210 SCIENCE OF COMMON THINGS 



Air consumed by a person in an heur. Vegetation purifies the air. 



about a cubic foot of oxygen per hour by respiration, 

 and consequently renders five cubic feet of air unfit for 

 breathing, since every five cubic feet of air contain one 

 cubic foot of oxygen. It is also calculated that two 

 wax or sperm candles absorb as much oxygen as an 

 adult. 



1400 To keep tJie air of a room pure, how much fresh air should be 

 allowed to pass in per hour f 



Five cubic feet for each person, and two and a half 

 cubic feet for each candle, should be allowed to pass in, 

 and an equal quantity to pass out. 



1401 Why do persons in a crowded church feel drowsy f 



1. Because the crowded congregation inhale a large 

 portion of the oxygen of the air, which alone can sus- 

 tain vitality and healthy action ; and 



2. The air of the church is impregnated with carbonic 

 acid gas, which (being a strong narcotic) produces 

 drowsiness in those who inhale it. 



14OS Why do persons who are much in the open air enjoy the best 

 health f 



Because the air they inhale is much more pure than 

 the air of close and confined rooms. 



1403 How does vegetation (trees and flowers] serve to purify the airf 



1. Because trees and flowers absorb the carbonic 

 acid generated by the lungs of animals, putrid sub- 

 stances, and other obnoxious exhalations ; and 



2. Trees and flowers restore to the air the oxygen 

 which man and other animals inhale. 



1404 Why is the air of cities generally less pure than the air of the 

 open country f 



1. Because there are more inhabitants to vitiate the 

 air; 



2. The sewers, drains, bins, and filth of a city very 

 greatly vitiate the air ; 



3. The streets and alleys prevent a free circulation ; 

 and 



4. There are fewer trees to absorb the excess of car- 

 bonic acid gas, and restore the equilibrium. 



