Salazar's Study: Book One

Harry finds Salazar Slytherin's old study, and apon talking to the portrait of the old Hogwarts founder, Harry learns a few new techniques when it comes to using magic. He finds one spell which allows him to go into spirit form, unseen by the world, and spy on whomever he likes.
He decides to spy on his potions professor, and what he finds shocks him to his core.

Warnings: mild swearing.
(roughly 6,815 words)