Check a really good science library. If you don't know where one is, ask your local librarian, or even better go to a local college and check with the chemistry or physics departments; if there's one around, they'll probably know where it is (if it's a large enough school, they may even have one).
If you don't have a science library nearby, try interlibrary loan. Generally you can get copies of old journal articles for the cost of making the copies (sometimes even free). You'll need more information than just "Proceedings of the Royal Society, London, 1870" to get an article, though. Like: title, author, page numbers, all that.