52 ELEMENTS OF GENERAL SCIENCE 



48. 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 who study these things 

 have formed a theory which seems to account for a great many 

 facts in nature. The theory is that the air is not a perfectly 

 continuous 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 email. 

 It is believed that they are in rapid motion, and that one mole- 

 cule 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 ques- 

 tion is more thoroughly investigated, more evidences are found 

 that agree with the theory, and as yet nothing has been found 

 that is opposed to it All scientific men have come to accept 

 it as a reasonable explanation and one that is probably true. 



49. Molecular theory applied to air. The molecular theory 

 will help us to answer some of our questions. In the case of 

 the diffusion of illuminating gas through the air of the room, 

 the molecules of the gas simply passed through the spaces 

 between the molecules of air. 



According to this explanation, gases are compressible be- 

 cause they are porous. Perhaps it may illustrate the condi- 

 tion of a gas if we imagine both the molecules and the spaces 

 to be greatly magnified. If the molecules should 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 particles are forced together. The force 

 with which air or any other gas resists compression makes it 

 appear that in some way the particles resist being pressed 

 together and are ready to fly apart again, as they do, in fact, 

 when the pressure is removed and the gas expands. 



