88 THE SUN'S GIFT OF HEAT 



different materials used in the experiments. One of the 

 reasons why we use iron for our radiators is that the heat 

 of the steam may readily pass from the inside to the out- 

 side of the radiator. , We cover our steam pipes with as- 

 bestos when we wish to retain the heat, because asbestos 

 is a poor conductor and will keep the heat in the pipes. 



On a cold day good conductors of heat feel colder than 

 other objects because they quickly conduct the heat away 

 from the hand. For that reason, a metal door knob seems 

 much colder than the door in winter. On a very warm day 

 good conductors feel hotter than other objects because they 

 conduct their heat to the hand rapidly. The metal knob, 

 therefore, seems much warmer than the door when the bright 

 sun is shining on them both in summer. 



This explains why tile and concrete floors feel cold, and 

 why we cover them with rugs, which are poor conductors 

 of heat. A woolen blanket feels warm, and a cotton sheet 

 cold, for the same reason. There is really no difference 

 between the warmth of these objects if they 

 are in surroundings of the same temperature. 



Experiment 33. Hold a piece of burning paper 

 under a bell jar held mouth downward. (Figure 

 42.) Notice the air currents as indicated by the 

 smoke. Paper soaked in a moderately strong 

 solution of saltpeter and dried burns with a very 

 FIGURE 42 smoky flame. 



Experiment 34. Fill a 500 cc. round-bottomed 

 flask half full of water and place on a ringstand above a Bunsen 

 burner, (figure 43.) Stir in a little sawdust. Some of it should 

 fall to the bottom of the flask. Gently heat the bottom of the 

 flask. Notice the currents. 



When the burning paper was held under the bell glass, and 

 when the water was heated at the bottom of the flask, cur- 



