The Royal society has a new website making freely available a selection of classic papers from the history of science.
I am just looking at an Isaac Newton’s letter about light and colors sent to the editor of Cambridge University Press 😉 in February 1671/72. It describes some Newton’s basic but fun diffraction experiments and his conclusions. … James Joule described the equivalence of heat and mechanical work in 1849 and the paper almost looks like a newly written preprint in LaTeX. … James Clerk Maxwell gives us his theory of electrodynamics in 1865. This is a fun 55-page preprint with lots of equations.
… and so on.