1 80 VEN TIL A TION. 



the " black hole of Calcutta." One hundred and forty- 

 six prisoners were shut up in a small room with two 

 narrow open windows. These windows would probably 

 have supplied abundant fresh air for ten or twenty per- 

 sons, but they were so insufficient for the needs of the 

 large number locked up in the room, that, in eight 

 hours, one hundred and twenty-three died. 



5, Ventilation. Most of us have to spend a large part 

 of our time within more or less closed rooms. In order 

 that the air in them may continue fit to breathe, it must 

 be changed all the time. This removal of the foul air 

 and its replacement by fresh, is known as ventilation. 

 Ventilation is "sufficient" when it renews the air fast 

 enough. It is good when, in addition to being sufficient, 

 it does not cool a room too much or cause injurious 

 draughts. 



6. The Amount of Ventilation Necessary depends of 

 course on many things. If there are two people living 

 in a room, they will require just twice as much fresh air 

 as one; and fifty will need fifty times as much. School- 

 rooms, churches, theatres, and other like places, where 

 many people collect, need very free ventilation. All 

 such burning things as fires or candles or gas or oil- 

 lamps, take valuable oxygen from the air and give 

 hurtful carbonic acid to it. In ventilating a room, allow- 

 ance must therefore be made for them. Ventilation just 



5. What is necessary that the air in inhabited rooms may continue 

 fit to breathe ? What is ventilation ? Sufficient ventilation ? Good 

 ventilation ? 



6. How does the number of persons in a room affect the amount of 

 ventilation necessary ? Examples of rooms which especially need 

 free ventilation? How do burning things alter the air? Why is 

 more ventilation necessary when the gas is lighted ? 



