What did phosphorus have to do with the mystery?

In Sir Arthur Conan Doyle's The Hound of the Baskervilles, Sherlock Holmes and Dr. John Watson work to solve the mystery surrounding the death of a man called Sir Charles Baskerville, and save his relative Sir Henry from the same fate. In the second chapter, aptly titled "The Curse of the Baskervilles," Dr. Mortimer shows Holmes and Watson a manuscript which tells the tale of Sir Charles's villainous ancestor Hugo Baskerville and the creature that killed him hundreds of years ago, referred to as the Hound of the Baskervilles. The hound is described as such:

...standing over Hugo, and plucking at his throat, there stood a foul thing, a great, black beast, shaped like a hound, yet larger than any hound that ever mortal eye has rested upon.

The last paragraph of the manuscript says that the beast has "plagued the family so sorely ever since," and many of the Baskervilles have died "sudden, bloody, and mysterious" deaths. While Sir Charles took the legend seriously, and Dr. Mortimer has good reason to believe in the possibility of the mythical hound being responsible for Charles's death, Watson and Holmes are able to get down to the truth and debunk this supernatural theory by the end of the story.
Sir Henry, Holmes, Watson, and Lestrade do eventually have an encounter with "an enormous coal-black hound," out on the moors. Watson tells us that the beast was "not such a hound as mortal eyes have ever seen," and that:

Fire burst from its open mouth, its eyes glowed with a smouldering glare, its muzzle and hackles and dewlap were outlined in flickering flame.

Luckily, Holmes is able to shoot and kill the hound before it gets the chance to hurt anyone or literally scare Sir Henry to death. Upon inspection of the the animal's body, they can see that the hound seemed to be on fire because someone covered it in a "cunning preparation" of phosphorus, an element that glows when it is exposed to oxygen. And so, this hound is not an immortal, devilish beast, but an uncommonly large dog that Stapleton, the true villain, made to look impossibly, supernaturally terrifying in order to use the legend to his advantage.