but demands absolutely, the constant diffusion in copious and
continuous floods of atmospheric oxygen, of the nerve-poisoning
carbonic acid of combustion (organic and inorganic), and of the
blood-poisoning bacteria of organic decomposition.

We find, then, as a matter both of experience and of philosophy, that
life or death, in the main and in the long run, turns on the single
pivot of atmospheric movement or obstruction. The resistance of mere
rising ground or dense vegetation to a free movement of the air from
low-lying levels performs an obstructive office similar to that of the
walls and roofs of houses, and with like effect. The invariable
condition of unhealthy _seasons_ and _days_ is a state of rarefaction
and stagnation of the atmosphere, when the poison-freighted vapor
cannot be lifted and dispersed, and every one complains of the sultry,
close, "muggy" (meaning _murky_) feeling of the air. Few reflect, when
fretted by the boisterous winds of March, upon the vital office they
perform in dispersing and sanitating the bacteria-laden exhalations
let loose by the first warmth from the soaked soil and the macerated
deposits of the former year.

The passing air, then, that we breathe so lightly, is on other
business, and carries a load we little think of, and that is not to be
trifled with. This grand carrier of nature, on business of life or
death, must not be detained, must not be hindered! or they who
interfere with the business by restraining walls and roofs will take
the consequences. It is a good deal like stopping a bullet, except as
to consciousness and suddenness of effect.

That men live at all in their obstructed and therefore poison-loaded
atmosphere, is a proof of the wonderful efficiency of the protective