COMPOSITION OF MATTER 



51 



between the platinum foils.) Two copper wires each having a 

 small piece of platinum foil attached to one end are so arranged that 

 the platinum foils extend up vertically in the water. 



Fill two test tubes with the water in the dish and invert them 

 over the platinum foils. To the ends of the copper wires attach 

 a battery consisting of several dry cells. Bubbles of gas will 

 begin to rise in the test tubes as soon as the battery is connected. 

 One of the tubes will fill twice as fast as the other. When this 

 tube is full quickly invert it and apply a lighted match to its mouth. 



FIGURE 21 



There will be a sharp explosion. This gas is hydrogen. Invert 

 the other tube and insert a splinter with a glowing spark at its 

 end. The spark will burst into flame. This gas is oxygen. 



Chemists have learned that every molecule of water 

 contains two particles of hydrogen and one particle of oxy- 

 gen. These particles are called atoms. An atom of hydro- 

 gen is hydrogen ; an atom of oxygen is oxygen no other 

 substance. For that reason, hydrogen and oxygen are 

 known as simple substances and are called elements. But 

 since the smallest particle of water a molecule is com- 

 posed of hydrogen and oxygen, water is not a simple sub- 

 stance but a compound of two other substances. Chemists 

 therefore call water a compound. 



Every kind of matter known to man is classified as either 

 an element or a compound. So far there have been dis- 

 covered only about eighty elements eighty substances that 

 cannot be reduced to simpler substances. Among these are 



