tailieunhanh - The complete guide to the toefl IBT reading part 3

Đọc để hiểu là một kỹ năng rất khó, nó không chỉ đòi hỏi ta có một vốn từ rộng mà nó còn đòi hỏi ta phải có một văn phạm thật chắc chắn để có thể hiểu nghĩa của từng câu. Tài liệu sẽ giúp ta luyện đọc một cách dễ dàng. | Lesson 1 Factual and Negative Factual Questions 31 Warren. When the general died at the battle of Breeds Hill Revere identified him by examining his teeth. This was the first known case of identification by means of dental records. Today of course dental records are commonly used as a means of identification. 3 By the early nineteenth century most communities in the United States had one or more dentists although not all of them had much training. In 1840 dentistry became a true profession. That s when the first dental school was opened in Baltimore Maryland. The course lasted sixteen weeks. There were only five students in the first class and only two of these graduated. This school has recently been restored as a museum of dental history. 4 The most common cure for toothaches was simply to pull out the offending tooth. Many dentists advertised painless extraction methods in the newspapers of the times. Negative Spray and Vitalized Air were two methods of reducing pain. It is not known today how these mysterious processes worked but it is unlikely that they worked very well. In 1844 dentist Horace Wills had patients inhale the gas nitrous oxide just before having a tooth pulled. The tooth could then be painlessly removed. Nitrous oxide mixed with oxygen is still used today to reduce pain during dental procedures. Two years later in 1846 the dentist William Morton gave a public demonstration of the effects of ether which could be used as anesthesia not only during dental operations but for surgeries of all kinds. 5 Another important development in dentistry was the discovery of X rays in 1895. X rays allow dentists to look inside teeth to discover defects. Early decay impacted teeth abscesses and bone loss are all things that dental X rays reveal. 6 The first dental drills appeared in the 1870 s. They were powered by foot pedals like the sewing machines of the time. Drills were given electric power in the late 1890 s. These power drills which were at first called .