The reason for this peculiarity of mercury is that the pull between the particles of mercury themselves is stronger than the pull between them and your finger or handkerchief. In scientific language, the cohesion of the mercury is stronger than its adhesion to your finger or handkerchief. Although this seems unusual for a liquid, it is what we naturally expect of solid things; you would be amazed if part of the wood of your school seat stuck to you when you got up, for you expect the particles in solid things to cohere--to have cohesion--much more strongly than they adhere to something else. It is because solids have such strong cohesion that they are solids. _APPLICATION 13._ Explain why mercury cannot wet your fingers; why rain falls in _drops_; why it is harder to drive a nail into wood than into soap; why steel is hard. INFERENCE EXERCISE Explain the following: 51. Ink spilled on a plain board soaks in, but on a varnished desk it can be easily wiped off. 52. When a window is soiled you can write on it with your finger; then your finger becomes soiled. 53. A starched apron or shirt stays clean longer than an unstarched one. 54. When you hold a lump of sugar with one edge just touching the surface of a cup of coffee, the coffee runs up the lump. 55. A drop of water on a dry plate is not flat but rounded. 56. It is hard to write on cloth because the ink spreads out and blurs. 57. If you roughen your finger nails by cleaning them with a knife, they will get soiled much more quickly than if you keep them smooth by using an orange stick. 58. When you dip your pen in the ink and then move it across the paper, it makes ink marks on the paper. 59. If you suck the air out of a bottle, the bottle will stick to your tongue. 60. You cannot break a thick piece of iron with your hands. SECTION 9. _Friction._ What makes ice slippery? How does a brake stop a car? Why do things wear out? It would not be such a calamity if we were to turn off friction from the world. Still, I doubt whether we should want to leave it off much longer than was necessary for us to see what would happen. Suppose we imagine the world with all friction removed: A man on a bicycle can coast forever along level ground. Ships at sea can shut off steam and coast clear across the ocean. No machinery needs oiling. The clothes on your body feel smoother and softer than the finest silk. Perpetual motion is an established fact instead of an absolute impossibility; everything that is not going against gravity will keep right on moving forever or until it bumps into something else.