Heat 



89 



FIG. 41. A thermometer. 



exactly how much an object expanded when it was heated 

 a certain amount ; exactly how much heat was needed 

 to change water to steam ; exactly how far a piece of 

 steel of a certain size and shape could bend without 

 breaking ; exactly how crystals form and so on and 

 so on. And they have always found that everything 

 acts as if it were made of moving molecules. Their 

 experiments have been so careful and scientists have 

 found out so much about what seem to be molecules, - 

 how large they are, what they probably weigh, how 

 fast they move, and even what they are made of, 

 that almost no one has any doubt left that fast-mov- 

 ing molecules make up everything in the world. 



