阅读理解

　　Doctors tell us not to smoke, but a lot of people smoke tobacco every day．Long ago, no one in Europe smoked; the tobacco plant was unknown there．It grew in America, and Christopher Columbus found it there．Later, he returned to Europe and told everyone about tobacco．He said that the American Indians often smoked it．Sir Walter Raleigh was a famous man in the days of Queen Elizabeth I．He traveled widely and learnt to smoke．Then he returned to England, but he did not stop smoking．He used to smoke two pieces every day secretly in his room．When someone came in, he quickly hid his pipe; but one day, he was not quick enough．A man came in and found clouds of smoke in the room．More smoke was coming from Raleigh's mouth, so the man got some water and threw it over Raleigh．He believed that Raleigh was burning．He ran out of the house and told everybody about it．After this, smoking was not a secret．Raleigh told Queen Elizabeth I about it．He added that he could weigh his smoke．She did not believe this, and she asked him to explain．．

　　“First,”he said,“I shall weigh the tobacco．Then I shall put it in my pipe and smoke it, then I shall weigh the ashes which remain．I can take them out of the pipe and weigh them on a balance．There will be a great difference between the two weights．The tobacco must be heavier than the ashes．The difference between them must be the weight of the smoke．

　　It was a good explanation：but Raleigh was wrong．When his tobacco burned, it did this, but the oxygen was not in the tobacco at the beginning．It was added when Raleigh smoked his pipe．So the smoke was heavier than the difference between the two weights．The weight of the oxygen was added．