SUMMARY 



73 



affin (white wax) are prepared. Coal is the chief source of 

 illuminating and fuel gas (cf. 124). 



c. Collision and Friction as Sources of Heat. If you slap your 

 hands together briskly, they become warm. When a bullet is stopped 

 by a rock, it becomes hot. What is the source of the heat in these 

 cases? The answer is that the moving bodies (the hands and the 

 bullet) have their motion changed into heat, the motion of the mole- 

 cules. The heat is due to the collision. When you rub your hands 

 together, they become warm, because their motion is partly changed 

 into heat. The loss of motion 



is due to the friction of one 

 hand against the other. 



Early man learned to use 

 friction to kindle his fires (Fig. 

 63), and thus exchanged mus- 

 cular energy for heat. Later 

 he obtained sparks by striking 

 together flint and steel, or 

 flint and iron pyrites ("pl-rl r - 

 tes"; also called " fools' gold/' 

 from its deceptive color). In 

 the modern match man is still 

 exchanging motion for heat; 

 for he uses friction to kindle 

 the match.- 



d. Electrical Resistance. 

 The energy of the electric 



current is easily changed into heat, and gives us electric lights, 

 heaters, and furnaces. We can understand these better after we have 

 studied Chapter VIII. 



FIG. 63. 



Batua Fire-Drilling, Congo Free State. Copy- 

 right, 1912; Frederick Starr. 



77. Summary. Bodies usually expand when heated, and con- 

 tract when cooled. This is explained by the theory that matter is 

 composed of molecules, and that heating separates them further, while 

 cooling causes them to come closer together. 



