COMPOSITION OF AIR: ATOMIC THEORY 59 



other. Both gases are colorless and look like air. One of them, 

 that of the greater volume, takes fire and burns with a pale 

 blue flame ; the other does not take fire, but a splinter burns 

 much more brilliantly in it than in the air. The one which 

 takes fire is called hydrogen ; the one which makes other 

 things burn more brilliantly is oxygen. They can be caused 

 to unite, and when they do so they form water. We may con- 

 clude that water is made up of two substances, hydrogen and 

 oxygen, and that the electric current causes them to separate. 



It appears strange that water, a liquid, should be composed 

 of two gases, but such is the case. Are the two gases merely 

 mixed ? If so, we should expect to find that the results of 

 the mixture would be a gas, and if we mix hydrogen and 

 oxygen, as we did air and illuminating gas, we have a mix- 

 ture of gases, nothing more. Water is something different 

 from a mixture. In fact, if we mix hydrogen and oxygen 

 and bring a flame to them, the mixture will explode. After 

 the explosion we cannot find either of the gases, but instead 

 we have water or water vapor. Making a mixture is quite a 

 different thing from forming a new substance. If we mix 

 hydrogen and oxygen, we have in the mixture two kinds of 

 molecules the molecules of hydrogen and the molecules of 

 oxygen. When water is formed, we have molecules of only 

 one kind the molecules of water. The two kinds of sub- 

 stances must combine to form the water molecules. 



60. The molecule of water. The theory as held by scien- 

 tific men about this matter is that the molecule of water is 

 not a simple thing that it is composed of two kinds of parti- 

 cles. These particles are called atoms. The atomic theory states 

 that each molecule is commonly made up of two or more 

 atoms, and that these atoms may be of the same kind or of 

 different kinds. 



Each molecule of hydrogen is believed to be made up of 

 two atoms of hydrogen ; each molecule of oxygen, of two 

 atoms of oxygen. Each molecule of water is believed to 



