In the night every body necessarily breathes night air and
no other. When admitted from without into a sleeping-room it is colder,
and therefore heavier, than the air within, so it sinks to the bottom
of the room and forces out an equal quantity of the impure air, warmed
and vitiated by passing through the lungs of inmates. Thus the question
is, Shall we shut up a chamber and breathe night air vitiated with
carbonic acid or night air that is pure? The only real difficulty about
night air is, that usually it is damper, and therefore colder and more
likely to chill. This is easily prevented by sufficient bed-clothing.
One other very prevalent mistake is found even in books written by
learned men. It is often thought that carbonic acid, being heavier
than common air, sinks to the floor of sleeping-rooms, so that the low
trundle-beds for children should not be used. This is all a mistake;
for, as a fact, in close sleeping-rooms the purest air is below and
the most impure above. It is true that carbonic acid is heavier than
common air, when pure; but this it rarely is except in chemical
experiments. It is the property of all gases, as well as of the two
(oxygen and nitrogen) composing the atmosphere, that when brought
together they always are entirely mixed, each being equally diffused
exactly as it would be if alone.
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