Of course, we all know that Griffin discovered the process through which he became invisible. Unfortunately for the mad scientist community, Mr. Marvel ended up with Griffin's journals. The last paragraph of The Invisible Man:
"So [Marvel] lapses into a dream, the undying wonderful dream of his life. And though Kemp has fished unceasingly, and Adye has questioned closely, no human being save [Marvel] knows those books are there, with the subtle secret of invisibility and a dozen other strange secrets written therein. And none other will know of them until he dies."
So, let's pretend Mr. Marvel has died, your a mad scientist, and you've acquired Kemp's books. What other "strange secrets" are in the books?
I would add astral projection and some form of clairvoyance for contemporary flavor, along with a limited but specific knowledge of alchemy, allowing for the periodic artificial creation of small amounts of gold or jewels.