222 
ON THE EXTERNAL CAUSES OP DISEASE. 
“ ? Tis surely God 
Whose unremitting energy pervades, 
Adjusts, sustains, and agitates the whole.” 
This is the reason why the purest air is always found at the 
bottom of an apartment, not because the oxygen is the heaviest, 
as we were formerly taught at the Veterinary College. 
The specific gravity of carbonic acid gas, is according to Guy 
Lussae, and Thenard - - - 1.5277 
The specific gravity of oxygen gas is only - - l.llll 
Whilst that of nitrogen, the lightest of the three gases, is only 0.9722 
Dr. Stevens was the first, we believe, who drew the attention 
of the scientific to the phenomena; he is incapable of explaining 
the manner in which it is accomplished any more than our¬ 
selves. 
Nitrogen, we have shewn, is the lightest of the three atmo¬ 
spheric gases ; but this agent possesses a sufficient attraction 
for the oxygen, to hold it in a state of suspension ; and by 
means of the same power the oxygen attracts the heavier car¬ 
bonic acid gas, lifts it up, and suspends it even in opposition to 
its greater gravity. But for the existence of the unknown power, 
this acid in the atmosphere would never rise. We know that it 
does ascend; and were it not so, the whole of that heavy gas 
would, from its greater gravity, fall down and occupy the lowest 
or that most important part of the atmosphere which is so im¬ 
mediately over the surface of the earth, so as to render it quite 
uninhabitable. 
Sir Humphry Davy tried to inspire a mixture of two quarts of 
common air with three of carbonic acid gas, without success ; but 
by increasing the proportion of common air to seven quarts, the 
mixture became respirable. When used in an undiluted state, 
a burning sensation at the top of the uvula and a violent feeling 
of suffocation were complained of: giddiness and torpor were 
its effects when mixed with common air. It is not to be ex¬ 
pected, therefore, that a gas of such deleterious properties should 
enter largely into the composition of a fluid so essential to life 
as atmospheric air ; accordingly it scarcely exceeds one per cent, 
and it is probable that even this quantity owes its presence 
more to chemical decomposition than original arrangement. 
As there are a number of bodies constantly abstracting oxygen 
from the air, it might be imagined that its relative quantity 
would decrease; but no such diminution takes place, except in 
instances, even from some local cause, where carbonic acid gas is 
evolved much faster than it can be removed. The fact is, that, 
if oxygen be absorbed from the air by one class of bodies, it is 
supplied by another. Plants yield it in large quantities. Thus 
