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.