gg PHYSIOLOGICAL EXPERIMENTS AND REMARKS. 



is effected, the same result uniformly follows ; if, for example, a porous mass saturated 

 with carbonic acid be exposed to an air-pump vacuum, in connexion with a tbermo- 

 metric arrangement, the gas, as it is liberated from the pores of the structure by the ac- 

 tion of the pneumatic machine, gives rise, by its expansion, to the production of cold. 

 Or, if the same porous mass, saturated in like manner with carbonic acid, be exposed to 

 an atmosphere of hydrogen, it absorbs but a small quantity of this latter substance, 

 while a very large amount of the former is liberated from its condensed state, and the 

 thermometer indicates a fall of temperature ; the resulting volume of the mixed gases 

 being much larger than the original volume of hydrogen. Again, if a porous mass, 

 which has absorbed its due volume of hydrogen, be immersed in an atmosphere of am- 

 monia, the resulting volume of the mixed gases is much smaller than the original amount, 

 and the porous mass becomes hot. 



176. The observations here made on the vicissitudes of temperature which an areo- 

 lar mass experiences, when successively immersed in an atmosphere of different kinds, 

 obviously apply when the exposures, instead of being consecutive, are simultaneous. If, 

 for example, a barrier separates carbonic acid and hydrogen gas, and absorbs the former 

 to a large amount, but exerts little or no action on the latter, then the opposite sides of 

 that barrier will be unequally heated. Suppose, for illustration, we call that surface of 

 the barrier which looks towards the carbonic acid, C, and the surface looking towards 

 the hydrogen, H: then, because of the condensing action of the barrier on the acid gas, 

 the surface C will become hot ; but because the gas, so soon as it has passed the barrier, 

 expands, as into a void, when it reaches the surface H that surface will become cold. 

 We see, therefore, that immediately after the action of the membrane or barrier is first 

 set up, the absorption of the carbonic acid takes place on a hot surface, and its evolu- 

 tion from a cold one ; whereas the absorption of the hydrogen takes place on a cold 

 surface, and its liberation from a hot one. A modified result of course happens when 

 both gases are absorbed in different degrees, and any prediction of the resulting action 

 becomes a matter of much difficulty. Where the barrier is very thin, or has a high con- 

 ducting power as respects caloric, this distinct surface action may not rigidly occur, but 

 the whole of the structure experiences some determinate rise or diminution ; a mean of 

 the condition of the two surfaces respectively. 



177. The obvious application of these results, in a physiological point of view, is to 

 the function of respiration. In no order of life, however, does the respiratory mechanism 

 coincide with the arrangements of two gaseous media separated from each other by a 

 barrier. In those tribes which breathe by lungs, the pulmonary vessels present themselves 

 on the remote bronchial cells, and the arrangement is, in effect, a liquid and a gas, parted 

 from each other by a membrane. Chemical physiologists have hastened to apply the 

 discovery of Dr. Mitchell in this case, and have done right. But still the chain of 

 evidence is incomplete, for we have not yet seen it proved that gaseous matter, in union 

 with a liquid, will leave it and pass through a barrier to join a gas on the other side. 

 The following experiment will supply this defect : A small jar, the mouth of which 

 was closed with India-rubber, and its opposite end made to terminate a tube one eighth 

 of an inch in diameter, while full of atmospheric air, was sunk in a vessel containing 



