Which do you think is the stronger, the pull of gravity which makes some of the water drip off, or the 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 particles 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 it by the force of adhesion. This is why your face gets much dirtier when it is perspiring than when it is dry. _APPLICATION 12._ Explain why cobwebs do not fall from the ceiling; why dust clings to a wet broom; why a postage stamp does not fall off an envelope. INFERENCE EXERCISE Explain the following: 41. There are no springs on the tops of high mountains. 42. People used to shake sand over their letters after writing them in ink. 43. People used to make night lights for bedrooms by pouring some oil into a cup of water and floating a piece of wick on the oil. The oil always stayed on top of the water, and went up through the wick fast enough to keep the light burning. 44. Your face becomes much dirtier when you are perspiring. 45. Ink bottles are usually made with wide bases. 46. When you spill water on the floor, you cannot wipe it up with wrapping paper, but you can dry it easily with a cloth. 47. Oiled mops are used in taking up dust. 48. Cake will stick to a pan unless the pan is greased. 49. Although the earth turns completely over every day, we never fall off it. 50. Signs are fastened sometimes to windows or to the wind shields of automobiles by little rubber "suction caps."