which we are exposed through the air are those of the breath, of which
we learned in a preceding lesson. We need plenty of fresh air to take
the place of the air which we poison by our breath. Every time we
breathe, we spoil at least _half a barrelful of air_. We breathe twenty
times a minute, and hence spoil ten barrels of air in one minute. How
many barrels would this make in one hour? We need an equal quantity of
pure air to take the place of the spoiled air, or not less than ten
barrels every minute, or _six hundred barrels every hour_.

~7. Ventilation.~--The only way to obtain the amount of fresh air
needed, when we are shut up in-doors, is to have some means provided by
which the fresh air shall be brought in and the old and impure air
carried out. Changing the air by such means is called _ventilation_.
Every house, and especially every sleeping-room, should be well
ventilated. School-houses, churches, and other places where many people
gather, need perfect ventilation. Ask your teacher to show you how the
school-room is ventilated; and when you go home, talk to your parents
about the ventilation of the house in which you live.

~8.~ Many people ventilate their houses by opening the doors and
windows. This is a very good way of ventilating a house in warm weather,
but is a very poor way in cold weather, as it causes cold draughts, and
makes the floor cold, so that it is difficult to keep the feet warm. It
is much better to have the air warmed by a furnace or some similar
means, before it enters the rooms. There ought also to be in each room a
register to take the foul air out, so that it will not be necessary to
open the windows. This register should be placed at the floor, because
when the pure air enters the room warm, it first rises to the upper part
of the room, and then as it cools and at the same time becomes impure,
it settles to the floor, where it should be taken out by the register.