60 ELEMENTS OF GENERAL SCIENCE 



sponge. Such a substance as iron does not appear to be 

 porous, and we do not find it easy to compress it into 

 smaller space. If air is porous, that may also explain how 

 the illuminating gas passed through the air and how it is 

 possible for air and other gases to expand and contract 



59. A theory about the air. In the attempt to answer the 

 questions that we have been asking about the air and also 

 to explain many other matters, scientists have formed a 

 theory which seems to account for a great many facts in 

 nature. This theory is that the air is not a perfectly con- 

 tinuous substance, but that it is formed of a great many 

 particles, each a little distance from the other particles. 

 These particles, called molecules, are believed to be extremely 

 small. It is believed that they are in rapid motion and that 

 one molecule moves in a straight line until it strikes another, 

 when it rebounds and again moves in a straight line. It is 

 supposed that molecules are continually moving and striking 

 one another at very short intervals. This theory of the 

 structure of the air and of other substances is called the 

 molecular theory. As the question is more thoroughly in- 

 vestigated more evidences are found that agree with the 

 theory, and as yet nothing lias been found that is opposed 

 to it. All scientific men have come to accept it as a reason- 

 able explanation and one that is probably true. 



60. Molecular theory applied to air. In the case of the 

 diffusion of illuminating gas through the air of the room, 

 the molecules of the gas simply pass through the spaces 

 between the molecules of air. According to this explanation 

 gases are compressible because they are porous. Perhaps it 

 may illustrate the condition of a gas if we imagine both the 

 molecules and the spaces to be greatly magnified. If the 

 molecules were to be magnified to the size of baseballs, they 

 would have to be about two feet apart to preserve the 

 proportion; that is, a gas consists of material particles with 

 spaces between them, and when the gas is compressed, the 



