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 About half a pint of water is given off through the lungs in a day. Minute quantities of injurious animal matter are also given off in the breath from even the soundest person. The air leaves the lungs warmer, damper, and with more carbon dioxid than when it entered (Exps. 3 to 9).

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How are the inlet and outlet situated with reference to the stove? ]

Persons with decayed teeth, catarrh, indigestion, diseased lungs, or other unsoundness give off still more of this material. When many people are assembled in a badly ventilated room, the amount of injurious animal matter in the air is much increased, and is called "crowd poison." Its odor is strong and repulsive to one who just enters the room, but the sense of smell becomes dull to it in a few minutes. It would seem that nature gives a fair warning against harm; but if we disregard the warning it soon ceases.

People who are really Unclean.—Nature's plan seems to be for us to live out of doors. Air once breathed is impure. It is just as unfit to enter our bodies as muddy water or decayed food. Yet many who call themselves cleanly and refined, and will not allow a speck of dirt to remain on their clothes, nor use a spoon just used by another, do not object to breathing into their lungs, over and over again, the cast-off air from the lungs of others. If a window is opened for ventilation, they are horror-stricken for fear of drafts. Drafts are injurious only to persons perspiring, or to those who have coddled the skin by continually overheating it. There are thousands of schools, churches, and theaters all over the land which reek daily with the malodorous particles from the lungs of their occupants. Although the air in them is odorless to those who occupy them, it is disgusting to any one who enters from the fresh air. Figure 80 shows the correct ventilation of a stove-heated schoolroom.

Dust causes catarrh of the bronchial tubes and chronic