MATTER AND ENERGY 69 



done plus the heat produced always bears the same 

 relation to the energy contained in the gasoline burned. 

 When heat is used to make water hot, the water will 

 give out as much heat on cooling as it took up "in becom- 

 ing heated. The heat required to change water to steam 

 is given out when the steam is condensed. This principle 

 is used in steam-heating plants. 



This universal experience is expressed in the law of the 

 conservation of energy, which may be stated thus : Energy 

 can neither be created nor destroyed, but it can be trans- 

 formed. It may change its form, but its total amount 

 is neither increased nor diminished. 



If coal is burned in a locomotive, it makes heat 

 which changes water into steam, and the steam moves 

 the locomotive and cars. This is one of the ways of 

 making use of the energy stored in coal. If the coal 

 were burned in a wrecked locomotive it would produce 

 heat just the same, but the heat would be wasted as far 

 as man's immediate use is concerned. So the law of 

 the conservation of energy does not mean that energy 

 cannot be wasted. It is man's business to use his energy 

 and all kinds of energy effectively to use it where it 

 will bring valuable results. 



QUESTIONS AND EXERCISES 



1. Examine several kinds of substances and note any differ- 

 ences. 



2. What are the three states in which matter can exist? 



3. Do all substances in the liquid state have the same tempera- 

 ture? 



4. In which of the three states are the following most useful: 

 water, iron, silver, mercury, sugar, gasoline, coal? 



5. What holds objects on the earth? 



