4.14 GENERAL SCIENCE 



tremendous force, but the molecules on the top of the water are pulled 

 up by only the molecules of air whose attraction is not as strong 

 as that of the molecules of water underneath; therefore, the mole- 

 cules on the surface of the water are pulled down into the water 

 with a greater force than they are pulled upward from the water, 

 which causes them to crowd together so closely as to form a tough- 

 ened place on the surface of the water on which the needle may rest. 



We call this force which pulls together molecules of any sub- 

 stance cohesion. Cohesion is very important to our life. Pieces 

 of wood stick together because the molecules of the wood attract 

 each other, and the stronger the cohesion is between the molecules, 

 the harder it is to break the body apart. If a piece of crayon is 

 broken, it is very difficult to stick the pieces together so that they 

 will hold. If we press very hard, we may get the molecules near 

 enough together to cause them again to attract each other. The 

 distance between the molecules before attraction takes place must 

 be within a millionth of an inch. 



Impenetrability. If we tear a sheet of paper in two we really 

 do not tear the paper but simply separate the molecules. This is 

 also true when we drive a nail into a board. The nail does not pass 

 through the board, but between the molecules, separating them, 

 sometimes sufficiently to cause the board to split. This property 

 of matter is called impenetrability, which means that nothing can 

 be penetrated. We do not drive nails into wood, strictly speaking. 



Adhesion. It is often necessary to use glue or cement to mend a 

 piece of wood or china which has been broken. Sometimes the 

 molecules of one substance are attracted with a great deal of force 

 toward the molecules of another substance. For this reason, two 

 pieces of wood which must be mended are covered with glue and 

 then tightly pressed together to get the molecules of glue as near 

 the wood as possible. This is called adhesion. 



If the molecules of one substance have a great attraction for the 

 molecules of another substance the two substances will adhere. 

 Water, for example, will separate over the table because the mole- 

 cules of the wood have a greater attraction for the molecules of the 

 water than the molecules of water have fqr each other. 



Mercury, however, will form into globules without moistening 



