August 1, 1774, polymath Joseph Priestley discovers "dephlogisticated air"

Experimento de Joseph Priestley

Joseph Priestley (1732-1804) was a scientist, theologian, dissident clergyman, philosopher, educator and political theorist who published more than 150 works and who on this date identified a gas that he initially called "dephlogisticated air" which, in contact with him, the candles burned brighter. Priestley believed that this was air from which the phlogiston had been removed.

The discovery of this gas was also attributed to the chemist Carl Wilhelm Scheele who called it "oxygen air" and to the chemist, biologist and economist Antoine-Laurent de Lavoisier who called it "oxygen" and refuted the phlogiston theory in the 18th century.

Lavoisier, known for his studies on the oxidation of bodies or the law of conservation of mass, showed that combustion was the result of a relatively fast and exothermic chemical reaction. Lavoisier's studies relieved the phlogiston theory to an obsolete place in the world of science.

Either way, Priestley was one of the first scientists to isolate oxygen in gaseous form and the first to recognize its critical role for living organisms.

What is phlogiston?

Phlogiston is the scientific theory of 1667, already refuted, that sought to provide an explanation for the combustion process and was postulated by the physician and alchemist/chemist George Ernst Sthal in the early eighteenth century, based on the previous work of his mentor Johann J. Becher.

Stahl assumed that heat can occur in two different ways: free and in combination. This second form is what he referred to as phlogiston, which means 'inflammable' in Greek, and which is inherent in all combustible bodies and, therefore, combustion would be the passage from this form of combined fire to the form free, where it could be appreciated with the senses. The resulting remains are unable to burn again because they have already used up all the phlogiston in the body. For example, Stahl believed that the metal was composed of phlogiston and lime, and when combustion was caused, only the lime remained; but if you added a substance that was rich in phlogiston (such as carbon) you could return to the original state.

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