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EXPERIMENTAL GENERAL SCIENCE 



also produces a large amount of heat. Burning gas jets and 

 oil lamps are instances in which both light and heat result 

 from chemical energy. In fact, all ordinary burning is due 

 to the union of the gases of the air with a combustible sub- 

 stance. Since heat causes molecular motion, it is not sur- 

 prising to find that motion may be turned to heat ; even rubbing 

 the hands together increases the heat in them. When a bullet 

 strikes a target, the motion of the bullet ceases, but both the 

 bullet and the target are heated. Brakes may be applied so 



FIG. 2. Test-tubes, beakers, flasks and stand used in making experiments. 



closely to car wheels as to cause sparks to fly from them. In 

 other days before matches were known, fire was made by 

 striking flint a glancing blow with steel. The motion of the 

 flint against the steel generated enough heat to make small 

 particles of the flint red-hot. Even the air, which at first 

 thought seems too light and thin to offer much resistance to 

 bodies passing through it, may nevertheless produce much 

 heat in this way as in the case of the so-called shooting stars, 

 which, in their fall to the earth become so hot by merely rub- 



