Newton's Cannon by J Gregory Keyes

I recently read Newton's Cannon, which is an alternate history story where the magical aspects of alchemy are real. The main characters are Ben Franklin and an Adrienne de Mornay de Montchevreuil, both 14 years old at the time.Isaac Newton had cracked the code for making alchemy years before, and now nations are using alchemy to build weapons of mass destruction.
The book has a lot more adult themes than I was expecting for a book about 14-year-olds, involving war, prostitution, and rape, which I would have preferred the book not to get into. What the book does best is demonstrate to the reader the mindsets of 19th century people, and it also does well at demonstrating the scientific method. But it gets silly near the end when one of the villains shows up, suddenly with a robot hand.
Has anyone else here read it? What did you think?