For much of the eighteenth century chemists believed that when a substance burned in air a substance called phlogiston was released.  It was thought that a flame eventually went out in a closed container because the air became saturated with phlogiston.

In August 1774, Priestley heated a red solid powder (mercury oxide) by using a lens to focus sunlight onto it.  He observed that a silvery liquid (mercury) was formed as well as some unknown gas.  On studying this gas he found that a candle burned much more brightly in it than in ordinary air.  Priestley had discovered oxygen but he called the gas ‘dephlogisticated air.’  He believed that a candle burned less brightly in ordinary air because air contained some phlogiston which was thought to extinguish flames.

In October of that year Priestley visited Lavoisier in Paris and told him about his experiment.  Lavoisier immediately began an experiment of his own in which he heated liquid mercury in a sealed, air-filled flask in a furnace.  Small red specks appeared on the surface of the mercury and the amount of this red substance slowly increased.  After 12 days Lavoisier extinguished the furnace and examined the gas left in the flask.  He found that it no longer supported combustion.  This made sense if large quantities of phlogiston had been released in the process.  However when he weighed the mercury plus red substance he found that its total mass was greater than the mass of the mercury with which he had started.  If the phlogiston theory were correct, the mass of the mercury should have decreased as it released phlogiston.  Some defenders of the phlogiston theory went so far as to conclude that phlogiston had negative mass but Lavoisier correctly concluded that, on heating in air, rather than releasing phlogiston, mercury combines with a part of the air and thus its mass increases.

Lavoisier next heated the red solid – mercury oxide.  He obtained mercury and a gas that supported combustion much better than air, just as Priestley had found.  Clearly this gas was one of the components of air.  Lavoisier correctly recognized it as an element and called it oxygen.  Thus, Priestley and Laviosier had discovered that the reaction of mercury with oxygen produces mercury oxide, and mercury oxide decomposes back to the elements mercury and oxygen.  These experiments spelled the end of the phlogiston theory and clearly showed for the first time that air was a mixture containing the element oxygen.