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. f Peagy 
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. 
