FIRE AND ITS USES 153 



The stove has numerous advantages. As its radiating sur- 

 face is relatively large, much more of the heat from the fire is 

 radiated into the room, and much less goes up the chimney. By 

 dampers set in the stovepipe and drafts below the fire box that 

 may be opened and closed, the flow of air through the fire can 

 be controlled and so the rate at which the fire burns. On the 

 cook stove the utensils may be heated by contact with the hot 

 surface, and not get covered with soot as they are in cooking over 

 an open fire. Certain metals are very good conductors of heat, 

 such as aluminium and copper, while others conduct it poorly. 

 It is an advantage to have the heat conducted rapidly to the 

 thing in the kettle or pan that is to be heated. So the teakettle 

 often has a copper bottom, and cooking utensils made of alumin- 

 ium are often used. 



You may readily demonstrate that there is a difference in 

 the heat conductivity of various substances. Take a piece of 

 No. 1 8 copper wire 8 inches long and one of iron wire of the same 

 size and length. Twist them together at one end so as to form 

 a V. Fix a little ball of paraffin or beeswax as big as a pea on 

 each wire halfway from the point of the V to the end. Holding 

 the V by its ends stick the point of the V in a flame, the arms 

 horizontal. Continue holding it thus until both wax balls fall off. 

 You will find that the one on the copper wire melts enough to 

 fall long before the one on the iron wire, for copper is a better 

 heat conductor than iron. We shall find that it is also a much 

 better conductor of electricity. 



We put coverings of poor conductors like asbestos felt on 

 steam pipes and furnace pipes to prevent loss of heat. We 

 build the fireless cooker (see Field and Laboratory Guide in 

 Physical Nature-Study, p. 58) by inclosing the pail in which the 

 cooking is to be done in a box packed with some non-conductor 

 like chopped straw or else surround it with several air spaces 

 separated by sheets of asbestos. Dry air is itself a very poor 

 conductor. So the thermos bottle is merely a bottle surrounded 

 by several air spaces, or, better still, spaces in which there is no 



