THE AIR AND VENTILATION 227 



quantity of carbonic acid gas is doubled, the air is mark- 

 edly oppressive. If the carbonic acid gas is increased to 

 three times its natural amount, the air is too oppressive for 

 comfort, and may contain enough germs of disease to be 

 dangerous to health. 



397. Computation of amount of fresh air. About ^ per 

 cent of fresh air is carbonic acid gas. When T ^ 7 per cent more of 

 carbonic acid gas has been added to the air, the air begins to be stuffy 

 and unfit for use. Suppose there is an air-tight room twenty feet square, 

 and ten feet in height, and in it one man is living ; the room will con- 

 tain 4000 cubic feet, yf^ per cent of 4000 cubic feet is T 8 ^ of a cubic 

 foot, which is nearly the average amount of carbonic acid gas breathed 

 out by the man each hour. Thus in an hour a man renders 4000 cubic 

 feet of fresh air stuffy. In reckoning the amount of fresh air to be 

 admitted to rooms, 4000 cubic feet per hour is the smallest amount 

 which can be safely allowed. Therefore, if only one person breathes 

 the air of a room twenty feet square, and ten feet high, the air needs 

 to be wholly renewed each hour, and yet it contains enough oxygen to 

 last a week. Fresh air is needed when the air of a room smells stuffy 

 to a person coming from pure air. 



398. Natural ventilation. When air is heated it ex- 

 pands so as to fill more space. While a cubic foot of air 

 at a temperature of 32 F. weighs about 1.2 ounces, at 

 80 F. it weighs about i.i ounces. So heated air, being 

 lighter, tends to rise. The air is slightly warmed in 

 breathing, and so tends to rise to the ceiling, while the 

 cool air which enters the room remains near the floor. 

 So the floor is usually cooler than the ceiling. If an 

 opening is made near the ceiling, and another near the 

 floor, the warm air of the breath will naturally pass out at 

 the upper opening, and the cool fresh air will enter the 

 lower opening. If only a few persons are in a room, 

 the openings about windows and doors may be sufficient 

 without special ventilation. If many persons are together 



