250 Dr. J. W. Draper's Remarks on the 



liberation of the gas is effected ; the same result uniformly 

 follows. If a porous mass, saturated with carbonic acid, be 

 exposed to an atmosphere of hydrogen, it absorbs but a small 

 quantity of this latter, whilst a very large amount of the 

 former is liberated from its condensed state, and the thermo- 

 meter indicates a fall of temperature ; the resulting volume of 

 the mixed gases being much larger than the original volume 

 of hydrogen. And if a porous mass which has absorbed its 

 due volume of hydrogen be immersed in an atmosphere of 

 ammonia, the resulting volume of the mixed gases is much 

 smaller than the original amount, and the porous mass be- 

 comes hot. 



The observations here made on the vicissitudes of tempera- 

 ture which a porous mass experiences when successively 

 immersed in an atmosphere of different kinds, obviously ap- 

 ply when the exposures instead of being consecutive are simul- 

 taneous. 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 toward 

 the hydrogen H ; then because of the condensing action of 

 the barrier on the acid gas, the surface C will become hot; 

 but because this gas as soon as it has passed through the bar- 

 rier expands, as into a void, when it reaches the surface H, 

 that surface because of the expansion will become cold. We 

 see, therefore, that immediately after the action of the barrier 

 is first set up, the absorption of carbonic acid takes place 

 on a hot surface, and its evolution from a cold one ; whereas 

 the absorption of the hydrogen takes place on a cold surface, 

 and its evolution 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. When the barrier is very thin, or has a high 

 conducting power as respects caloric, this distinct surface ac- 

 tion may not rigidly occur, but the whole structure experiences 

 some rise or diminution, a mean expressive of the condition 

 of the two surfaces respectively. 



On the 22nd of November 1837, on analysing atmospheric 

 air at this place, there was found in it only 19-60 per cent, of 

 oxygen, corroborative results being obtained by the use of 

 deutoxide of nitrogen and hydrogen gas ; but the day previous 

 and the day following, the proportion was almost 21 per cent. 

 This is not an insulated result; I know that on several occa- 

 sions during the last four years the proportion of oxygen in the 



