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. Thus the carbonic acid from the skin
and lungs, being warmed in the body, rises as does the common air,
with which it mixes, toward the top of a room; so that usually there
is more carbonic acid at the top than at the bottom of a room.
[Footnote: Prof. Brewer, of the Tale Scientific School, says: "As a
fact, often demonstrated by analysis, there is generally more carbonic
acid near the ceiling than near the floor."] Both common air and
carbonic acid expand and become lighter in the same proportions; that
is, for every degree of added heat they expand at the rate of 1/480
of their bulk.

Here, let it be remembered, that in ill-ventilated rooms the carbonic
acid is not the only cause of disease. Experiments seem to prove that
other matter thrown out of the body, through the lungs and skin, is
as truly excrement and in a state of decay as that ejected from the
bowels, and as poisonous to the animal system. Carbonic acid has no