Molecular Attraction 43 



pull of adhesion which makes some of the water cling 

 to your finger? 



If the pull of gravity is stronger, would not all the 

 water drop off, leaving your finger dry? If the pull of 

 adhesion is the stronger, would not all the water stay on 

 your finger, none dropping off? 



The truth of the matter is that gravity is stronger 

 than adhesion unless things are very close together ; 

 then adhesion is stronger. The part of the water that 

 is very close to your finger clings to it in spite of gravity ; 

 the part that is farther away forms drops and falls 

 down because of the pull of gravity. 



Adhesion, then, is the force that makes things cling 

 to each other when they are very close together. 



Why it is easier to turn a page if you wet your finger. 

 Water spreads out on things so that it gets very close to 

 them. The thin film of water on your finger is close 

 enough to your finger and to the page which you are 

 turning to cling to both ; so when you move your finger, 

 the page moves along with it. 



Why dust clings to the ceiling and walls. The fine par- 

 ticles of dust are wafted up against the ceiling and walls 

 by the moving air in the room. They are so small that 

 they can fit into the small dents that are in plaster and 

 paper and can get very close to the wall. Once they 

 get close enough, the force of adhesion holds them with 

 a pull stronger than that of gravity. 



Oily and wet surfaces catch dust much more readily 

 than clean, dry ones, simply because the dust can get 

 so much closer to the oil or water film and because this 

 film flows partly around each dust particle and holds 



