SCIENCE OF COMMON THINGS. 159 



How hot substances are cooled. Blowing hot food cools it 



Because the sun heats the surface of the earth, and 

 the air (resting on the earth) is heated by contact / as 

 soon as it is heated it ascends, while its place is sup- 

 plied by colder portions, which are heated in turn also. 



1030 If air be a bad conductor, why does hot iron become cold by expo- 

 sure to the airf 



Because it is made cold 1. By " convection ;" and 

 2. By " radiation." 



1031 How is hot iron made, cold by convection ? 



The air resting on the hot iron (being intensely heated) 

 rapidly ascends with the heat it has absorbed ; colder 

 air succeeding, absorbs more heat and ascends also ; and 

 this process is repeated till the hot iron is cooled com- 

 pletely down. 



ICpSS How is broth cooled by being left exposed to the air f 



It throws o&some heat by radiation but it is mainly 

 cooled down by convection. 



1033 How is hot broth cooled down by convection 1 



The air resting on the hot broth (being heated) as- 

 cends ,\\ colder air succeeding absorbs more heat, and 

 ascends also ; and this process is repeated till the broth 

 is made cool. 



1 The particles on the surface of the broth sink as they are cooled down, 

 and warmer particles rise to the surface, which gradually assist the cool- 

 ing process. 



1034 Why are hot tea and broth cooled faster by being stirred about f 



1. Because the agitation assists in bringing its hottest 

 particles to the surface 



2. As the hotter particles are more rapidly brought 

 into contact with the air, therefore cqnvection is more 

 rapid. 



1035 How does bloiving hot food make it coolf 



It causes the air (which has been heated by the 

 food) to change more rapidly F , and give place to fresh 

 cold air. 



1036 If a shutter be closed in the daytime, the stream of light (pierc- 

 ing through the crevice) seems in constant agitation : why is this f 



Because little motes and particles of dust (thrown into 



