d enter a Room.=--The air laden with waste coming
out of the lungs quickly mixes with the other air of the room. In this
way all of the air in the room soon becomes impure. Forty children
will give out nearly two barrels of air in one minute. In another
minute this air has made all of the other air in the room unclean. It
can still be breathed, but it makes children feel drowsy and lazy and
may cause headache. They then do poor work.

To keep the air pure in a room, fresh air must be let in from the
outside. If there are many in the room, the openings must be large or
fans on a wheel must be used to force the air in. In the New York
schools a little over a cubic yard of fresh air is forced into the
room for each child every minute.

=How to get Fresh Air into a Room.=--When air is warmed it becomes
lighter and rises. In many public buildings, fresh air heated by a
furnace is forced into the rooms through pipes entering several feet
above the floor. By a fan or heated flue the impure air is sucked out
of the room through openings near the floor.

[Illustration: FIG. 65.--How the windows of your bedroom should be
open to get the most fresh air.]

Changing the air in a room is called _ventilation_. To get plenty of
fresh air in a room there must be one or more places for it to enter
and one or more places for it to pass out. Where there is no furnace
or fan, windows on one side of the room may be opened at the bottom to
let in the air and the same windows opened at the top to let the
impure air escape. _Do not sit in a draft_, but use a board or curtain
to throw the air upward as it enters the window. _A room should not be
kept too warm._ Sitting in a very warm room weakens the body and
prepares it to take cold. The temperature of a living room should be
between 65 and 70 degrees.

=Fresh Air while you Sleep.=--Thousands of people have weakened their
bodies and brought on disease by sleeping in bad air. Many persons
keep their windows so tightly closed during the night that the air
smells bad in the morning. I knew a family who always slept with
windows closed except in the very warmest weather. Three of the
children died of tuberculosis, and a fourth one took the disease but
was saved by keeping his windows wide open.

Bad air in the sleeping room makes one feel drowsy in the morning
instead of refreshed by sleep. _Your windows should always be open
while you sleep._ In cold weather a window should be open a foot a