Because qualities could vary, it was possible to change lead into gold. Aristotle was the teacher of the alchemists and his ideas dominated chemistry for nearly two thousand years after his death. Phlogiston explained how air initially supports combustion and then does not. It also addressed some of the shortcoming of Aristotle's theory, particularly its vague notions of chemical change. Charcoal leaves little residue upon burning because it is nearly pure phlogiston.
Mice die in airtight space because air saturates with phlogiston.
When heated, metals are restored because phlogiston transferred from charcoal to calx. Stahl attributed the weight increase to air entering the metal to fill the vacuum left after phlogiston escaped. This suggested that combustion, fermentation and metabolism all have a common chemical basis. Black quantitatively explained all the changes in weight for these processes without needing to invoke the mysterious phlogiston. Black's work laid the foundation for modern chemistry and the arrival of Lavoisier.