THE ATMOSPITEKE AS RELATED rO VEGETATION. 101 



stnntly, bubbles begin to escape rap:;lly from the bottom 

 of the tube throiauh the water of the wine-glass, thus 

 demonstiMiiiig that hydrogen ])asses into the cup faster 

 ihan air can escMpe outwards 

 ihrongli it-: porc-<. If tin- btll be 

 removed, ihc cuj) is at once butlied 

 Mgain externally in common air, the 

 light hydrogen floating instantly 

 uj)wards, and now the water begins 

 to rise in the tube in consequence of 

 the return to the outer atmosphere 

 of the hydrogen which before had 

 diffused into the cup. 



It is the i)erpetual action of tliis 

 diffusive tendency which maintains 

 the atniosplierc in a state of such 

 uniform mixture that accurate ana- 

 lyses of it give for oxygen and 

 nitrogen almost identical figures, at 

 all tnios of the day, at all seasons, 

 all altitudes, and all situations, ex- 

 cept near the central surface of 

 large bodies of still water. Here, 

 the fact that oxygen is more largely 

 absorbed by water than nitrogen, 

 diminishes by a minute amoimt the 

 usual proportion of the former gas. 

 If in a limited volume of a mixture of several gases a 

 solid or liquid body be placed, which is capable of chemic- 

 ally uniting witli, or otherwise destroying the aeriform 

 condition of one of the gases, it will at once absorb those 

 particles of this gas which lie in its immediate vicinity, 

 and thus disturb the uniformity of the remaining mixture. 

 Uniformity at once tends to be restored by diffusion of a 

 portion of the unabsorbed gas into the space that has been 

 deprived of it, and thus the absorption and the diffusion 



