1、科普知识 11Hunger and thirst are two feelings we all know well. There are two kinds of hunger. If you miss a meal or two, your empty stomach presses and moves. This gives you hunger pains. These are easily studied and measured by scientists and doctors. Another kind of hunger comes when there is not eno
2、ugh sugar in the blood to feed the billions of cells in the body. Thirst comes when the total amount of water in your body drops by 3 percent. You feel a dryness in your mouth and throat. When this happens, it means that all your body tissues(组织) need water. If you do not eat, the feeling of hunger
3、slowly goes away. But thirst does not go until you body has had the water it needs.2It may seem strange that ice itself sometimes can protect crops from frost! Some growers actually spray their crops with water on a freezing night. Water freezes quickly on the plants and then a strange thing happens
4、. As long as ice stays wet, it cant get colder than 0, a temperature many plants can stand. If the ice over became entirely frozen and dry, it might drop many degrees lower, ruining the plants. But continually spraying water on the ice, the growers keep it from going below 0C even if the air is much
5、 colder. This may be unbelievable but it saves the plants.This strange kind of “ice blanket” works only on plants that are strong enough to stand the weight of frozen spray. The system is used even to protect banana plants on some Central American plantations.3Refrigeration is one of the most import
6、ant means of preserving(保存) food. Refrigeration makes anything cold. Scientists and engineers have discovered many means of making things cold.One way is to allow water, or another liquid, to evaporate(蒸发). Evaporation cools a liquid and the liquid cools the food. Most household refrigerators contai
7、n a liquid which evaporates.Cold can also be produced by passing a gas through a small hole. Gases are made up of very small particles(微粒) called molecules(分子). Before the molecules pass through the small hole they are close together. They are also moving quickly. Afterwards they are further apart a
8、nd moving more slowly. Because the molecules are moving slowly, the gas is cool. The slower the molecules, the cooler the gas. If the molecules move very slowly the gas changes into a liquid. Liquid oxygen can be passing it through a small hole.4In the past 20 years, scientists have learned a great
9、deal about sleep and dreams. They have discovered, for example, that sleep, generally, makes a person happier in the morning.Dreams, also, have a strong effect on the moods of people. However, who appears in a dream is more important than what happens. Every person has a special dream character. If
10、this special character appears in dreams, people are happier after sleeping. If there are many people in ones dreams, this has a good effect. A person will wake up happier.The moods of people affect their performance throughout the day. The level of ones moods rises and falls during the day. The les
11、s sleepy people are, the better they perform. And, usually, they are more friendly, more active, and more clear thinking.Some day scientist hope to be able to know more about sleep in order to control the way people feel and perform.5Why does pouring hot water over a tight jar lid loosen it?Hot wate
12、r makes the metal of the jar lid expand; this makes the cover loose.Most objects expand when heated because they are made of molecules(分子) that are in constant motion. Heat makes molecules move faster. The hotter the molecules become, the faster they move and the larger space they need.This explains
13、 why we hear a click as we travel by train or a gentle thump as we drive on a concrete(水泥) highway. The clicks and thumps tell us that we are passing over expansion joins put in to allow room for the metal rails and the concrete to expand. These joints prevent the rails and the roads from buckling(弯
14、曲) as they expand under the sun. Like the molecules of the metal jar lid, the molecules of these materials need more space when they are hot.6Charles Darwin proposed the theory of natural selection in 1859. The theory holds that those species that are best adapted to the conditions under which they
15、live will survive, and those that are not adapted will perish.Take, for example, the experience of the moth(蛾) that once lived near Manchester, England, in industrial city.Before the Industrial Revolution, the lightcolored form of this month was common. As Manchesters environment became increasingly
16、 polluted, however, the tree trunks(树干) on which the moth lived became blackened by soot(烟尘). The lightcolored form of the moth stood out against the dark tree trunks and was eaten by birds, which destroyed 99 percent of the lightcolored moths.In the absence of the lightcolored moths, the dark moths
17、 became more common and took the place of lightcolored moths. In the 1950s, however, Manchesters industries were cleaned up, the tree trunks became lighter, and the lightcolored moth became more common type again.7Every human being has unique(独一无二的) fingerprints in his finger skin and the fingerprin
18、ts are unchangeable. Scientists and experts have proved the uniqueness of fingerprints and discovered that no exact similar pattern(同样类型) is passed on from parents to children, though nobody knows the reason.A persons fingerprints do not change with growing and is not affected by superficial injurie
19、s(表皮损伤). Burns, cuts and other injuries to the outer parts of the skin will be replaced in time by new one which has the same prints. It is only when the inner skin is injured that the prints will be destroyed. Some criminals(罪犯) make use of this fact to get rid of their own fingerprints but this is
20、 dangerous step to take.Fingerprints can be made very easily with printers ink. They can be recorded easily. With special methods(方法), identification(鉴定) can be achieved successfully within a short time. So fingerprints have often been used as a method to prove who a person is even if his appearance
21、 has been changed by age or accident.8Ask three people to look out the same window at a busy street corner and tell you what they see. Chances are that you will receive three different answers. Each perceives something different about it.Perceiving goes on in our minds. Of the three people who look
22、out of the window, one may say that he or she sees a police officer giving a motorist a ticket. Another may say that he or she sees a rush hour traffic jam. The third may tell you that he or she sees a woman trying to cross the street with four children. For perception(感知觉) is the minds interpretati
23、on of what the senses in this case our eyestell us.Many psychologists today are working to try to determine just how a person experiences or perceives the world around him. Using a scientific approach, these psychologists set up experiments in which they can control all of the factors. By measuring
24、and charting(制图) the results of many experiments, they are trying to find out what makes different people perceive totally different things about the same scene.9My secret for staying young is simple. Put all attention to the part of youyour brain and keep it young and growing. Keep your mind awake
25、and youll stay young all over. These are exciting times. Take an interest in the world around you, and stick to learning at least one new thing every day.No matter how old you are, its not too late to make your life more interesting. I know a housewife with no knowledge before who made herself into
26、a wellknown industrial engineer. I know a retired(退休的) electrical engineer who has become a highlypaid fashionable dress designer.Get over the idea that you are too old to go back to school. I know a man who entered medical college at 70. He got his degree with honors and became a famous doctor. Ano
27、ther man went to law school at 71 and now an active lawyer.Staying young is easy for those, old or young, who live in the future. You can do it if you care enough to try. Keep your mind awake and active: thats the only elixir(灵丹妙药).10When the weather is hot, you go to a lake or an ocean. When you ar
28、e near a lake or an ocean, you feel cool. Why? The sun makes the earth hot, but it can not make the water very hot. Although the air over the earth becomes hot, the air over the water is still cool. The hot air over the earth goes up. Then the cool air over the water moves in and takes the place of
29、the hot air.Then you may ask, “Why is the ocean full of salt?” Scientists will say that the salt comes from the rocks. When the rock gets very hot or very cold, it cracks(爆裂). Rain falls into the cracks. The rain then carries the salt into the earth and into the rivers. The rivers carry the salt into the ocean. But then you ask, “But the ocean does not get more salty every year.” Scientists are not sure about the answer to this question.We know a lot about our world. But there are still many answers that we do not have, and we are curious.
