CONSTITUTION OF MATTER 9 



over the ball, but when the ball is heated, it no longer 

 passes through the ring. 



Thus we say, solids expand with heat and contract 

 with cold. This statement means, speaking in terms of 

 the Molecular Theory, that the motion of the molecules 

 is accelerated by an increase in temperature and, mov- 

 ing more rapidly and with greater force, they collide, 

 strike one another harder, and rebound from one 

 another a greater distance. The inter-molecular spaces 

 are thus enlarged and the Avhole object takes up more 

 space. 



On the other hand, when, heat is withdrawn, the 

 motion of the molecules is retarded; they are drawn 

 more closely together, the inter-molecular spaces 

 become smaller, and the object takes up less space. 



Thus, we see that a change in temperature produces a 

 pronounced change in the activity of molecules and 

 consequently in the size of the inter-molecular spaces. 



What has been said of the effect of the change of 

 temperature upon solids is equally true for most 

 liquids and gases. 



Molecules in liquids. In liquids the molecules are 

 farther apart than in solids and have greater freedom 

 of movement. This is shown by the fact that liquids 

 can be poured, while solids, as we know, can not. In 

 pouring a liquid, the molecules glide over each other 

 with freedom and ease. The greater freedom of mole- 

 cules in liquids is shown in diffusion. Diffusion is the 



