COMMON THINGS FIRE. 



and which therefore produces in the chimney a much stronger 

 upward current. 



Thus the effect of closing the opening A I is to stimulate the fire 

 not only hy causing to pass through it all the air which previously 

 entered the opening A I, but also by augmenting the draught in 

 the chimney. 



32. From what has been explained above, it will be perceived 

 that an open fireplace such as is represented in fig. 4 serves the 

 double purpose of warming and ventilating. 



All the air which enters the chimney, whether it passes through 

 the grate or through the opening above the grate, must be replaced 

 by an equal volume of fresh air from without, which must find its 

 way through the interstices of doors and windows, or through 

 other openings provided expressly for its admission. That part of 

 the air which passes through the grate subserves the double 

 purpose of warming and ventilation. It warms by stimulating 

 and maintaining the combustion of the fuel, and it ventilates by 

 leaving in the room a void into which an equal volume of fresh 

 air must enter. That portion of air which enters the chimney 

 through the opening above the grate has no effect direct or indirect 

 in warming, but its effect in ventilating is just so much greater 

 than that of the air which passes through the grate, as the magni- 

 tude of the opening above the grate is greater than the magnitude 

 of the spaces between the bars in the front and bottom of the grate. 



33. The necessity for ventilation is so much the greater as the 

 room is smaller and lower, and as the causes of the pollution of 

 its air are more numerous and active. The air of a room is de- 

 prived of its oxygen and rendered unfit for respiration by several 

 causes. Each person who is present in the room absorbs oxygen 

 by respiration. It is calculated that an adult of average size 

 absorbs about a cubic foot of oxygen per hour by respiration, and 

 consequently renders five cubic feet of air unfit for breathing. It 

 is also computed that two wax or sperm candles absorb as much 

 oxygen as an adult. It follows, therefore, that to keep the air of 

 a room pure, five cubic feet for every person, and two and a half 

 cubic feet for every candle in the room should pass per hour into 

 the chimney, or through some other opening, and an equal volume 

 of fresh air should be admitted. 



34. Plants give out oxygen by day, but absorb it by night. 

 Their presence in a room by day is therefore innocuous, but at 

 night they have the effect of polluting the air, and should never 

 be admitted except where there are ample means of ventilation. 



35. A crowded room, illuminated with many candles and lamps, 

 and, as generally happens, without a fire, soon becomes filled with 

 air in which there is a deficient pr-oportion of oxygen and a 



204 



