68 GENERAL SCIENCE 



for doing work. Energy exists in various forms, according 

 to the kind of body in which it is found, such as heat 

 energy, mechanical energy, electrical energy. 



Heat energy can be changed to mechanical energy, 

 and mechanical to electrical energy, and electrical energy 

 can be changed back to mechanical energy and heat. 

 When fuel is burned in the cylinder of a gas engine the 

 energy that the fuel contains is transformed, part appear- 

 ing in the form of mechanical energy which does work, 

 and a considerable part taking the form of heat. If the 

 engine is run without pulling a load, all the energy of 

 the fuel used is changed into heat, in part directly in the 

 cylinder and in part through the heating of the bearings 

 by friction. Such an engine is spoken of as a trans- 

 former of energy because it changes one form of energy 

 into another. 



The process in our bodies is similar to that in the engine. 

 The materials digested from our food are carried by the 

 blood to the working tissues and there virtually burned. 

 Part of the energy may be used to do visible external 

 work, but even then much of it is converted into the 

 form of heat. While our bodies are in a state of so-called 

 rest, the work done by the internal organs finally results 

 in the generation of heat, somewhat as does the motion 

 of the engine when it is not pulling a load. 



In a great number of investigations it has been found 

 that whenever one form of energy disappears an equiva- 

 lent amount appears in some other form. Thus, if heat 

 is produced by mechanical means, such as the fall of an 

 object from a height, friction, or other means, the heat 

 which appears is always exactly proportional to the 

 mechanical energy used in producing it. If the stored-up 

 energy of gasoline is used to run an engine, the work 



