The phlogiston theory once put forward the belief that there was a fire-like element called phlogiston inside combustible bodies which was released during combustion. When the substances burned, the phlogiston escaped leaving a solid ash called calx. This theory explained why the mass after burning fuels was less than the original mass.
Careful experiments that measured the masses of gasses showed that the phlogiston theory was wrong. This led to the discovery of oxygen. However, some scientists continued to believe in phlogiston. They thought this explained the heat and light given off in combustion reactions. Today we see this as evidence of the transfer of energy to the surroundings.