题目内容
Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brilliant flash of an explosion. A typical combustion process is the burning of gasoline in an automobile engine. The vaporized fuel is mixed with air, compressed in the engine"s cylinder, and ignited by a spark. As the fuel flame up, the heat produced flows into the adjacent layer of unburned fuel and ignites it. In this way a zone of fire spreads throughout the fuel mixture is called a combustion wave. The speed at which such a combustion wave travels through a fuel mixture is called the burning velocity of the mixture. The burning velocity of a gas such as methane quietly burning in air is only about one foot per second. By comparison, the burning velocity of more reactive combinations such as the rocket Fuels, hydrogen and fluorine, can be hundreds of feet per second. If the fuel flows at the same speed as the combustion wave, the result is a stationary flame, like the one in your kitchen gas burner. In the kitchen burner a jet of gas mixed with airflows from the opening in the head of the burner. If the velocity of the fuel mixture flowing from the opening is greater than its burning velocity, the flame blows out. In jet engines speeding through the air at 500 to 600 miles per hour, the engine"s flame is sometimes blown out by the blast of air entering the combustion chamber at high speeds. Jet pilots call this condition "flameout". Combustion can sometimes occur very slowly. A familiar example of slow combustion is the drying of ordinary oil-based paint. In this chemical reaction, called oxidation, the oxygen in the air reacts with the drying oil in the paint to provide a tough film. The linseed oil molecules link together, forming an insoluble coating. How can the chemical reaction involved in such a quiet process as the drying of paint also produce spectacular flames and explosions The main difference between the two is the temperature at which they occur. At lower temperatures the reaction must take place over a long time. The heat which is slowly produced is dissipated to the surroundings and does not speed up the reaction. When the heat produced by the low-temperature reaction is retained instead of being dissipated, the system breaks into flame. In a flame or explosion, the reactions are extremely fast. In many chemical processes, however, such a rapid oxidation process would be extremely destructive. Rocket fuels are more explosive than methane gas because of ______.
查看答案
搜索结果不匹配？点我反馈
更多问题
Today the study of language in our schools is somewhat confused. It is the most traditional of scholastic subjects being taught in a time when many of our traditions no longer fit our needs. You to whom these pages are addressed speak English and are therefore in a worse case than any other literate people. People pondering the origin of language for the first time usually arrive at the conclusion that it developed gradually as a system of conventionalised grunts, hisses, and cries and must have been a very simple affair in the beginning. But when we observe the language behavior of what we regard as primitive cultures, we find it strikingly elaborate and complicated. Stefansson, the explorer, said that "In order to get along reasonably well an Eskimo must have at the tip of his tongue a vocabulary of more than 10,000 words, much larger than the active vocabulary of an average businessman who speaks English. Moreover these Eskimo words are far more highly inflected than those of any of the well-known European languages, for a single noun can be spoken or written in several hundred different forms, each having a precise meaning different from that of any other. The forms of the verbs are even more numerous. The Eskimo language is, therefore, one of the most difficult in the world to learn, with the result that almost no traders or explorers have even tried to learn it. Consequently there has grown up, an intercourse between Eskimos and whites, a jargon similar to the pidgin English used in China, with a vocabulary of from 300 to 600 uninflected words, most of them derived from Eskimo but some derived from English, Danish, Spanish, Hawaiian and other languages. It is this jargon which is usually referred to by travelers as "the Eskimo language". And Professor Thalbitzer of Copenhagen, who did take the trouble to learn Eskimo, seems to endorse the explorer"s view when he writes: "The language is polysynthetic. The grammar is extremely rich in flexional forms, the conjugation of a common verb ending. For the declension of a noun there are 150 suffixes (for dual and plural, local cases, and possessive flexion). The derivative endings effective in the vocabulary and the construction of sentences or sentence-like words a mount to at least 250. Not withstanding all these constructive peculiarities, the grammatical and synthetic system is remarkably concise and, in its own way, logical." Some of the evidence about language in the passage is taken from the observations of ______.
The 1990s have been designated the Decade Against Drug Abuse by the United Nations. But, (1)_____ less than three years to go before the end of the decade, governments and health organizations (2)_____ that they have made (3)_____ progress in reducing drug, alcohol and tobacco abuse. Today, consumption of all these substances is increasingly steadily worldwide. (4)_____ every country now has problems with (5)_____ drugs. And the world is producing and consuming more alcohol and tobacco than ever. Between 1970 and 1990 beer production (6)_____ rose by over 80 per cent. And, (7)_____ the number of smokers keeps on (8)_____,by the second or third (9)_____ of the next century there could be 10 million deaths each year (10)_____ smoking related illnesses. Drugs are also a huge burden (11)_____ the world economy. In the United States, for example, it"s estimated that alcohol and illegal drug use costs the country tens of billions of dollars each year, mainly (12)_____ health care. When the cost of tobacco related illnesses is added, (13)_____ total more than doubles. Drugs are also closely (14)_____ crime. Many police forces no longer (15)_____ between illegal and legal drugs when fighting crime. In Australia, for example, experts (16)_____ that police in some parts of the country spend between 70 and 80 percent of their time dealing with alcohol-related incidents. One explanation for the increase in drug (17)_____ is simply that people have more money to spend. Tobacco and alcohol companies are now (18)_____ much more on developing countries to take (19)_____ of greater wealth there. And criminals involved in the illegal drug trade are following (20)_____, introducing drugs into countries where they were previously hardly use.
男性，23岁，学生，因选拔校运动员来体检，平素无自觉症状，每日体育锻炼。体检：BPl60／50mmHg，脉率60次／分，可触及水冲脉，毛细血管搏动征阳性，心尖部可闻及隆隆样舒张早期杂音，主动脉瓣第二听诊区可闻及叹气样舒张期杂音。最可能的诊断是
The 1990s have been designated the Decade Against Drug Abuse by the United Nations. But, (1)_____ less than three years to go before the end of the decade, governments and health organizations (2)_____ that they have made (3)_____ progress in reducing drug, alcohol and tobacco abuse. Today, consumption of all these substances is increasingly steadily worldwide. (4)_____ every country now has problems with (5)_____ drugs. And the world is producing and consuming more alcohol and tobacco than ever. Between 1970 and 1990 beer production (6)_____ rose by over 80 per cent. And, (7)_____ the number of smokers keeps on (8)_____,by the second or third (9)_____ of the next century there could be 10 million deaths each year (10)_____ smoking related illnesses. Drugs are also a huge burden (11)_____ the world economy. In the United States, for example, it"s estimated that alcohol and illegal drug use costs the country tens of billions of dollars each year, mainly (12)_____ health care. When the cost of tobacco related illnesses is added, (13)_____ total more than doubles. Drugs are also closely (14)_____ crime. Many police forces no longer (15)_____ between illegal and legal drugs when fighting crime. In Australia, for example, experts (16)_____ that police in some parts of the country spend between 70 and 80 percent of their time dealing with alcohol-related incidents. One explanation for the increase in drug (17)_____ is simply that people have more money to spend. Tobacco and alcohol companies are now (18)_____ much more on developing countries to take (19)_____ of greater wealth there. And criminals involved in the illegal drug trade are following (20)_____, introducing drugs into countries where they were previously hardly use.
微信支付 
支付宝支付