What is phlostigen?

Answer 1

Phlostigen was the initial name for the gas: Oxygen

Phlostigen was believed to be the element that made things burn. It has since been thoroughly debunked by numerous experiments

When they first created a vacuum, they called it "De-phlogisticated Air" - as what they assumed was that all of the "Phlostigen" had been removed and that was why a flame would not burn in the vacuum.