Rebel with a cause
Joseph Priestley was born in Yorkshire, the eldest son of a maker of wool cloth. His mother died after bearing six children in six years. Young Joseph was sent to live with his aunt, Sarah Priestley Keighley, until the age of 19. She often entertained Presbyterian clergy at her home, and Joseph gradually came to prefer their doctrines to the grimmer Calvinism of his father. Before long, he was encouraged to study for the ministry. And study, as it turned out, was something Joseph Priestley did very well.

Aside from what he learned in the local schools, he taught himself Latin, Greek, French, Italian, German, and a smattering of Middle Eastern languages, along with mathematics and philosophy. This preparation would have been ideal for Oxford or Cambridge, but as a Dissenter (someone who was not a member of the Church of England) Priestley was barred from England's great universities. So he enrolled at Daventry Academy, a celebrated school for Dissenters, and was exempted from a year of classes because of his achievements.

After graduation, he supported himself, as he would for the rest of his life, by teaching, tutoring and preaching. His first full-time teaching position was at the Dissenting Academy in Warrington. (Although obviously brilliant, original and outspoken-and by one report, of "a gay and airy disposition" Priestley had an unpleasant voice and a sort of stammer. That he made a living through lectures and sermons is further evidence of his extraordinary nature.)

In 1762, he was ordained and married Mary Wilkinson, the daughter of a prominent iron-works owner. She was, he noted, "of an excellent understanding, much improved by reading, of great fortitude and strength of mind, and of a temper in the highest degree affectionate and generous; feeling strongly for others and little for herself."

Priestley traveled regularly to London, and became acquainted with numerous men of science and independent thought, including an ingenious American named Benjamin Franklin, who became a lifelong friend. Franklin encouraged Priestley in his research, one result of which was The History and Present State of Electricity. For that work, and his growing reputation as an experimenter, Priestley was made a Fellow of the Royal Society in 1766.

The history book was too tough for a popular audience, and Priestley determined to write a more accessible one. But he could find no one to create the necessary illustrations. So, in typical fashion, he taught himself perspective drawing. Along the way, he made many mistakes, and discovered that India rubber would erase lead pencil lines—-a fact he mentioned in the preface.

By the age of 34, Priestley was a well-established and respected member of Britain's scientific community. He was still paying a price for his religious nonconformity, however. When the intrepid explorer Captain James Cook was preparing for his second voyage, Priestley was offered the position of science adviser. But the offer was rescinded under pressure from Anglican authorities who protested his theology, which was evolving into a strongly Unitarian position that denied the doctrine of the trinity.

In retrospect, the Cook affair may have been all for the best. In 1773, the Earl of Shelburne asked 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 access to social and political circles Priestley could never have gained on his own, while leaving ample free time for the research that would earn him a permanent place in scientific history.

He 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." Thus he observed that plants release oxygen into the air—the 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. (The year before, Swedish apothecary Carl Wilhelm Scheele isolated the same gas and observed a similar reaction. Scheele called his material "fire air." But his findings were not published 1777.)

Whatever the gas was called, its effects were remarkable. "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."

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