CONSTITUTION OF MATTER. 25 



space of one inch, and the weight of a million million mole- 

 cules of hydrogen gas (the lightest substance known) is 

 supposed to be equal to about three pennyweights. Or, if 

 you prefer another way of looking at it, imagine a drop 

 of water the size of a pea magnified to the size of the earth, 

 then the molecules in it increased in the same proportion 

 would be coarser than fine shot, but probably not so coarse 

 as cricket-balls. These speculations may seem rather extrav- 

 agant ; but three trains of thought have led philosophers 

 to nearly similar conclusions. These molecules are con- 

 ceived to be continually in motion, so that the interior of a 

 body presents to our imagination some resemblance on a 

 small scale to the vast system of the universe. In fact, we 

 see in the latter case stars held in certain positions deter- 

 mined by the law of universal attraction, and revolving one 

 about another. Repeating briefly what we have told you 

 in this section, masses of matter are made up of excessively 

 small particles, called molecules, separated from each other 

 by void spaces, mutually attracting and constantly in mo- 

 tion. 



21. Advantages of this Theory. We have already alluded 

 to the fact that this theory enables us to explain intelli- 

 gently the divisibility of matter, bftt besides this it accounts 

 in a satisfactory manner for many of the facts of physical 

 science. When you heat iron, it expands ; this you remem- 

 ber was explained fully in Part I., just as we do now by 

 saying that the particles are put farther apart by the heat, 

 which is really but an increase of motion imparted to them. 

 When any thing is cooled down, on the other hand, it con- 

 tracts that is, the particles approach each other. This 

 theory, then, accounts for the changes of volume caused by 

 changes of temperature. Then, again, the very existence of 

 three states of matter solid, liquid, and gaseous depends 

 on the relative position of the molecules. In solid bodies, 



B 



