The Composition of Matter 23 



unite with each other in certain fixed proportions to form mole- 

 cules of different kinds of matter. 



Now let us use our imagination and try to picture a molecule 

 of water. Scientists tell us that this molecule of water is com- 

 posed of two atoms of a gas called hydrogen and one atom of 

 a gas called oxygen. These particles or atoms of oxygen and 

 hydrogen cannot be divided into other kinds of material. Water, 

 then, must be made up of these two substances. Substances which 

 cannot be broken up into other kinds of matter we call elements. 



By means of electricity, it is possible to separate a molecule of 

 water into its two elements, hydrogen and oxygen. When these 

 elements separate, the water vanishes, and in place of a molecule 

 of water we have two atoms of hydrogen and one atom of oxygen. 

 Substances which, like water, can be divided into elements, we 

 call compounds. According to the atomic theory, which is ac- 

 cepted by scientists as true, the molecules of all compounds are 

 made up of atoms in various proportions. 



Having learned that an element is a substance which cannot 

 be broken up into anything simpler, we shall now take up the 

 study of some of the most common of them. Before beginning 

 such a study, we must first collect or prepare the elements to 

 be studied. 



Oxygen. Oxygen is a gas, named by Lavoisier. He discov- 

 ered that it exists as an element in air and in many compounds, 

 and that it is the true cause of burning. It is the most widely 

 distributed element. One-third of water, about one-fifth of air, 

 and a large per cent of the crust of the earth are oxygen. It is 

 absolutely necessary to all life. The most important charac- 

 teristic of oxygen is its ability to sustain combustion, that is, to 

 produce heat and light when it unites with other elements. 



Experiments with Oxygen. 



Preparation of Oxygen. Mix equal parts of manganese 

 dioxide and potassium chlorate in a flask. Close the opening 

 of the flask with a rubber stopper through which a bent glass 

 tube passes, the other end of the tube reaching under the surface 

 of water in a tray. Place an inverted bottle filled with water 

 over that end of the glass tube which is in the water, being 



