Laval University From the SelectedWorks of Fathi Habashi December, 2019 The aN ture of Fire Fathi Habashi Available at: https://works.bepress.com/fathi_habashi/588/ The Nature of Fire Introduction Fire remained a mystery for about two thousand years. Early people were much more intimately acquainted with fire than they are in the all-electric homes of today where fire is rarely seen. To the ancient Greeks fire was a magical element stolen from the gods and given to mankind by Prometheus. Some civilizations worshipped fire just as others worshipped the sun. In the early days of his evolution, man did not know how fire worked, but used it for hundreds of centuries. He was familiar with the volcano, lightning, forest fires, etc., but the mysterious flame, the similarity of its effects with that of the sun, its intimate connection with light, its terrible and yet genial power, easily account for the reverence in which it was held in ancient times. Fire was included in the four elements according to the Greek philosopher Empedocles about 440 BC, or earlier in the writings of the Persian prophet Zarathustra (630–553 BC) as a component of destruction as well as its utility in the cooking of foods. The Zoroastrians still keep fire burning in their temples since ancient times while modern Christians light candles in churches. Fire has its origin in the East even before Zarathustra. The Cambodian four elements is said to be described in the Sanskrit books and is well known to the people. Metallurgists of the Middle Ages like Vannoccio Biringuiccio (1480-1539) used fire to melt ores and cast metals and did not bother about its nature. It was in the middle of the 17th century that fire was the problem of the day - - what was its nature. The Four Elements A Zarathustra’s priest keeping fire burning in temple Phlogiston theory It was the alchemist Johann Joachim Becher (1635-1682) who first attempted to solve this problem. He was born at Speyer in Germany and was for a time in England. He published Physica Subterranea in 1669 and introduced the idea of an inflammable earth (terra pinguis) to explain combustibility, which was extended later by his student Georg Ernst Stahl (1659-1734). Johann Joachim Becher Georg Ernst Stahl A fire was a process by which something escaped from the burning substance such as wood. Burning was interpreted as separation of matter into the basic elements earth, water, air, and fire. The latter three went up in smoke and flame called phlogiston (phlox in Greek means flame) leaving the earth component behind as ashes or calx. This can be represented as: Heat Wood → Phlogiston + Calx Substances varied in their phlogiston content. Those with a high content burned readily and left little calx. Those with little phlogiston burned only with difficulty and left much calx. Since charcoal burned almost completely, leaving very little calx, it had been considered to be nearly pure phlogiston. On the other hand, when metals burned in air they left so much calx (oxide), hence they had been considered to be poor in phlogiston: Metal Phlogiston + Calx It was therefore,⟶ argued that if phlogiston were added to the calx, the metal could be obtained back, which actually happened: when charcoal (pure phlogiston) was heated with a calx (an ore), the metal was obtained: Ore + Charcoal Metal (oxide) Stahl further indicated ⟶that the air (gas) which escaped during the smelting process (CO2) was phlogisticated air, i.e., air saturated with phlogiston and consequently, it would no longer support combustion, nor was it fit to breathe. He also thought that on calcination of limestone, phlogiston is absorbed from the fire and this phlogiston was regarded as the cause of making the lime so vigorous in its reaction with water: Limestone + Φ → Caustic lime Cavendish The phlogiston was never isolated. It was Henry Cavendish (1731-1810) who determined to try. He noted in the work of Robert Boyle (1627-1691) that a flammable air was evolved when iron was treated with sulfuric acid. In 1766 he repeated Boyle’s experiment, extended it on zinc and tin, using sulfuric and hydrochloric acids, and collected the air formed in these experiments in a separate bladder. He noted that the flammable air produced from different acids by different metals was identical burning with a flame, and that its density was far less than that of atmospheric air. He called this gas “inflammable air” (hydrogen). He interpreted his experiments in terms of the phlogiston theory. He came to the erroneous conclusion that the gas came from the metal and not from the acid. He thought he had isolated the phlogiston and published his findings before the learned men of the Royal Society. Cavendish Lavoisier Priestley Lavoisier and Priestley By the mid-18th century laboratory apparatus had become more sophisticated. One example was the collection of gases. At first gases were collected in bladders that were not so 'air-tight' and made it impossible to obtain measurements such as volume and weight. Researches began to collect gases over water using a pneumatic trough, a bell jar and a specially shaped retort flask. Soon mercury replaced the water so that soluble gases could be collected. Gases could now be accurately measured and tested. Many famous scientists turned their attention to the study of gases. Soon after the time of his discovery of dephlogisticated air - - air that sustains respiration (oxygen), the English chemist Joseph Priestley (1733-1804) met Antoine Laurent Lavoisier (1743-1794) in Paris in 1774 and told him of his discovery. Lavoisier, however, had invented a very accurate balance and was able to confirm that when inflammable air (hydrogen) reacted with dephlogisticated air (oxygen) to give water, there was no mass present except that the reactants and products of the reaction. In 1775 he argued that since the mass of the reactants equals the mass of the products, in the reactions he investigated, there was no mass that could account for phlogiston. Phlogiston therefore could not exist. Lavoisier was able to explain combustion as an oxidation process, thus directing the final blow to the phlogiston theory. It was Lavoisier who gave the gas its name from the Greek words Oxys = acid, and genēs = I beget, because many substances like carbon or sulfur burn in the gas to give an acid. Epilogue [1] Fire is the phenomenon of combustion [2] Acids are not all formed from oxygen as Lavoisier had claimed because many acids like HCl and HF do not contain oxygen. In German language for example, oxygen is called “Suaerstoff”, i.e., acidic substance. [3] The phlogiston in modern language is the electron. The oxidation of a metal would be represented by: 2+ - M M + 2e - 2- ½ O2 + 2e O M2+ ⟶+ O2- MO ⟶ where M is a divalent⟶ metal and MO will be the calx. Similarly, the dissolution of a metal in an acid can be represented in modern terms as follows: 2+ - M M + 2e + - 2H + 2e H2 ⟶ ⟶ [4] Cavendish is the discoverer of hydrogen. References F. Habashi, "Zoroaster and the Theory of Four Elements", Bull. Hist. Chem. 25 (2), 109–115 (2000) F. Habashi, “Cambodia’s Four Elements,” Bull. Hist. Chem. 29 (2), 97–98 (2004) F. Habashi, “Phlogiston and Modern Chemistry,” Bull. Hist. Chem. 31 (1), 31 (2006) F. Habashi, “The Theory of Four Elements. Water, Air, Earth, and Fire,” pp. 15–20 in Proceedings First International Symposium on Aqua Science, Water Resource, and Innovation Development of Countryside, Sakuraza, Sakawa, Kochi Prefecture, Japan, November 26–30, 2007 .