CHAPTER IV 

 ENERGY 



23. Movements of Matter. Matter is seldom, if ever, in a 

 complete state of rest. Suns and planets are ever in motion 

 through the heavens, wind blows, smoke rises, water evapor- 

 ates, rain falls, and grass grows. Even when the substance 

 does not move as a whole, its molecules are in rapid motion. A 

 rock or a piece of metal expands when heated and contracts 

 when cooled, thus changing both its size and temperature. 

 Matter, however, cannot move of itself. All the movements 

 which the various forms of matter exhibit are due to the effects 

 of energy upon them. 



24. Heat and Molecular Motion. All of the molecules of a 

 substance, as we have already learned, are supposed to be in 

 constant and rapid motion. The speed of a molecule of 

 hydrogen gas at ordinary temperature is more than a mile a 

 second. Oxygen molecules travel about one-fourth as fast, and 

 the molecules of other substances have similar speeds. If the 

 molecules of the air moved continuously in one direction, 

 instead of vibrating back and forth, it would produce a breeze 

 of more than fifteen miles a minute, a velocity sufficient to 

 blow away everything in its path. While we cannot follow 

 the molecules in their flight, we may, by proper experiments, 

 see the results of such motion. If a small quantity of india-ink 

 be mixed with water and viewed with a microscope, each par- 

 ticle is seen to be in rapid oscillation. Since these particles 

 cannot move of themselves, their motion is regarded as being 

 caused by the multitudes of molecules colliding with them. 

 The motion of the molecules is caused by the heat they contain ; 



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