It's been a while since I've played that part, but I believe this is how it goes. First, you have to solve the astonomy puzzle to remove the seal from the wall in that really big room. Next, you need to find the Mandragora in the Torture Chamber, and then place it at the base of the wall where the seal was. Then, go back for the nitro, and...this is very important....make it back to that big room without getting hit or jumping, otherwise you'll die. Once in the room, place it with the mandragora, an explosive reaction will accure, clearing away the wall, and allowing you to place the special jewel which will turn on the elevator to the next level. Be ready when you do this, a big suprise is in store after the elevator is activated.