what is the significance of Doctor Browns phosphorous experiment?

Dr. Brown shows Tita how phosphorous combined with oxygen can create a flame. He shares with her one of his grandmother’s old sayings that everyone has a box of matches inside them that can only be lit by oxygen and a candle. The oxygen, says Dr. Brown is a lover and the candle can be any other pleasure in life. He claims that finding the elements to ignite the matches within is a task that each person must complete in order to nourish his soul. If a person does not discover what ignites their matches then the matchbox dampens and the soul leaves the body in search of something to nourish it.

Tita begins to fear that her matches are too damp to ever be lit again but Dr. Brown tells her that there is a cure. He warns her to stay away from those who might try to extinguish her fire. As Dr. Brown describes his dead grandmother, Tita realizes that her ghost was the woman who gave her tea.