126 ELEMENTARY SCIENCE 



motion. The motion of the molecules is a cause, of which 

 heat is one of the effects. 



If we explain heat as a phenomenon of molecular motion, 

 it is easy to understand that it is a form of energy. This is 

 further evident when we consider what results from the 

 transmission of heat to the water in the boilers of a steam- 

 engine. We know that the heat in the boiler of a locomo- 

 tive, by means of the pressure of the steam it produces, 

 can be changed into motion. We also know that motion 

 can be changed into heat again. We change motion into 

 heat every time we strike a match; even a slight rub 

 develops enough heat to set fire to the sensitive chem- 

 icals that compose the "head" of a match. Even safety- 

 matches require a slight rub on the specially prepared 

 surface, and this rub increases the heat just enough to 

 make the match ignite. This principle is also evident 

 when a train gets a "hot box." We know that this heat 

 is a result of friction. We know that lubricants (oil, graph- 

 ite, vaseline, etc.) must be used in machines wherever there 

 is danger of overheating on account of friction. Friction 

 is the rubbing together of two bodies. Evidently, there 

 can be no friction unless there is motion. It was probably 

 by means of friction that man first obtained fire. We do 

 not know this to be true, but it seems probable. 



Think then of the surprise and terror of that ancient 

 savage who first discovered fire ! Picture him as he may 

 have squatted in the sun on some hot beach. About him 

 lay plenty of driftwood, dry as tinder, as you yourself 

 may have seen it on lake or ocean beaches. He rubbed the 

 sides of two pieces briskly together, perhaps to fashion 

 one into a shape he wanted, perhaps just to be doing some- 

 thing. As he rubbed hard and fast, suddenly he noted a 



