阅读理解

　　When you see or hear the word ‘glass’, most of you will think first of what is used for windows and bottles, which is easily broken．But great changes have taken place in the making of glass．Now it can be made as hard as steel or as soft as silk, as light as paper or as heavy as iron．

　　We don’t know just when or where man first started making glass, but the oldest man-made pieces of it have been found in the Middle East．It was soon discovered that many different kinds of glass could be made by adding different kinds of chemicals．

　　As centuries passed, people could make many useful things of glass, and almost everybody came to make some use of it．Not long ago, plastics, metal and paper began to take the place of many glass bottles, drinking glasses, containers, and so on．So glassmakers realized they needed to improve their products and find new uses for it．It is often said that necessity is the mother of invention．This has proved to be true, because they are now making glass that will bend, stretch, resist great heat and pressure, and do many other useful and interesting things．Today there are over one hundred thousand methods for the making of glass．

　　There is an old saying,“People who live in glass houses shouldn’t throw stones．”But now it is safe to live in a glass house, because we have a type of glass that can hardly be broken．It is so strong that it is being used for the wall of large buildings．The walls can be of any colour and they need no care．

　　Another type of glass becomes stronger as the pressure on it increases．It can even be used in space travel．The heads of missiles(导弹)are made of a type of glass．

　　So glass is proving to be the most valuable in many uses．We can hardly do without it in the modern world．

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