20、No one is glad to hear that his body has to be cut open by a surgeon and part of it taken out. Today, however, we needn't worry about feeling pain during the operation. The sick person falls into a kind of sleep, and when he awakes, the operation is finished. But these happy conditions are fairly new. It is not many years since a man who had to have operation felt all its pain.
Long ago, operation had usually to be done while the sick man could feel everything. Soon after 1770, Josept Priestley discovered a gas which is now called "laughing gas". Laughing gas became known in America. Young men and women went to parties to try it. Most of them spent their time laughing, but one man at a party, Horace Wells, noticed that people didn't seem to feel pain when they were using this gas. He decided to make an experiment on himself. He asked a friend to help him.
Wells took some of the gas, and his friend pulled out one of Well's teeth. Wells felt no pain at all.
As he didn't know enough about laughing gas, he gave a man less gas than he should have. The man cried out with pain when his tooth was being pulled out.
Wells tried again, but this time he gave too much of the gas, and the man died. Wells never forgot this terrible event.