30 HEAT. 



Section 2. Communication of Heat. 



54. Heat is communicated by conduction, convection, 

 and radiation. These three modes of communication 

 will be considered in the order in which they are 

 named. 



CONDUCTION. 



Explain the 55. Conduction is the passage of heat 



%* uction f through a body by communication from 



particle to particle. An iron wire, one 



end of which is held ' 



o o o o o o ^ 



in aflame, soon grows Q 



hotter at the other, by conduction of the heat of the 

 flame. The progress of heat along a wire may be 

 shown by fastening marbles to it with wax, as rep- 

 resented in the figure, and then heating one end by a 

 lamp. The marbles drop off successively, as the heat 

 in its progress melts one bit of wax after the other. 

 The communication of heat from one body to another 

 in contact with it is also a case of conduction. 



56. WHEN CONDUCTION CEASES. Con- 



When docs 



conduction duction proceeds toward the cooler por- 

 tions of a body until all its particles be- 

 come equally hot, just as the absorption of water by a 

 sponge continues until all its pores are filled. This 

 point being reached, there is no tendency to further 

 motion in the case of the heat more than in water. 



57. THE METALS ARE THE BEST CON- 



W hat substan- 

 ces are the best DUCTORS. -The earths and wood conduct 



very slowly; fine fibrous substances, like 



