board under a window (as shown in Figure 27) while another window on the same side of the room is lowered from the top. Often by lowering all the windows slightly at the top a great deal of the hot, moist air in a crowded room can be got rid of without causing cold draughts. Schoolrooms should be filled with fresh air while they are empty, and at

a room.

noons and recesses the windows should be raised and the fresh air allowed to pour in; for no one can be expected either to learn his lessons or to keep his health in a room that is stuffy and close and filled with air that has already been breathed.

Ventilating sleeping rooms. Sleeping rooms are harder to ventilate than living rooms, because we are all the while moving about through our living rooms, and the opening and closing of doors

sets the air in motion. We spend so much of our time in sleeping rooms, however, that it is of the greatest importance that the air in them be pure. Do not sleep in a room where you wake with a stuffy feeling in the morning, but open the windows, or in some other way get fresh air into your bedroom. Do not be afraid of night air, for long ago it was proved to be harmless. A current of fresh air will do no harm if your body is warmly covered, or if you are protected from a direct draught by a window board

FIG. 28. The best kind of sleeping room is out-of-doors. This one was planned when the house was built. It is open on three sides and in summer is screened to keep out flies and mosquitoes.

or a screen.

The

Outdoor sleeping. best place of all to sleep is out in the fresh air, where the warm air that comes from the lungs is blown away from the face. Usually an upper porch is the

best place for outdoor sleeping, and houses should be built with porches that can be used for this purpose. That great benefits come from open-air sleeping is shown by the fact that the health of persons who are sick with consumption or pneumonia is often greatly improved when they begin this practice.

Methods of heating and ventilation. Gas and oil heaters that have no pipes for carrying away the gases give off great volumes of impurities; and to heat a sleeping room with one of these stoves is unhealthful. Stoves and furnaces that leak coal gas also are unhealthful. Fireplaces give good ventilation because they send a current of air up the chimand this draws more air into the room. Vessels of water should be kept on stoves and on or behind radiators to add moisture to the air. When plants grow well in a room the air is not dry enough to be harmful to the health.

ney,

Questions: 1. How much of the air is oxygen? 2. Why must the body get rid of carbon dioxid? 3. What are the three reasons why the body must have air? 4. Why is ventilation necessary? 5. What are the important points in ventilation? 6. At what temperature should a crowded room be kept? 7. What trouble is there with the ventilation of buildings that are heated with hot air? 8. How may this be remedied? 9. Explain how a schoolroom may be ventilated without causing draughts. 10. What may be done at recess to change the air in a room? II. Why is it hard to ventilate sleeping rooms? 12. Why is it important that they be well ventilated? 13. What is the best of ali sleeping places? 14. How is this proved? 15. What methods of heating bring fresh air into a house?

Suggestions and topics for development: Ritchie's Primer of Physiology gives a more complete presentation of the newer ideas on ventilation than is possible in the limited space in this book.

Of all the organs of the body, the lungs and air passages are most frequently attacked by disease germs. Colds, catarrh, bronchitis, and grip are so common that no one entirely escapes them, while pneumonia and consumption kill thousands of persons every year. Yet every person can do much to avoid these diseases by taking a reasonable amount

of care of his breathing organs and by securing for himself an abundance of fresh air. We have learned some ways by which we may secure pure air; now we are going to learn how to care for the organs that get rid of carbon dioxid and take in oxygen for the body.

The air passages. The air enters the nose through the nostrils and passes down into the throat through two openings at the back of the mouth. It then goes down the windpipe (trachea), which divides and enters the two lungs. These large branches of the trachea divide into smaller and smaller branches, as a tree divides into small limbs and twigs, and these smallest branches end in little air sacs. The lungs are mainly composed of millions of these little tubes and the air sacs at their ends. The air which we breathe passes down the windpipe and out through the tubes into every one of these sacs.

The blood purified in the lungs. In the thin, delicate walls of the air sacs of the lungs are great numbers of very small blood vessels. As the blood passes through these vessels in fine little streams, it takes up oxygen from the air in the sacs and gives off carbon dioxid. The carbon dioxid is then breathed out of the body, and when the next breath is taken in, more oxygen is drawn down into the lungs.

The danger of breathing dust. Most of the diseases of the air passages and lungs are germ