Constitution of the Atmosphere. 249 



other in the barrier, 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 restraint, expands freely into a void, and when there 

 measured, gives a resulting volume totally different to what 

 it would have given had not the tissue compressed it. Sup- 

 pose, for example, we place on one side of a barrier carbonic 

 acid gas, of which it could condense its own volume, and on 

 the other atmospheric air on which it exerted no action. 

 Whilst the two gases were engaged together in the barrier 

 the one would be presented to the other under an elastic 

 force double of that which it would have had, if no absorp- 

 tion had gone on ; but since its density is directly proportional 

 to its elastic force, the continual velocity with which it rushes 

 into the other gas is the same as though no compression 

 whatever had occurred ; the rate of exchange in the barrier 

 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 dimen- 

 sions. 



This explanation satisfies all the facts, and reduces these 

 experiments to the operation of the hypothesis of gaseous 

 action ; I would not here be understood to say that there are 

 no other disturbing actions going on in barriers except those 

 that result from their absorbing power. A great disturbance 

 often arises from the circumstance that when two gases are 

 absorbed together they experience a greater condensation 

 than each would in a separate state. It is therefore impos- 

 sible to foretell what the result of diffusing one gas into an- 

 other will be, by simply ascertaining how many volumes of 

 either alone will be absorbed by the tissue, inasmuch as a 

 greater or less condensation may happen when both are em- 

 ployed together. 



Variations of temperature, which probably affect the power 

 of absorption, and thereby the diffusive volumes, are experi- 

 enced by all barriers. When charcoal, or any other porous 

 mass, is placed in an atmosphere of gas which it can rapidly 

 condense, its temperature rises, the effect apparently depend- 

 ing more on the velocity of absorption than on the final 

 amount. In the case of ammonia, it does not even require a 

 thermometer to discover the increase of temperature, for it is 

 very sensible to the touch. On the other hand, when this 

 condensed gas makes its escape, a corresponding diminution 

 of temperature happens : it is immaterial by what means the 



