The writeup in Mayr's book "The Growth of Biological Thought" is really enjoyable. Miescher started by working with pus collected from hospital patients, but quickly moved to Rhine salmon "because their sperm is filled with the substance". His work was so surprising that his advisor Hoppe-Seylor (a well-known biologist for many other things) redid all his experiments. It's actually one of the great examples of protein bias- they couldn't understand why the molecule they purified didn't contain sulfur (DNA doesnt' have sulfur, while many proteins do).