54 CO-ORDINATION OF DALTON'S, GRAHAM'S, AND MITCHELL'S RESULTS. 



with a vacuum by means of an aperture, it is immaterial whether the air be allowed to 

 flow into the void without any pressure, or whether it be urged by a direct action on 

 the piston, its velocity as it goes into the void will be in both cases the same ; for, if it 

 be compressed, the immediate action of the force exerted on the piston is to reduce the 

 air in the cylinder to such a density that its elasticity shall be equal to the compressing 

 force, and because the elasticity varies directly as the density, the density of the air in- 

 creases with the impelling force. The matter to be moved is increased, therefore, in 

 the same proportion with the pressure, and therefore the final velocity is the same. Now 

 what is here said of a cylinder of compressed air, applies evidently to the action of a cellu- 

 lar tissue, which is nothing more than a perpetual and equable condensing engine. If it 

 increases the elastic force of one of the gases by compressing it, at the same time it in- 

 creases its density ; and, therefore, its velocity of transit is the same as though it had 

 not suffered any action of compression. 



170. Such is the case while the gases are engaged with each other in the tissue; 

 but as soon as they are passed from it, and are beyond the reach of its attractive force, 

 a new condition of things takes place : the condensed gas being no longer under re- 

 straint, expands freely into a void, and when there measured, gives a resulting volume 

 totally different to what it would have been had not the tissue compressed it Sup- 

 pose, for example, we placed on one side of a tissue carbonic acid, of which it would 

 condense its own volume, and on the other atmospheric air, on which it exerted no 

 action. While the two gases were engaged together in the tissue, one would be pre- 

 sented to the other under an elasticity double of that which it would have had had no 

 absorption gone on ; but since its density is directly proportional to its elastic force, the 

 continual velocity with which it would rush into the other gas is the same as though 

 no compression had occurred ; the rate of exchange in the cellular tissue is the same 

 as under normal circumstances ; that is to say, every volume of air replaces 0-8091 

 of compressed carbonic acid ; but so soon as this gas has reached the opposite side of 

 the barrier, and there escapes, its elastic force being restrained by no compression, 

 causes it to assume its original dimensions. 



171. It will be readily perceived that the theory here given depends on the princi- 

 ple, that however much a gas is condensed, it will at all pressures rush into a vacuum 

 with the same velocity. The elasticity of a gas in any state is measured by the force 

 under which it exists, and this is ordinarily the pressure or weight of the atmosphere ; 

 it follows, therefore, that though the density of gases may vary, yet they have all the 

 same elastic force ; but, when pressure is exerted upon them, the density and elasticity 

 increasing together, their velocity in rushing into a void is always, and under all pres- 

 sures, a constant quantity. 



172. We may now apply this reasoning to certain practical cases. Mr. Graham 

 found that the absorption of carbonic acid by a porous plug of stucco was very small 

 in amount, and the absorption of atmospheric air is equally minute. Accordingly, 

 when these two gases are separated from each other by a screen of that substance, they 

 diffuse according to the law of the square roots of their density. One volume of air, 

 replacing 0-8091 of carbonic acid, the gas, therefore, on that side of the screen where 



