110 HOW WE USE HEAT 



water. Gravity therefore pulls with greater force on a 

 given volume of cold water than it does on the same vol- 

 ume of warm water. Because of this greater pull, the cold 

 liquid is drawn under the lighter one and pushes it up, 

 thus causing a convection current. A similar explana- 

 tion accounts for convection in gases. 



Demonstration 3. To See if Peat Will Travel Along a Metal Rod. 



Take a metal rod ; attach it to a stand, as shown in the diagram. 

 Tie threads to each of six tacks. Attach to the rod each of the 



threads about two inches apart 

 by means of melted wax. Now 

 apply heat to the end of the 

 rod. What happens? 



/!/ 



tacks 4 



fastenecC 



Conduction. This dem- 

 -i ^ onstration shows that heat 

 travels from one end of the 

 rod to the other. The flame 



\viU2 "\vax. heats the tiny molecules of 



iron, causing them to vi- 

 brate or move faster. They 

 hit neighboring molecules 

 harder blows and thus cause 

 them to move. In this way, from molecule to molecule, 

 heat travels by conduction. Most metals are very good 

 conductors of heat. Substances like glass, water, and 

 many rocks are fair conductors ; while air, paper, linen, 

 silk, and wool carry heat so poorly that they are called 

 nonconductors. ' Poor conductors of heat are sometimes 

 called heat insulators. Metals are the best and gases the 

 poorest conductors of heat. 



Radiation. The sun is 93,000,000 miles away from us, 

 and we know there is very little matter in all of this space. 

 Yet heat comes from the sun to us. This method of heat 

 transfer is known as radiation. Experiments show that 

 rough, black, dull substances absorb heat better than 



