20 - Young Diana by Marie Corelli

Poor Diana May finds herself at a crossroads; her fiancé has abruptly left her, her parents view her as a burden, and her brilliance goes unnoticed. Feeling unloved and unappreciated, she’s on the brink of despair—until she takes a bold step to reclaim her life. It all starts with a mysterious ad for an assistant to an eccentric scientist hiding away in a secret laboratory in Switzerland... - Summary by Newgatenovelist