Before I start this I'd like to say this theory has been disproven, phlogiston is not real. Phlogiston is an old theory used to explain heat back in the 1600s and has been disproved. This is more of a little history lesson then anything and I just think it's a funny little theory.
The theory of phlogiston started in 1667 with a scientist named Johann Becher. Phlogiston is an attempt to explain heat. It was believed that phlogiston was an element that existed inside of anything that can burn or elplode (aka combustible bodies). When these combustible bodies did explode the phlogiston was released into the air. Substances with phlogiston in them were refered to as "phlogisticated" and then "deplogisticated" when burned. Testing the theory of phlogiston in enclosed spaces soon revealed that combustion would not last long in closed areas. This led scientists to believe the air had the ability to absorb only a finite amount of phlogiston. Once the phlogiston limit was reached the air could not support life or combustion. Since air filled with phlogiston could not support life it was believed that respiration existed to remove phlogiston from the body. Therefore phlogiston was believed to be anti-oxygen. In 1753 experiments revealed problems with the phlogiston theory. Some metals gained mass after being burned even though they lost phlogiston. Some scientists concluded that phlogiston had negative mass while others tried to explain that it is simple lighter then air. Unfortunately neither of these explinations really covered things and the phlogiston theory was eventually replaced with the oxygen theory that is accepted today.