30 CAUSES INTERFERING WITH 
“ duly to appreciate, in the vast economy of ter- 
restrial adaptations, the importance of this me- 
chanism, by which gases and vapours rapidly 
permeate each other’s hulks, and become equally 
diffused. The atmosphere which surrounds the 
globe consists of a mixture of several aeriform 
fluids, in certain fixed proportions, upon the 
proper maintenance of which, by measure and by 
weight, the welfare of the whole organic creation 
depends. The processes of respiration and of 
combustion are perpetually tending to the de- 
struction of the vital air, and the substitution of 
another which is a deadly poison to animal life ; 
and yet, by the simple means which we have 
described, the poisonous air is not allowed to 
accumulate, but diffuses itself instantly through 
surrounding space, while the vital gas rushes, by 
a counter tendency, to supply the deficiency 
which the local consumption has created. Hence 
the invariable uniformity of this mixture, which 
is one of the most surprising phenomena in a 
system where all is admirable. The most accu- 
rate examination has been made of air which has 
been taken from localities the most opposed to 
each other in all the circumstances which can be 
conceived to affect its purity ; by means of a 
balloon, from a height of 22,000 feet above the 
level of the sea ; from the surface of the ocean ; 
