COMPOSITION OF AIR ATOMIC THEORY 67 



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 

 mixture 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 mix- 

 ture 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 hydrogen and oxygen com- 

 bine to form the water molecules. 



70. The molecule of water. The 

 theory held by scientific men is that 

 the molecule of water is not a simple 

 thing that it is composed of two 

 kinds of particles. These particles are 

 called atoms. The atomic theory states 

 that each molecule is commonly made 



FIG. 37. Decomposition 



of water 



By use of an electric current 

 passed through the water 

 in the tubes the water is 

 decomposed into the hydro- 

 gen and oxygen of which 

 it is composed. The upper 

 clear region in the right- 

 hand tube contains hydro- 

 gen, and the upper part of 

 the left-hand tube contains 

 oxygen. The volume of 

 hydrogen is twice that of 

 the oxygen 



