Phlogiston Theory - Challenge and Demise

Challenge and Demise

Eventually, quantitative experiments revealed problems, including the fact that some metals, such as magnesium, gained weight when they burned, even though they were supposed to have lost phlogiston. Mikhail Lomonosov attempted to repeat Robert Boyle's celebrated experiment in 1753 and concluded that the phlogiston theory was false. He wrote in his diary:

"Today I made an experiment in hermetic glass vessels in order to determine whether the mass of metals increases from the action of pure heat. The experiment demonstrated that the famous Robert Boyle was deluded, for without access of air from outside, the mass of the burnt metal remains the same."

Some phlogiston proponents explained this by concluding that phlogiston had negative weight; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it was lighter than air. However, a more detailed analysis based on the Archimedean principle and the densities of magnesium and its combustion product shows that just being lighter than air cannot account for the increase in mass.

During the eighteenth century, as it became clear that metals gained weight when they were oxidized, phlogiston was increasingly regarded as a principle rather than a material substance. By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked to hydrogen. Joseph Priestley, for example, in referring to the reaction of steam on iron, whilst fully acknowledging that the iron gains weight as it binds with oxygen to form a calx, iron oxide, iron also loses “the basis of inflammable air (hydrogen), and this is the substance or principle, to which we give the name phlogiston.” Following Lavoisier’s description of oxygen as the oxidizing principle (hence the name oxygen: oxus = sharp, acid; geneo = generate, produce), Priestley described phlogiston as the alkaline principle.

Phlogiston remained the dominant theory until Antoine-Laurent Lavoisier showed that combustion requires a gas that has weight (oxygen) and could be measured by means of weighing closed vessels. The use of closed vessels also negated the buoyancy which had disguised the weight of the gases of combustion. These observations solved the weight paradox and set the stage for the new caloric theory of combustion.