HEAT 



2749 



HEAT 



end becomes hot; that is, the molecules begin 

 to move rapidly. In moving rapidly they hit 

 against the molecules next to them and this 

 raises the temperature of the next part, and 

 so on down the i , 



entire length of 

 the poker. One 

 of the most in- 

 teresting exam- 

 ples of the power 

 of metals to con- 

 duct heat is the 

 safety lamp which 



Tninprs n<5P Onp 

 miners use. Une 



BY CONDUCTION 

 The feet of a stove may 

 serve as an example of heat- 

 ing by conduction. The un- 



derside of a stove and the 



of the great feet become very hot, because 



. . . iron is a good conductor of 



dangers in mines heat. The stove must be set 



i that nf pvnln- on a platform made of some 



expl substance through which heat 



sions. But the passes less readily, to prevent 



, i scorching the floor. 

 miners must have 



light, and so are obliged to carry lamps. These 

 lamps are made safe by surrounding the flame 

 with a screen of metal gauze and solid metal 

 pieces. The surrounding metal conducts the 

 heat of the flame away and actually makes the 

 flame cooler, so that it cannot set fire to any- 

 thing. It has been found by experiment that 

 all solids that can conduct electricity are good 

 heat conductors. Silver and gold, copper and 

 iron, steel and aluminum, are very good heat 

 conductors. Stone, glass and paper are poor 

 conductors. Mercury is the only good con- 

 ductor among the liquids. Water is poor, and 

 so are all gases, including air. The asbestos 

 coverings on locomotive boilers and steam 

 pipes are poor conductors of heat, and are put 

 on to prevent its escape. Wool is a poor con- 

 ductor, whether in its natural state on a sheep's 

 back or made into garments. Air is so poor a 

 conductor that air spaces are left between the 

 walls of refrigerators to dispute the passage of 

 heat. The reason that down comforters are 

 warm is that the air entangled with the down 

 keeps the heat from escaping. Wood is a poor 

 conductor, and that is why it is used for 

 handles on pots and pans. 



Radiation. In both of the processes of heat 

 transference that have been described, the sub- 

 stance heated is in contact with the body that 

 heats it. But there must be some other process 

 of transference, for we all know that a hot 

 stove will heat any object that is near it. We 

 know that cloth can be scorched or burned 

 without touching the stove. We know that 

 the heat from the sun somehow succeeds in 

 reaching the earth. This process is known as 

 radiation. Through radiation heat is trans- 



BY RADIATION 

 One illustration of heat 



ferred from one point to another by means of 

 waves. These waves can transfer heat across a 

 vacuum or from the sun and stars to the earth. 

 Therefore there must be some medium which 

 fills all the spaces, large and small, in the 

 universe. This 

 medium is called 

 ether (see 

 ETHER). 



Just as water 

 waves spread out 

 in rings from the 

 spot where a 

 stone falls into a 



pool, so heat traveling by radiation is 



shown in the preparation of 



waves spread in toast. Heat is carried from 



all directions the nre by the ether waves ' 

 from the vibrating molecules of hot bodies; 

 and just as water waves break and give up their 

 energy to pebbles on the shore of the pond, so 

 the heat waves strike and give up their energy 

 to particles of matter in their vicinity. Suppose 

 the fire in the stove is reduced so that the 

 temperature of the stove falls as low as that of 

 your body. The stove does not cease to send 

 out heat waves, for if we put a piece of ice near 

 the stove it will melt. As a matter of fact, the 

 stove will continue to send out neat waves 

 until it becomes of the same temperature as 

 the air of the room and of all the objects in 

 it. Then, if the temperature of the room is 

 raised, the stove, in common with other things 

 in the room, will begin absorbing heat. This 

 is true of all bodies; they are either radiating 

 heat or absorbing it at all times. If, in a 

 given time, the amount of energy that a body 

 absorbs is greater than that which it radiates, 

 its temperature rises, while if the amount of 

 energy that it absorbs is less than that which 

 it radiates, its temperature falls. This prin- 

 ciple is known as Prevost's theory of exchanges. 



What Heat Does. Among the most familiar 

 heat effects are change in volume, change in 

 temperature (which has been discussed), and 

 change in molecular arrangement, such as 

 melting or boiling. 



(1) Makes Bodies Larger. If any solid body 

 is warmed over a fire or in other ways, it 

 becomes larger. This is the theory on which 

 an ordinary thermometer is made. A bulb con- 

 necting with a slender tube is filled with mer- 

 cury, which is a good conductor of heat. When 

 the temperature of the air rises, the mercury 

 expands and rises in the tube. In laying rail- 

 road tracks, this property of steel has to be 

 taken into account. The rails, if they are laid 



