COMPOSITION OF MATTER 9 



11. Molecular Theory Now because these molecules 

 are too small to be discovered, no one knows exactly 

 how they are put together to form any body of matter. 

 There are several like cases in scientific study, when 

 things cannot be definitely known but are only explained 

 by guess. Such guesses are not rash, however, or hasty; 

 they are made by men who have studied and thought 

 deeply, and may generally be taken as probably true 

 explanations. An explanation of this sort is called a 

 theory. The explanation of the composition of matter 

 is called the molecular theory ; while it cannot of course 

 be proved by ordinary methods of proof, it is generally 

 believed by scientists. 



The molecular theory states that the molecules in any 

 body are separated from each other by spaces called 

 pores. These pores are larger than the molecules them- 

 selves, and in them the molecules are supposed to move 

 rapidly to^ and fro. The rapid to-and-fro motion is 

 called vibration. 



Experiment 6. The existence of pores may be shown by dip- 

 ping a piece of gold (one of the densest of solids) into a cup of 

 mercury (quicksilver). The molecules of mercury will fill the 

 pores between the molecules of gold. (The mercury can be 

 removed from the gold by dipping into nitric acid.) Similarly 

 iron will take water into its pores. 



QUESTIONS 



1. Name the three states of matter. Define each. Give exam- 

 ples of each. 



2. What features do most gases have in common ? What is 

 meant by diffusion ? How is diffusion caused ? What is a vapor ? 

 What is smoke? 



