I first heard about the petermen in the book Alchemy of Air. I found the topic interesting, so I decided to learn more about it online. I discovered an article in the Firearms History, Technology & Development blog about the petermen.
My sources for my saltpeter page are salt.org's article on saltpeter, the Alchemy of Air, and Wikipedia on saltpeter.
My sources for my fixed nirogen page are Alchemy of Air, and Wikipedia on fixed nitrogen.
My sources for my guano islands page are Alchemy of Air, and Wikipedia on the guano islands.