80 HEAT AND ENERGY 



Experiment 71. Put an iron rod or wire in a hot fire. After 

 a few minutes, try its temperature at different points, beginning 

 with the end that is farther from the fire. Let it remain and see 

 if it grows hotter throughout. 



Substances which allow heat to pass through them 

 easily in this way are called conductors of heat. In gen- 

 A eral, solids and liquids are good con- 



ductors as compared with gases, which 

 are very poor. Metals are usually very 

 good conductors, while wood and cloth 

 conduct heat but poorly. Stove lifters 

 and pokers often have wooden handles, 

 FIG. 59 for this reason ; felt also is used around 



steam and water pipes to keep the heat in. 



Experiment 72. Arrange four metal wires (e.g. iron, copper, 

 brass, and German silver), as in Fig. 59. Apply heat at A, and 

 note the order in which the other ends become hot. Compare 

 the conducting power of the different metals. 



Experiment 73. Find the temperature of the air in the room, 

 and of water which has been in the room a long time ; they 

 should be the same. Now put your 



hand into the water. How does it feel ? "*?^ 



Which takes heat out of your hand 

 faster, air or water? Which is the 

 better conductor? 



Experiment 74. Boil the top of 

 water in a test tube, as in Fig. 60. 

 Note how long it is before the bot- 

 tom becomes hot, and compare with 

 a similar length of iron. Fl - 6 



91. Convection. Convection is the transfer of heat 

 from place to place by the change of position of heated 

 particles. Since the, molecules of solids are not free to 



