In the mid-18th century, the most pressing issue in chemistry and physics was to determine what exactly happens when something burns. The prevailing theory was that flammable materials contained a substance called “phlogiston” (from the Greek word for burn) that was released during combustion. The theory held that when a candle burned, for example, phlogiston was transferred from it to the surrounding air. When the air became saturated with phlogiston and could contain no more, the flame went out. Breathing, too, was a way to remove phlogiston from a body. A typical test for the presence of phlogiston was to place a mouse in a container and measure how long it lived. When the air in the container could accept no more phlogiston, the mouse would die.
In 1773, the Earl of Shelburne asked Joseph Priestley to serve as a sort of intellectual companion, tutor for the earl’s offspring, and librarian for his estate, Bowood House. The position provided ample free time for the chemical research. During this time, Priestly systematically analyzed the properties of different “airs” using the favored apparatus of the day: an inverted container on a raised platform that could capture the gases produced by various experiments below it. The container could also be placed in a pool of water or mercury, effectively sealing it, and a gas tested to see if it would sustain a flame or support life.
In the course of these experiments, Priestley made an enormously important observation. A flame went out when placed in a jar in which a mouse would die due to lack of air. Putting a green plant in the jar and exposing it to sunlight would “refresh” the air, permitting a flame to burn and a mouse to breathe. Perhaps, Priestley wrote, “the injury which is continually done by such a large number of animals is, in part at least, repaired by the vegetable creation.” Of course, we know today that what Priestly observed is the byproduct of a process known to us as photosynthesis.
On August 1, 1774, he conducted his most famous experiment. Using a 12-inch-wide glass “burning lens,” he focused sunlight on a lump of reddish mercuric oxide in an inverted glass container placed in a pool of mercury. The gas emitted, he found, was “five or six times as good as common air.” In succeeding tests, it caused a flame to burn intensely and kept a mouse alive about four times as long as a similar quantity of air.
Priestley called his discovery “dephlogisticated air” on the theory that it supported combustion so well because it had no phlogiston in it, and hence could absorb the maximum amount during burning. He then went on to experiment on the gas by breathing it into his OWN lungs. “The feeling of it in my lungs,” Priestley wrote, “was not sensibly different from that of common air, but I fancied that my breast felt peculiarly 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.“
Priestly later met Antoine Lavoisier in France and described his discovery. It turned out to be the clue Lavoisier needed to develop his theory of chemical reactions. Burning substances, Lavoisier argued, did not give off phlogiston; they took on Priestley’s gas, which Lavoisier called “oxygen” from the Greek word for acid-maker.
By then, however, Priestley had returned to England, where he escalated his support for the American Revolution and for highly unorthodox religious views. Those positions were a source of embarrassment for Lord Shelburne. Priestley left his service in 1780, moving to Birmingham and taking a position as head of a liberal congregation called New Meeting.
His new location brought him into contact with numerous luminaries including Erasmus Darwin, grandfather of Charles, the great architect of evolutionary theory. James Watt and Matthew Boulton — who were about to transform society with their steam engine — were there, as was Josiah Wedgwood, the famous potter, who supported Priestley’s chemical experiments. Birmingham also boasted a distinguished scientific discussion group, the Lunar Society (from which we get the term lunatics), which met on nights of a full moon so that the members could see their way home.
Priestley’s encouragement of the French Revolution, together with his increasingly controversial theology and attacks on the doctrine of the trinity, eventually became too notorious for safety. In 1791, an alcohol-fueled mob of royalists burned the New Meeting house, and then Priestley’s home. The scientist and his family barely escaped. They fled to London, but eventually it proved no safer. Priestley’s sons could not find work and emigrated to Pennsylvania, where they hoped to found a center for free-thinking Englishmen.
Priestley turned down the offer of a teaching position at the University of Pennsylvania in Philadelphia, and instead built a house in the remote hamlet of Northumberland to be near his sons. There Priestley continued his research, isolating carbon monoxide (which he called “heavy inflammable air”) and founding the Unitarian Church in the United States.
On February 3, 1804, Priestley began an experiment, but found himself too weak to continue. He went to his bed in his library, never again to emerge. On February 6, he summoned one of his sons and an assistant. He dictated some changes in a manuscript. When he was satisfied with the revisions, he said “That is right. I have now done.” Minutes later he died painlessly, ending what Thomas Jefferson called “one of the few lives precious to mankind.”
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