The Blackthorn Key

Christopher wakes up to Tom urgently knocking on the shop's front door. Tom tries to tell Christopher about the latest murder, but Christopher explains that he already knows. Tom believes local gossip that the murderer is part of a cult participating in human sacrifices and worries that the weather is a bad omen. 

Tom has brought Christopher an apple pie from his father's bakery for his birthday. After they eat it, Christopher shows Tom the antimony cube. He explains that the circles on the top show the universe. Tom is surprised to find that the Sun is at the center and shows him the book that accompanied the cube—Systema Cosmicum by Galileo Galilei—to prove it. Christopher shows him that each symbol on the sides represents a planet, but Mercury is missing. Where it should be, there is a black hole. Christopher believes that this is the keyhole and that the missing symbol will help them find the key. He looks in the quicksilver jar for the key, because quicksilver's true name is mercury, but sees nothing. Tom points out that the keyhole is round and that round keys do not exist. Christopher realizes that they must pour the quicksilver into the hole; when they do, the cube opens.