constitutional complaint, consumption, simply from breathing the warm
breath of their friends below. But since the cages have been altered,
and made to range side by side from top to bottom, consumption--I
understand--has vastly diminished among them.

The first question in ventilation, therefore, is to get this carbonic
acid safe out of the room, while it is warm and light and close to the
ceiling; for if you do not, this happens--The carbonic acid gas cools and
becomes heavier; for carbonic acid, at the same temperature as common
air, is so much heavier than common air, that you may actually--if you
are handy enough--turn it from one vessel to another, and pour out for
your enemy a glass of invisible poison. So down to the floor this heavy
carbonic acid comes, and lies along it, just as it lies often in the
bottom of old wells, or old brewers' vats, as a stratum of poison,
killing occasionally the men who descend into it. Hence, as foolish a
practice as I know is that of sleeping on the floor; for towards the
small hours, when the room gets cold, the sleeper on the floor is
breathing carbonic acid.

And here one word to those ladies who interest themselves with the poor.
The poor are too apt in times of distress to pawn their bedsteads and
keep their beds. Never, if you have influence, let that happen. Keep
the bedstead, whatever else may go, to save the sleeper from the carbonic
acid on the floor.

How, then, shall we get rid of the foul air at the top of the room? After
all that has been written and tried on ventilation, I know no simpler
method than putting into the chimney one of Arnott's ventilators, which
may be bought and fixed for a few shillings; always remembering that it
must be fixed into the chimney as near the ceiling as possible. I can
speak of these ventilators from twenty-five years' experience. Living in
a house with low ceilings, liable to become overcharged with carbonic
acid, which produces sleepiness in the evening, I have found that these
ventilators keep the air fresh and pure; and I consider the presence of
one of these ventilators in a room more valuable than three or four feet
additional height of ceiling. I have found, too, that their working
proves how necessary they are, from this simple fact:--You would suppose
that, as the ventilator opens freely into the chimney, the smoke would be
blown down through it in high winds, and blacken the ceiling: but this is
just what does not h