Here is the story of rubber. From the earliest time it was common knowledge to the Peruvians(秘鲁人)that when a cut was made in the outside skin of a rubber tree, a white liquid like milk came out, and that from 【21】______ a sticky mass of rubber might be made. This rubber is 【22】______ when warm, so that it is possible to give it any 【23】______ . The Peruvians made the 【24】______ that it was very good for keeping out the wet. Then in the early 1800's, the Americans made use of it for the first time. First they made overshoes to 【25】______ their feet dry. Then came a certain Mr. Mackintosh, who made coats of cloth covered with natural rubber. From that day to this we have been coating cloth with rubber as Mr. Mackintosh 【26】______ , and our raincoats are still named after 【27】______ . These first rubber overshoes and raincoats were all soft and sticky 【28】______ summer, and hard and inelastic (没有弹性的)in the winter 【29】______ it was cold. 【30】______ the rubber we have today is soft and elastic, it is very strong 【31】______ in the warmer summer and the colder winter. This was made possible by a man 【32】______ Goodyear. After many 【33】______ , he found that nitric acid(NHO3) made the rubber 【34】______ better, but it is not hard and strong enough. Then strange thing 【35】______ . A friend of his, Nicholas Hayward, had the 【36】______ in his sleep that rubber might be made hard and strong if mixed 【37】______ sulphur and put in the sun. Goodyear put this idea to the test, and saw that it did have more or less the desired 【38】______ though somewhat less than more. The only effect it had was on the outside of the rubber. 【39】______ is common knowledge now that the way to make rubber hard and strong is by heating it with sulphur. It took Goodyear four more years to find this heating method. When 【40】______ he did it, he had nothing at all. Everything of the smallest value had been used to get money, even his sons' school-books. 【21】