听力原文：W: Excuse me, I've been using this old book for a research project and I notice that a lot of pages are turning brown and becoming brittle. M: Yes. Unfortunately, that's a common problem with books made from wood pulp. W: I suppose that to make paper from wood you have to add a lot of chemicals and acids to make it turn white. M: Exactly, and it's the acid that eventually eats away the paper: W: Oh, that actually makes sense, but this book's not even 75 years old and I've seen books in museum that are hundreds of years old and they're in free condition. M: Well, you see, books have been made from wood pulp only since the 1850s, before that they were made from materials mostly animals' skins, and no chemicals were added. W: It's a shame those older wood pulp books are going to fall apart some day. Is there anything that can be done to preserve them? M: Eh, currently the only way to stop the books from decaying is to remove the binding and treat each page individually to remove the acid. W: That doesn't sound very economical. M: No, it isn't. It's not practical to treat a large number of books with this process, so we only try to rescue the most valuable edition books in our collection. W: Well, thanks for the explanation. I'd better get back to my project. M: Good luck and I hope the old book will hold long enough for you to finish it. (23)