CHAPTER II 

 WHAT GOES ON IN THE WORLD 



7. Things change. The world that we know is made up of 

 things that are constantly becoming different, or cJianging. 

 The weather changes from hour to hour, from day to day, from 

 season to season. Non-living objects are constantly changing — 

 moving, burning, rusting, fading, crumbling. Living objects 

 change from day to day, from season to season ; they move 

 about, they fight, they build up, and they destroy — that is, they 

 are themselves constantly changing, and they are constantly 

 bringing about alterations in other objects. 



8. Physical changes. In the course of these various changes 

 we see materials take on new forms, as when clay is pressed 

 into bricks, or when bricks are assembled into houses. We see 

 substances change their state, as when solid butter melts to a 

 liquid, or when liquid water evaporates into a gas or freezes 

 into solid ice. We see solids dissolve, and we see their condi- 

 tions change in other ways, as when an electric current, pass- 

 ing through a platinum wire, makes it hot, or when an electric 

 current passes around a piece of iron and makes it magnetic. 



In all these changes, and in many others, the material appears 

 to remain essentially the same stuff. These are examples of 

 physical changes. 



9. Chemical changes. In the course of other changes, cer- 

 tain of the substances involved seem to disappear entirely, while 

 new materials make their appearance. For example, in a fire 

 that destroys a house or a forest, much that formerly existed 

 ceases to exist, and smoke and ashes appear as new substances. 

 In the souring of milk, or in the cooking of food, or in the 



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