However, there are some strong clues that suggest the landlady uses carbon monoxide poisoning to kill her guests. Here's why:
* The Smell: Billy notices a "peculiar, sweetish smell" in the house, which could be associated with the odorless and colorless carbon monoxide.
* The Gas Fire: The landlady keeps a "gas fire" constantly lit in the sitting room, which could be a source of carbon monoxide if not properly vented.
* The "Sleeping" Guests: Billy sees stuffed animals that look remarkably like her previous guests, implying they died in their sleep. This is consistent with the effect of carbon monoxide poisoning, which can cause drowsiness and eventually death.
* The Lack of Other Signs: There are no signs of struggle or obvious wounds, making poisoning a more likely cause of death than any other method.
Ultimately, the specific poison remains ambiguous, adding to the unsettling nature of the story.