vicar arrives in a panic, informing Holmes that Mortimer Tregennis has now died in the same way as his sister. The two men, along with Watson, rush to Mortimer's room, and find it foul and stuffy, even though the window has been opened; the servant who opened it is now ill. A lamp is lit, and smoking, on the table beside the dead man. Holmes finds gravel on an upstairs windowsill; he also scrapes some ashes out of the lamp and puts them in an envelope, leaving the rest for the local police to find. Holmes tells the police to contact him if they wish his help, but they never do. Holmes deduces that a poison, activated by combustion and affecting the cognitive functions, is the murder weapon; there were fires burning in both murder rooms, and people who entered them either felt ill or fainted, further suggesting a poisoned atmosphere. He tests his hypothesis by buying a lamp like the one in Tregennis's room, lighting it, and putting some of the collected "ashes" on the smoke guard. The smoke from this powder is so potent a poison that Holmes is immediately struck down. Watson is able to resist and drags Holmes out of the room just in time. Once recovered, Holmes explains that Mortimer Tregennis is the only one who could be guilty of the first crime. Based on footprints, the gravel on the windowsill, and other clues, Holmes further deduces that Dr. Sterndale was Mortimer's murderer. Dr. Sterndale arrives in answer to Holmes' summons, and confesses; he used the gravel to wake Mortimer, held him at gunpoint and forced him to breathe the poison. It comes from a plant called Radix pedis diaboli ("Devil’s-foot root" in Latin); when powdered and burned, the plant is extremely toxic, causing madness or death. Sterndale once described the powder to Mortimer Treginnis, who later stole some from Sterndale's collection of African curiosities, then murdered his siblings by throwing it on the fire just before he left. Mortimer had hoped Sterndale would be at sea before news reached Plymouth, but the symptoms of the victims, described in the vicar's telegram, showed Sterndale his poison had been used. Fearing a rural jury would never believe such a fantastic story, Sterndale "executed" Mortimer himself. The motive for the crime lay in the fact that Sterndale was passionately in love with Brenda Tregennis; however, he had