222 SCIENCE OF COMMON THINGS. 



The burning point rust flame fire. 



The burning point. 



14*75 Is this point different for different substances f 



It is ,' thus phosphorus will combine slowly at 77 F., 

 but does not enter into rapid combustion until the tem- 

 perature is raised to 104 F. Charcoal burns slowly 

 below a red heat. 



14*70 Is the quantity of heat given out, when a body combines slowly 

 with oxygen, the same as when it combines rapidly with it, the relative 

 quantities of the combining bodies remaining the same in both cases ? 



The total quantity of heat given out is the same, 

 whether the combustion takes place slowly or quickly ; 

 but in the case of slow combustion, the heat is much 

 less intense, and often becomes insensible, because 

 during the long time occupied in combination the 

 greater part is carried away by conduction. 



147"7 Is the quantity of light given out the same, whether the combus- 

 tion be rapid or slow ? 



No : the quantity of light given out during the com- 

 bination of oxygen with a given quantity of a com- 

 bustible body varies greatly, according to the rapidity 

 of the combustion. 



1478 What is rust f 



The oxidation of iron in moist air. 



14*79 When iron rusts in the air is heat given outf 



Certainly ; but the process of rusting takes place so 

 slowly that the amount of heat given out at any one 

 time is imperceptible to our senses. 



148 O What is flame f 



Burning gas or vapor. 



1481 What is fire? 



Heat and light produced by the combustion of in- 

 flammable substances. 



1483 What does the brightness or illuminating power of flame depend 

 fnf 



It depends on the degree of heat in part, but mainly 

 on the presence or absence of solid particles in the 

 flame, which may act as radiating points. 



1483 Are there solid particles in every illuminating flume t 



