Meet oxygen I: history

As it often is with science, it is unclear as to who first discovered oxygen. http://nautilus.fis.uc.pt/st2.5/scenes-e/elem/ claims that 13^th century Chinese already knew that what we now call oxygen was one of the constituents of water, and that the role of the air in the combustion was observed by Leonard da Vinci (1452-1519) and in again 1669 by John Mayow who stated that the spiritus nitro-aereus (oxygen) caused a mass increase in the metals when heated up. Determining the oxygen content in the air, he proved that it was consumed during the combustion and during the breathing of the animals, and that the two processes had the same purpose. In 1678 the oxygen was extracted from saltpetre by Borch; in 1731, from the same substance by Hales.

However, most other sources begin the history of oxygen between 1771 and 1773, when Carl Wilhelm Scheele discovered that red-hot manganese oxide (MnO₂) produces a gas. He called the gas "fire air" because of the brilliant sparks it produced when it came in contact with hot charcoal dust. He recognized that heating mercuric oxide produced the same gas. The reaction is now known to be

2 HgO(s)

2 Hg() + O2(g)

He collected the gas in pure form using a small bag. He explained the properties of fire air using the phlogiston theory and carefully recorded the experiment in his notes, but they were not published until 1777. (see 1777.)

In April 1774, pharmacist Pierre Bayen. Bayed discovered that heating mercury oxides resulted in a discharge of gas and a loss of mass. He collected the gas, and noted that it was slightly denser than air. Bayen was timid about publishing anything but direct observations. He did not interpret his results and he did not examine the gas further. Had he done so, he would have realized that the gas he had collected was not ordinary air.

On August 1, 1774, Joseph Priestley independently repeats the production of oxygen from mercuric oxide in his laboratory at Lord Shelburne's country mansion near Calne, England. He wrote, "What surprised me more than I can well express, was that a candle burned in this air with a remarkably vigorous flame..." Priestley carefully documented his work, recognized the significance of what he had done, and published his results promptly. Posterity gives Priestley credit for the discovery of oxygen.

On September 16, 1774, Antoine Lavoisier began to muddy the water. He observed that heating mercuric oxide produces metallic mercury. He thought the reaction might have been caused by contact with iron; he made no note of gas evolution in his notebook. Lavoisier later announced to the Academy of Science in Paris that he had isolated a component of air that he called "eminently breathable air" by decomposition of mercuric oxide.

In 1777, Scheele reported his preparation of oxygen in his Treatise on Fire and Air. He was aware of Priestley's and Lavoisier's claims, but he did not make any claim of his own. Lavoisier biographer Jean-Pierre Poirier describes Scheele as "self-effacing" and speculates that Scheele did not want to be accused of plagiarism – even though the discovery was rightly his! Scheele does receive parenthetical credit today.

Gallingly, in 1789, Lavoisier describes the preparation of oxygen in detail by heating the red oxide of mercury in his Traité Élémentaire de Chimie, Lavoisier refers to oxygen as "this air, which Mr. Priestley, Mr. Scheele, and I discovered about the same time". (Priestley denounced his claim.)

Nonetheless, it must be said that Lavoisier concluded, although improperly, that oxygen was the fundamental constituent of all the acids and gave its actual name, which in Greek means “producing of acids”. (It was a mistaken belief at the time that oxygen was necessary for the formation of all acids. Today we know that certain acids, such as hydrochloric acid, can be formed without oxygen.)

Joseph Priestley

Joseph Priestley’s first interest was always theology, but he also excelled in physics, philosophy, algebra, mathematics and a variety of ancient and modern languages, and devised an improved system of shorthand. It was his work as a scientist that he would best be remembered for. (Joseph Priestley also discovered photosynthesis, and discovered and described the properties of ammonia, sulphur dioxide, hydrogen sulphide and carbon monoxide.)

Priestley lived next to a brewery and one day noticed that the gas given off from the fermenting vats drifted to the ground, implying that it was heavier than air. Moreover, he discovered that it extinguished lighted wood chips. He had discovered carbon dioxide, which he called ‘fixed air’. Devising a method of making the gas at home without brewing beer, he discovered that it produced a pleasant tangy taste when dissolved in water. By this invention of carbonated water, he had become the father of fizzy drinks!

In 1772 he discovered the respiration of plants by placing a shoot of a green plant in a sealed container, in which he lit a candle that he left burning until it went out. Later, he discovered, the candle could be lit again. In the same year he invented a piece of apparatus that was to be of crucial importance in his experiments with gases. These experiments involved floating various materials on mercury inside a sealed glass container, which he then heated using a burning glass. 

With this apparatus he discovered nitrous oxide (laughing gas) in 1772 and then, in 1774 he made the discovery for which he is best known. Placing a piece of mercuric oxide in his apparatus, he discovered that a candle burned very brightly in the gas produced. After finding that mice survived quite happily in the gas he took a deep breath of it himself: 'The feeling of it to my lungs was not sensibly different from that of common air; but  I fancied that my breast felt particularly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury? Hitherto only two mice and myself have had the privilege of breathing it.' He had discovered oxygen.

Remarkably, as Oxygen by Nick Lane states,

Quote: Priestley was uncannily prescient. He foresaw not only the medical applications of oxygen but also its potential danger.

After all, in his Experiments and Observations on Different Kinds of Air, 1775, he mused,

Quote: …that though pure dephlogisticated air might be very useful as a medicine, it might not be so proper for us in the usual healthy state of the body; for, as a candle burns out much faster in dephologisticated than in common air, so we might, as may be said, live out too fast and the animal powers be too soon exhausted in this pure kind of air. A moralist, at least, may say, that the air which nature has provided for us is as good as we deserve.

Unfortunately, Priestley's mind was encumbered with some outdated and, as we now know, highly erroneous ideas. Along with his contemporaries, he believed that air, like water, was an element that could become polluted with phlogiston as a result of fire or of animals' exhalations, and which (he discovered) could be restored to purity by agitating air in water or by the respiration of plants. (Phlogiston theory: the idea that burning releases an invisible substance.) He consequently thought that the oxygen he had isolated was dephlogisticated air - air with less phlogiston than ordinary air. In fact, Priestley's report on his experiments with gases is a fascinating account of one man's struggle to make sense of what he had discovered, from which we can see that the leaps of imagination and understanding that would have been needed to truly appreciate the significance of his discovery were quite beyond him.