HEAT 



145 



greater specific heat. .In estimating the quantity of heat taken 

 up or given out by a body, we must find the product of three 

 quantities, viz. , its mass, its capacity for heat, and the range of 

 temperature through which it is heated or cooled respectively. 



In considering any question on capacity for heat, it should be 

 borne in mind that the amount of heat given out depends upon 

 the weight of the substance involved, its change of temperature, 

 and the heat capacity of unit mass, or its specific heat. 



Conduction of Heat. By touching a succession of things in 

 a room, say the marble mantel-piece, the fender, the back of a 

 chair, the hearth-rug, we obtain a succession of sensations the 

 first two we say are cold, the 

 chair-back not quite so cold, 

 while the rug feels quite 

 warm, and yet they are one 

 and all under the same con- 

 ditions and there is no reason 

 why they should not be at 

 the same temperature. The 

 explanation of these different 

 sensations is really very 

 simple. In all those cases 

 where the hand receives heat 

 we feel the sensation of 

 warmth, while in those where 



the hand gives out heat we say the body is cold or cool. Fig. 

 61 will enable the student to remember this. Now we see why 

 the fender feels colder than the hearth-rug. The fender takes 

 more heat from the hand than the hearth-rug, and it does so 

 because it is a better conductor of heat. 



We shall do well to consider this expression a little. Put one 

 end of a poker in the fire and hold the other. Soon the poker 

 begins to feel warm, and as time goes on it gets warmer and 

 warmer, until at last you can hold it no longer. Heat has 

 passed from the fire along the poker, or has been conducted 

 from the fire by the poker. 



The process by which heat passes from one particle of a 

 body to the next is called conduction, and the body along 

 which it passes is known as a conductor. 



Good and Bad Conductors of Heat. As we have learnt, 

 heat is conveyed by conduction from one particle of a body to 

 the next, the heat travelling from the hotter to the colder parts. 



L 



Fio. 61. Hot and Cold Bodies. 



