When humans first learned about fire, it allowed them to cook food, get warm and be protected from wild animals, but people knew very little about the process of combustion till the Scientific Revolution. Progress was limited because people in these days considered measurement to be unimportant. By 1700, combustion was assumed to be the decomposition of a material into simpler substances. People saw burning substances emitting smoke and energy as heat and light. To account for this emission, scientists proposed a theory that combustion depended on the emission of a substance called phlogiston, which appeared as a combination of energy as heat and light while the material was burning which was not able to be detected beforehand. An extraordinary stupid theory called phlogiston which is a colorless, odorless, tasteless and weightless substance, became the new explanation of why flammable materials burned.
The phlogiston theory was used to explain many chemical observations of the day. For example, a lit candle under a glass jar burned until the surrounding air became saturated with phlogiston, at which time the flame died because the air inside could not absorb more phlogiston. There never was a widespread coherent theory of phlogiston. The word is based upon a Greek word used by Aristotle in his writings on matter. The Greeks thought that phlogiston was released by the combustion of flammable matter and the respiration of living organisms. These processes formed phlogisticated air, which was absorbed by plants. They said that charcoal was the residue that was left from a burning process, which was almost pure phlogiston. They thought that when burned with a metal’s calx (a powdery metallic oxide formed when an ore or mineral has been heated), it would be restored to its compound state. Thus rusting iron was slowly releasing phlogiston and so returning to its elemental state.
German chemist Johann Joachim Becher (1635–1682) brought the term “phlogiston” into use among European chemists in the middle 1600s, which he formulated in an attempt to explain such processes as rusting and combustion. Becher suggested metals and minerals were compounds which, when burned, released terra pinguis (fatty, flammable earth) into the air. This left behind the metal’s true form, the calx, composed of terra lapida (body – stony earth) and terra mercurialis (weight and color – mercurial earth). Becher believed substances to be composed of three earths, the vitrifiable, the mercurial, and the combustible. He supposed that when a substance burned, a combustible earth was liberated.
German chemist Georg Stahl (1660–1734) further articulated the phlogiston theory in the early 1700s and he renamed the Becher term “terra pinguis” to “phlogiston”. Phlogiston was the postulated substance of fire, the active principle of acids, and the driving force behind chemical reactions. According to this theory, the air has an ability to absorb only a limited amount of phlogiston. When the air becomes phlogisticated, it supports neither burning, nor breathing.
Daniel Rutherford (1749-1819) a Scottish physician, chemist and botanist is famous for his discovery of nitrogen gas and working with thermometers. In 1772, Rutherford trapped a mouse in a confined place and he observed that once the air ran out the mouse died. Rutherford then burnt a candle in the rest of the air. When the flame also died out, he burnt a piece of phosphorous in the container till it stopped burning. This air was then passed via a solution that absorbed the rest of the carbon dioxide. Rutherford had removed oxygen and carbon dioxide from this air mixture. He named the remaining, isolated gas as noxious air or phlogisticated air, which we call Nitrogen today. He believed that this gas was given out by the mouse while it was breathing, as when the air could hold no more phlogiston, the mouse died.
Joseph Priestley (1733-18-4) was an English chemist who is known for discovering oxygen gas and being the man behind the discovery of carbonated water. After Rutherford’s work, Joseph Priestley obtained a reddish powder when he heated mercury in the air. He assumed that the powder was mercury devoid of phlogiston. But when he heated the powder, an unexpected result occurred, where Metallic mercury along with a gas that allowed a candle to burn, formed. Following the phlogiston theory, he believed this gas that supports combustion to be “de-phlogisticated air.” Priestley conducted an experiment by focusing the Sun’s rays on a sample of mercuric oxide. He first tested it on mice, who surprised him by surviving quite a while entrapped with the air, and then on himself, writing that it was “five or six times better than common air for the purpose of respiration, inflammation, and, I believe, every other use of common atmospherical air.” In this oxygen gas a candle burned extra brightly and a mouse lived twice as long as it would in ordinary air.
In 1789, Antoine Lavoisier (1743-1794) a French scientist published a famous treatise on Chemistry, which amongst other things, demolished the then prevalent phlogiston theory of combustion. In place of phlogiston theory, Lavoisier proposed the first reasonably sensible scientific interpretation of heat and this became central to the 18th-century chemical revolution. Lavoisier’s caloric theory was accepted because it was inherently simpler than the phlogiston theory. The crudest version of this idea says that the phlogiston theory unnecessarily complicated things by postulating the existence of phlogiston which was an unobservable substance. The rationality of the phlogiston theory was very thin, and as stated in Occam’s razor, simpler is always better. Lavoisier arrived at a attractive new hypothesis about the composition of water, namely that it was a compound of oxygen and inflammable air, or hydrogen as it is now known, not an element as the ancient Greeks and phlogiston theorists had assumed.