14 GETTING ACQUAINTED WITH THINGS 



down into still smaller particles called atoms and those 

 into still tinier bits of matter which he calls electrons and 

 protons. We see a good deal in the papers and magazines 

 about these tiny electrons, which the physicist says are a 

 form of electrical energy, but nobody has ever seen one. 

 Our present idea of the nature of matter is only a theory, 

 but so sure is the scientist of his experiments that the 

 modern world has come to accept this theory as a basis 

 on which we build all our knowledge of matter. 



Elements and Compounds. In order to understand 

 a little about what goes on in the world we must know 

 something about how changes in matter are brought 

 about. To do this we can make a little excursion into 

 the field of chemistry. The chemist says that all matter 

 is compounded of simple substances called elements. 

 The entire universe is made up of these substances, of 

 which there are about 90, and he says that these elements 

 can combine to form compounds which are so numerous 

 that everything, living or dead, is made up of these sub- 

 stances. Some elements we know : oxygen, for example, 

 is the gas we need if we are to breathe. You have per- 

 haps seen the experiment made in which a red powder, 

 red oxide of mercury, is heated in a test tube. As the 

 substance gets hot, we see the red substance change to 

 glistening drops of mercury such as you see in the bulb of 

 a thermometer. If you insert a glowing match into the 

 test tube, it bursts into flame, showing the presence there 

 of a gas which supports the flame. This is oxygen, a gas 

 which helps things to burn. This experiment shows that 

 we can break down a compound into its original elements, 

 which in this case are the elements oxygen and mercury. 



Energy. A lighted match gives out heat and light. 

 Exploding gasoline can move an automobile. A thrown 

 stone may break a window. When matter is in a con- 

 dition of motion it can exert a force it does not have when 



