ELEMENTS AND COMPOUNDS 93 



other example of diffusion.) In burning, the molecules of 

 wood have ceased to be wood. They have changed into 

 molecules of other substances. Any molecules when they 

 burn change into molecules of other substances. 



The early scientists studied just such common things as 

 the burning of wood, and scientists still study such things, 

 for there is much yet to be found out about them. Since 

 wood in burning produces two or more other substances, 

 it seemed reasonable to infer that the molecules of these 

 other substances are not so large as the molecules of wood. 

 Does this seem reasonable to you? 



Remember that to the early scientists molecules were a 

 theory. That means that they had good reason to believe 

 in them, but they could not absolutely prove their exist- 

 ence. But now the existence of molecules has been proved. 

 Similarly, the early scientists formed the theory that in 

 burning larger molecules break up into smaller molecules. 

 This, too, has been proved to be a fact. 



It is thus that science advances. A scientist studying 

 a certain problem considers all the facts about it that he 

 knows. He then forms an explanation of these facts. If 

 this explanation cannot be proved it is called a theory. 

 As more facts are learned, theories are either proved or 

 disproved. All depends upon what the facts are, not upon 

 what any one's theory is. So scientists constantly seek 

 for undiscovered facts, and are always ready to change 

 their theories in the light of facts newly learned. 



As they tested various substances and noted what hap- 

 pened, the early scientists observed that some substances 

 apparently could not be changed into other substances. 

 These they called elementary substances or elements, 

 They formed the theory that all other substances are com- 



