Nitre is the British spelling of the word niter, which is the mineral form of potassium nitrate. It's common to find it in the crystallized form in caves. It is an irritant, and breathing it can cause coughing and sneezing.
But in Edgar Allan Poe's story "The Cask of Amontillado," the niter has a symbolic meaning, as well. As Fortunato is led deeper into the catacombs of Montresor's family, the niter increases. Montresor has premeditated the murder of Fortunato, and he plays with his victim the way a cat sometimes plays with its prey before killing it. Montresor is pretending to be concerned with Fortunato's health when the truth is that he has absolutely no regard for Fortunato. The symbolic meaning of the niter is that Fortunato is being led further into Montresor's web, just like the web-like niter on the walls is increasing.
From the beginning, Montresor plays with his victim. First, he plants the seed of rivalry to taunt Fortunato into following him into the catacombs. He says...
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