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Carl Scheele (1742–1786) was a Swedish chemist who was almost certainly the first person to isolate oxygen, producing it chemically in about 1771. Unfortunately, Scheele did not publish his results until 1777, well after both Priestley and Lavoisier had published their work. Their findings were at least partially based on Scheele's results, which he had communicated to them. Scheele also died at an early age of 44, almost certainly poisoned by some of the chemicals that he worked with, which he liked to taste and sniff.

Joseph Priestley (1733–1804) made many important discoveries, especially in relation to the properties and handling of gases. Priestley was an eighteenth‐century English country minister who had a passion for science (Jaffe, 1976). His formal education was limited primarily to theology and languages, whereas his scientific knowledge was mostly self‐taught. He was deeply influenced by a meeting with Benjamin Franklin, whom he met on a trip to London. He lived next to a brewery in Leeds, which provided a constant supply of carbon dioxide, so initially, he studied that gas's properties. One of his first results was the invention of seltzer water, for which the British Royal Society awarded him a gold medal.

We remember Priestley especially for his discovery of oxygen, first in an indirect way, by observing the action of plants in 1771, and then in pure form in 1774 by heating mercuric oxide and collecting the gas given off. He described the 1771 experiments thus:

Finding that candles would burn very well in air in which plants had grown a long time … I thought it was possible that plants might also restore the air which has been injured by the burning of candles. Accordingly, on the 17th of August, 1771, I put a sprig of mint into a quantity of air in which a wax candle had burned out and found that on the 27th of the same month another candle burnt perfectly well in it.

Priestley's interpretation of this and related experiments (Fig. 3.1) was that the candle (or mouse – he had a plentiful supply of mice and often used them as test subjects) produced large amounts of phlogiston, which was the basis for interpreting all chemical processes at that time. Phlogiston, which is derived from the Greek word for “to set on fire,” was thought to be a flammable substance possessed by all substances that can burn. Upon combustion or respiration, the phlogiston was released into the air and contaminated it. Plants had the unique ability to recapture the phlogiston that had been released by burning. In his later experiments, Priestley was able to prepare and analyze in some detail significant quantities of pure oxygen, which he called “vital air.” Priestley was a brilliant experimentalist, whose abilities to generate and manipulate gases were far better than those of most of his contemporaries. However, his skills at interpreting the observations he made were not as strong. He interpreted all his observations in terms of the phlogiston theory, even after most other scientists had abandoned it. He was usually content to simply describe his results with little interpretation. Priestley traveled to France in 1774 and met with Lavoisier, to whom he openly described his many experiments. Lavoisier used this information, as well as many of his own experiments, to overturn the phlogiston theory and establish the modern study of chemistry.

Figure 3.1 Joseph Priestley and the discovery of oxygen.

Priestley was both a religious and a political nonconformist and vocally supported both the American and the French revolutions. He was often in trouble for his unorthodox views, was attacked by the press and denounced in Parliament, and the scientific establishment increasingly shunned him. In 1791, an angry mob burned his house to the ground. In 1794, he fled to the USA and settled in Pennsylvania, where he was warmly welcomed by both religious and scientific leaders. He lived there until he died in 1804.