It is matter that was thought to be intimately involved with gases and combustion. In closed containers, burning extinguished when the air was saturated with phlogiston. Respiration in living organisms removed phlogiston as well. This material was used to explain the weight loss observed when something is burned. A problem with this theory arose when it was shown that all things do not lose weight when burned.
Gases can be a complicated system to explore, test, and understand.
Think about these questions as you perform the following activities. Set up a candle in the petri dish and light it. Let the candle burn for two or three minutes. Scientists of the 17th century believed that candles and wood weigh less after combustion because phlogiston was given off. Now blow out the candle and reweigh it and the dish.