At the time of Leeuwenhoek the microscope was not well developed, but with it he discovered bacteria. The study of these tiny forms of life which looked like specks to him was not practical until more than 150 years later, when microscopes were much improved. However, the discovery led some medical men at the time to think that contagious diseases were due to germs passed from the sick to the well.
Dr. Edward Jenner, a young English physician, overheard a milkmaid say that she was not afraid of smallpox because she had just recovered from an attack of cowpox. This gave Jenner the clue, and in 1796 he proved that a person vaccinated with cowpox germs is quite certain to escape from getting smallpox. At this time smallpox was so common that about only one person in a hundred escaped.
In Aristotles days, it was the common belief that air caused foods to spoil. People believed that this was true until the seventeenth century. In 1800, Napoleon offered a prize for the successful invention of a container which would keep foods from spoiling in war times. The prize was won by Francois Appert, who had worked all his life on this problem. He packed foods in glass or china jars, poured in enough water to cover the food, corked and sealed them, then placed the jars in a container of water which was gradually heated to the boiling point. He thought, however, that it was air that caused the foods to spoil. It was not until fifty years later that Louis Pasteur proved that it was not air that spoiled foods, but tiny living organisms that float about in the air. The first tin-can container was made in 1807 by Peter Durand. The canning industry in the United States began in 1819 when salmon, lobster, and oysters were first canned.
【英语六级考试快速阅读提高练习11】相关文章:
最新
2016-10-18
2016-10-11
2016-10-11
2016-10-08
2016-09-30
2016-09-30