Long ago, an operation had usually to be done while the sick man could feel everything. The sick man had to be held down on a table by force while the doctors did their best for him. He could feel all the pain if his leg or arm was being cut off, and his fearful cries filled the room and the hearts of those. who watched. Soon after 1770, Joseph 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 Wells' 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.