For thousands of years, people thought of glass as something beautiful to look at. Only in large glass windows. Glass bottles and jars that hold food and drink allow us to see the contents. Glass is used to make eyeglasses, microscopes, telescopes, and many other extremely useful and necessary things.

    Until the Second World War, most of the glass used for optical (光学的) instruments was imported from Europe. However, during the war Americans could not get European glass, and they were forced to make their own. Therefore, new kinds of glass were developed that had been previously unknown. These new effects were achieved by mixing other chemical elements with the sand. Some of the new glass is very strong and can resist many kinds of shocks. Legend (传说) has it that a kind of very hard glass was invented by a Roman who showed his discovery to the Emperor. When the Emperor saw the glass he feared that it would become more valuable than gold and sliver, making his treasure worthless. Therefore, he had the glass-maker killed, and the secret was not discovered again for hundreds of years.

    In the present century, safety glass was invented for use in modem cars and planes. Safety glass is made by placing a layer of plastic between two layers of plate glass. When the outside layer of glass is broken, the pieces do not scatter (散开) and injure people. Some glass of the type is strong enough to resist bullets (子弹).

    Although nowadays plastics have replaced glass under conditions where glass might be easily broken, there are new uses being developed, for the greatest advantage of glass is that its component (组成的) parts are inexpensive and can be found all over the world.