230 SCIENCE OF COMMON THINGS. 



Benefit of stirring a dull fire. How to extinguish a chimney on fire. 



goes fermentation, which produces carbonic acid gas / 

 and heat is evolved through a species of combustion. 



153G On what does the intensity of fire depend ? 



The intensity of fire is always in proportion to the 

 quantity of oxygen with which it is supplied. 



1537 Why does stirring a dull fire serve to quicken it ? 



Because it breaks up the compacted cinders and 

 coals, making a passage for the air into the very heart 

 of the fire. 



1538 Why is the fiame of a candle extinguished when bloion by the 

 breath, and not made more intense, like a fire f 



Because the flame of a candle is confined to a very 

 small wick, from which it is severed by the breath, and 

 (being unsupported) must go out. 



1539 When a chimney with an open fireplace gets on fire, and burns so 

 as to endanger the house, how may it at once be extinguished? 



By throwing a quantity of brimstone or sulphur into 

 the fire, and closing up the fireplace with a fireboard or 

 screen. The sulphurous acid soon fills the chimney, and 

 taking up all the oxygen from the contained air, ex- 

 tinguishes the fire. Even the fire, after it has extended 

 into the woodwork of the house, may be extinguished 

 by this simple method. 



1540 Fire in a chimney may be also extinguished by closing the top of 

 thefiue with a damper : how does this extinguish the fire f 



It cuts off the draught, and the carbonic acid gene- 

 rated by the combustion soon puts an end to the fire. 



1541 What is meant by spontaneous combustion ? 



Combustion produced without contact with fire or 

 flame. 



1543 Give me an example of spontaneous combustion. 



Oiled cotton and rags imbued with any drying oil, 

 when packed in mass in a barrel, take fire, after a time, 

 at ordinary temperatures. Mixed lampblack and lin- 

 seed-oil cake take fire at ordinary temperatures, if the 

 lampblack is in excess, or a portion of it is dry. 



1543 What is generally the cause of spontaneous combustion ? 



The absorption of oxygen. Porous bodies, that are 



