Constitution of the Atmosphere. 247 



To me, however, it seems that an explanation can be given of 

 them, which will lead us entirely to a different conclusion, 

 and furnish a beautiful illustration of the truth of that hypo- 

 thesis. 



It appears there is abundant and conclusive evidence, that 

 under ordinary circumstances of temperature and pressure, 

 any given gas bears the same relation to one which is perco- 

 lating into it that a vacuum would do, for the law of dis- 

 charge is identically the same. For the purpose of illustration, 

 we may therefore regard it to all intents as a vacuum, and 

 reason accordingly. If the particles of heterogeneous gases 

 possess no repulsive tendency as respects each other, but are 

 perfectly quiescent and neutral, then it is immaterial how 

 many of such particles are condensed together into a given 

 space, for owing to the want of repulsive action in those par- 

 ticles, that space will be as much a vacuum to any other gas 

 as it ever was. Now it has just been shown that certain 

 gases will diffuse into others even though the latter may be 

 condensed into a space twenty times less than that which they 

 would ordinarily occupy. The vacuum is not the less a 

 vacuum because it is contained under smaller dimensions, 

 any more than a Torricellian vacuum is less perfect when the 

 mercury is made to rise nearly to the top of the barometric 

 tube, than it was when there was a vacant space many inches 

 in length. Theory would therefore indicate, that tnese dif- 

 fusions might take place under all pressures, provided the 

 gaseous condition subsists. 



Moreover, the foregoing experiments do not actually furnish 

 any proof that gases diffuse themselves into one another with a 

 force greater than one atmosphere. It is a mistake to adduce 

 them as examples in point, for the fact is that the barrier or 

 tissue, far from being passive, exerts a very remarkable action 

 in virtue of its absorbing power, a property pre-eminentl}' pos- 

 sessed by charcoal, and some other porous bodies. This 

 seems to afford an explanation of the whole phaenomenon, 

 and furnish an important fact in a physiological point of 

 viev/, — that membranes and tissues are occasionally the origin 

 and seat of powers of uncommon intensity. 



It will be convenient, for the better understanding of these 

 actions, to consider them under two heads. First, where the 

 barrier between the media exerts no absorbent action on the 

 media ; this will include most of the results of Prof. Graham : 

 secondly, where one of the media is absorbed to a much 

 greater extent than the other ; this will include all the fore- 

 going experiments. 



In the first case, the velocities with which any two gases 



