THE MAMMOTH CAVE. 45 
no existence in this subterranean world. In 
fact, there is an eternal sameness here, which is 
without a parallel. 
In many parts of the Cave, time itself is not 
an element of change; for where there is no 
variation of the temperature, no water, and no 
hght, the three great forces of geological trans- 
formation cease to operate. 
The atmosphere of the Cave, contrary to 
what might be generally supposed, is remark- 
~ ably pure and wholesome. 
The proportions of oxygen and nitrogen bear 
the same relation to each other in the Mam- 
moth Cave that they do in, the external air. 
The proportion of carbonic acid gas 1s less than 
that observed in the atmosphere in the sur- 
rounding country, upon an average of many 
observations. This noxious gas is one of the 
necessary constituents of vegetable existence ; 
and, as there is no vegetable life within the 
Cave, its comparative absence is a natural in- 
ference. 
In the dry parts of the Cave the proportion 
of carbonic acid is said to be about 2 to 10,000 
of air; in the vicinity of the rivers, something 
less. Not a trace of ammonia can be detected 
in those parts of the Cave not commonly visited. 
