The Propagation of Heat 185 



from one part of a solid body to another. No motion 

 of the body as a whole or of any parts of it as seen 

 by the eye takes place, the transmission being effected 

 by the molecules of the substances nearest the heating 

 source giving up part of the heat supplied by the heating 

 agent to their neighbours, which again pass on part of 

 the heat thus received in a similar fashion. The best 

 conductors are the .metals, and among these the list is 

 headed by silver, closely followed by copper. Brass, 

 iron, lead, and bismuth come next in the order of 

 naming. The conducting power of silver is over 50 

 times that of bismuth. Compared with the metals, 

 other solids are relatively poor conductors. This may 

 be shown practically by repeating Experiment 65 with 

 a stick of slate-pencil and a metal rod of equal diameter. 

 The end of the pencil will be found to remain cool 

 enough to be handled long after the metal rod is too 

 hot to touch. 



122. Conduction takes place also in liquid bodies, 

 but all liquids come under the heading of bad con- 

 ductors. That this is so in the case of water may be 

 illustrated by a striking experiment. 



EXPERIMENT 67. Surround a small lump of ice with 

 copper or iron gauze in sufficient 

 quantity to make the ice sink when 

 placed in water. Drop the ice thus 

 prepared into a large test-tube, and 

 nearly fill the tube with water. 

 Holding the tube near the middle 

 of its length, heat the water in the 

 upper portion in a flame. The 

 water in this part of the test-tube 

 may be raised to boiling-point before ^8- 56> 



the ice at the bottom of the tube has melted. 



