46 THE MAMMOTH CAVE. 



The amount of the vapor of water varies. Thus, 

 in those avenues at a great distance from the 

 rivers, upon the walls and floors of which there 

 is a deposit of the nitrate of lime, the air is 

 almost entirely destitute of moisture, from the 

 hygroscopic properties of that salt ; and animal 

 matter mummifies instead of undergoing putre- 

 factive decomposition. For the same reason, no 

 matter what state of division the disintegrated 

 rock may attain, dust never rises. In portions of 

 the Cave remote from the localities in which the 

 bats hibernate, no organic matter can be recog- 

 nized by the most delicate tests. Not a trace of 

 ozone can be detected by the most sensitive re- 

 agents. 



From what has been stated, it will be ob- 

 served that the atmosphere of the Mammoth 

 Cave is freer from those substances which are 

 calculated to exert a depressing and septic in- 

 fluence on the animal economy than that of any 

 other locality on the globe. This great differ- 

 ence is observed by every one on leaving the 

 Cave, after having remained in it for a number 

 of hours. 



In such instances, the impurity of the external 

 air is almost insufferably offensive to the sense of 

 smell, and the romance of a "pure country air" 

 is forever dissipated. 



