96 RESPIRATION, ETC. [CH. IX 



In the daytime, especially if sunny, he would find the 

 atmosphere rapidly getting drier and richer in oxygen, 

 because the chlorophyll -corpuscles are consuming the 

 carbon-dioxide and turning out free oxygen, while the 

 water-vapour is rapidly evaporating into the drier air 

 outside : during the darkness of night, he would find the 

 atmosphere getting damper and damper, because evapora- 

 tion is slower, while the cells are continually pumping 

 more and more water from the afferent vessels, and the 

 living cells are pouring out carbon-dioxide as the 1'esult 

 of their respiration. For each living cell acts like an 

 animal in so far that it consumes all the oxygen it can 

 absorb from the air, and burns off some of the sugar-like 

 carbonaceous bodies in its substance in order to employ 

 the energy thus obtained for its life-purposes. 



In reality this process of respiration goes on day and 

 night, continuously, as long as the cell is alive ; but in 

 bright sunshine the breaking up of the carbon-dioxide 

 (with evolution of oxygen) goes on so much more vigor- 

 ously than the consumption of oxygen (with evolution of 

 carbon-dioxide) that the latter process is more than com- 

 pensated by the former, and our prisoner in the tunnel 

 would find the atmosphere get richer and richer in oxygen 

 as described. 



