HEAT AND COLD 137 



solid. We do not have to look at it to tell this; it is indi- 

 cated by the fact that the handle of the freezer gets harder 

 and harder to turn. Evidently, then, the withdrawal of 

 heat, which means the reduction of molecular motion, 

 also involves the transformation of this liquid into a solid. 



Heat Conduction. But how did this heat get out 

 through the wall of the metal cy Under? You may accept 

 the idea of heat being due to molecular motion, but it 

 may be a little difficult to see how heat, if this be its nature, 

 is transmitted through such a solid substance as the metal 

 of this cy Under. But did you ever sting your hands by 

 striking a basebaU with a bat? Or suppose that your 

 body were protected by tight-fitting metal and some one 

 were to strike you hard with a ball or with a bat. Wouldn't 

 you feel the effect of the blow ? Then think of the milUons 

 of milUons of blows which the molecules of a Uquid or a 

 gas are constantly striking against the waUs of any vessel 

 which contains them. These waUs, like the wood of a 

 ball bat, are capable of transmitting the effects of blows 

 struck upon them, and the phenomenon of the transmission 

 by soUds of molecular blows struck upon them is caUed 

 the conduction of heat. Some substances are better con- 

 ductors of heat than others; that depends upon the nature 

 of their molecular construction. Wood is a poor conductor 

 of heat, while metals are good conductors. You would 

 have a great deal of trouble in freezing ice-cream if a wooden 

 cyUnder were substituted for the metal one. Why do 

 teakettles and coffee-pots usually have wooden handles ? 



But what becomes of the heat that passes out through 

 the metal cylinder? Evidently, according to the law of 

 the conservation of energy, something must happen to 



