9 6 GENERAL SCIENCE 



heat travels with such great speed through the air with- 

 out heating it. 



Glass will permit radiant heat to pass through it 

 without itself being heated. For this reason the sun 

 shining through the windows will warm a room. When 

 the sun shines on a greenhouse the heat passes through 

 the glass and makes it much warmer inside than the air 

 is on the outside. Not much of the heat inside the 

 greenhouse can pass out by radiation, as the objects 

 are not hot enough to give off much radiant heat. 



(b) Conduction is the process of transferring heat in 

 an object by the activity of the molecules in the object 

 itself or by two objects touching each other. We learned 

 in Chapter XI that heat is a form of molecular activity. 

 This molecular activity can be transmitted the entire 

 length of an object or from one object to another if they 

 are in contact. We have experienced the results of con- 

 duction by placing a spoon, knife, or fork in a hot sub- 

 stance and feeling the handle become warm ; or by holding 

 the end of an iron poker in the fire until the end in the 

 hand became almost too hot to hold. How does the 

 handle of a smoothing iron get hot? Why do we use a 

 cloth on the handle of a smoothing iron? 



The heat from the fire in a stove passes through the 

 stove by conduction and keeps it almost as hot on the 

 outside as it is on the inside. How does the heat pass 

 from the stove into the room? The hot steam in a 

 radiator makes the iron hot and the heat passes through 

 the radiator by conduction, but it passes from the radia- 

 tor into the room by radiation. 



Silver is the best conductor of heat of any substance 

 known. Iron conducts heat about one-eighth as easily 

 as silver. Glass is a poor conductor of heat, only about 



