ry the balance ordained in nature has its compensating power.
The poisonous carbonic acid thrown off by lungs and body is absorbed by
vegetation whose food it is, and which in every waving leaf or blade of
grass returns to us the oxygen we demand. Shut in a close room all day, or
even in a tolerably ventilated one, there may be no sense of closeness;
but go to the open air for a moment, and, if the nose has not been
hopelessly ruined by want of education, it will tell unerringly the degree
of oxygen wanting and required.

It is ordinarily supposed that carbonic-acid gas, being heavier, sinks to
the bottom of the room, and that thus trundle-beds, for instance, are
especially unwholesome. This would be so, were the gas pure. As a matter
of fact, however, being warmed in the body, and thus made lighter, it
rises into the common air, so that usually more will be found at the top
than at the bottom of a room. This gas is, however, not the sole cause of
disease. From both lungs and skin, matter is constantly thrown off, and
floats in the form of germs in all impure air. To a person who by long
confinement to close rooms has become so sensitive that any sudden current
of air gives a cold, ventilation seems an impossibility and a cruelty; and
the problem becomes: How to admit pure air throughout the house, and yet
avoid currents and draughts. "Night-air" is even more dreaded than the
confined air of rooms; yet, as the only air to be had at night must come
under this head, it is safer to breathe that than to settle upon carbonic
acid as lung-food for a third, at least, of the twenty-four hours. As
fires feed on oxygen, it follows that every lamp, every gas-jet, every
furnace, are so many appetites satisfying themselves upon our store of
food, and that, if they are burning about us, a double amount of oxygen
must be furnished.

The only mode of ventilation that will work always and without fail is
that of a warm-air flue, the upward heated air-current of which draws off
the foul gases from the room: this, supplemented by an opening on the
opposite side of the room for the admission of pure air, will accomplish
the desired end. An open fire-place will secure this, provided the flue is
kept warm by heat from the kitchen fire, or some other during seasons when
the fire-place is not used. But perhaps the simplest way is to have ample
openings (from eight to twelve inches square) at the top and bottom of
each room, opening into the ch