94 THE SUN'S GIFT OF HEAT 



expand; withdrawal of heat causes most substances to 

 contract. 



Mass is the amount of matter in a body. Volume is the 

 amount of space a body occupies. Density depends on the 

 amount of matter in a given volume. Weight is the measure 

 of the earth's attraction, or gravity, for any mass. 



Heat is molecular energy. Sufficient heat will change 

 solids to liquids and liquids to gases. The most common 

 way of producing heat is by burning. Burning is a chemical 

 process in which atoms of oxygen unite with atoms of fuel 

 elements, such as carbon and hydrogen. Heat may also 

 be produced by chemical, mechanical, or electrical action. 

 The temperature to which a substance must be brought be- 

 fore it will burn is called its kindling temperature. Keep- 

 ing fuel elements in a furnace at their kindling temperature 

 and providing just the right oxygen supply are the two 

 problems to be solved in saving fuel and abating the smoke 

 nuisance. Fire can always be extinguished if the supply of 

 air that reaches it can be shut off. 



In gas, metal, and liquid thermometers, substances that 

 expand and contract uniformily through ordinary tempera- 

 tures are employed. The two most commonly used ther- 

 mometer scales are the Centigrade and the Fahrenheit. Some 

 substances require more heat than others to raise their 

 temperatures. Water absorbs more heat than almost any 

 other known substance. When a solid changes to a liquid 

 or a liquid to a gas, a tremendous amount of heat is ab- 

 sorbed which does not raise the temperature. When the 

 changes are reversed, this heat is given out. 



Heat may be transferred by conduction, convection cur- 

 rents, and radiation. The principle of heat transference 

 accounts for the efficiency of stoves and furnaces, as well as 



