WORK AND MACHINES 161 



is no destruction of energy. When energy is expended 

 upon bodies without causing any apparent motion in them, 

 or not enough motion to account for all the energy ex- 

 pended, these bodies become heated, i.e., molecular motion 

 in them is increased and their temperature rises. 



Rubbing a match head on a rough surface raises its tem- 

 perature sufficiently to cause ignition. The muscular energy 

 expended is much in excess of what is needed to move the 

 match through the air from its one position to the other. 

 The excess energy that is used in moving the match over- 

 comes the resistance to motion offered by friction. It 

 appears as heat, and ignition occurs. Electricity in passing 

 through the filaments in electric light bulbs produces heat, 

 or molecular motion, instead of causing any motion from 

 place to place in the lamp parts. When one attempts to 

 break a piece of iron or copper wire by bending it back 

 and forth rapidly, the wire may become so hot as to burn the 

 fingers. Part of the muscular energy used appears as heat 

 in the wire. A piece of metal subjected to a vigorous pound- 

 ing becomes too hot to hold in the hand. The use of oil 

 and other lubricants in machinery is to lessen the waste of 

 mechanical energy through its conversion into heat. 



The sun is considered the primary source of the energy that 

 causes all changes in the material world about us. Upon 

 this solar energy all life upon the earth is dependent. It is a 

 matter of great interest to learn by readings in Astronomy 

 how scientists account for the tremendous annual output of 

 energy from the sun, and what beliefs are entertained as to 

 the period through which this may continue. Some of the 

 ways in which plant life appropriates it, and how in turn 

 it becomes available for animal life, will be discussed later. 

 (Pages 226 and 268.) 



To account for the transmission of this energy from the 

 sun to the earth it is necessary to believe in the existence of a 



