英语时差：旧书为什么变黄（上）

Open an older book and you may find the paper yellowed around the edges. Even older books fall apart as you turn the pages. But you might be surprised to learn that the pages in some of the oldest books are still white and turn easily without cracking. During the last 500 years, the quantity of paper being produced has increased dramatically, but the chemical composition of modern paper causes it to turn yellow and crumble faster. Paper can be made from almost any form of cellulose that can be shredded into tiny fibers and mixed into a watery pulp. Since cellulose is found in all plants, most plant matter can be used to make paper. In the pulp, the cellulose fibers are partly dissolved so that when the sheet of paper is dried, the molecules of different fibers bond to each other.