atmospheres and a half, it took place, apparently, as 

 readily as if no such resistance had been opposed to it. The question at once arises, 

 Whence is this powerful impulsive force derived I clearly not from the action of one 

 gas upon the other, for there is great probability, as we have already seen, that that 

 force would not be able to lift more than thirty inches of mercury. The porous tissue 

 or barrier alone can be regarded as the seat of this power. This fact, that systems of 

 capillary tubes, or thin tissues, have in themselves certain powers, capable of producing 

 high mechanical action, and operating successfully against the severest pressures that 

 can be brought to bear against them, is worthy of the serious contemplation of physiol- 

 ogists ; it is a great error to impute the forces producing these phenomena to the gas- 

 eous media. In the tissue itself we must admit a source of power, a source far trans- 

 cending that which solicits the gases to penetrate each other. Let us next inquire 

 into the nature of this power. 



151. It is well known that porous substances 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 480 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 on the surface of 

 the particles of the absorbing medium, in a state of very high compression. And the rea- 

 soning which here applies, applies also in the case where the two gases are separated by 



