![]() ![]() In 1754, Joseph Black identified what he called "fixed air" (now known to be carbon dioxide) because it could be returned, or fixed, into the sort of solids from which it was produced. The steam engine was in the process of transforming civilization, and scientists of all types were fascinated with combustion and the role of air in it.īritish chemists were especially prolific. The second half of the 1700s witnessed an explosion of interest in such gases. The principal method for altering the nature of air, early chemists learned, was to heat or burn some compound in it. Nobody knew what it was, and researchers kept finding that it could be converted into such a variety of forms that they routinely spoke of different "airs." ![]() It wasn't clear how air fit into that system. Researchers had distinguished no more than two dozen or so elements, depending on who was doing the counting. In the mid-18th century, the concept of an element was still evolving. They comprise 78 and 21 percent of the atmosphere, respectively. Scientists now recognize 92 naturally occurring elements-including nitrogen and oxygen, the main components of air. ![]() It is hard to overstate the importance of Priestley's revelation. Among them was the colorless and highly reactive gas he called "dephlogisticated air," to which the great French chemist Antoine Lavoisier would soon give the name "oxygen." In a series of experiments culminating in 1774, Priestley found that "air is not an elementary substance, but a composition," or mixture, of gases. In the process, he helped dethrone an idea that dominated science for 23 uninterrupted centuries: Few concepts "have laid firmer hold upon the mind," he wrote, than that air "is a simple elementary substance, indestructible and unalterable." The world recalls Priestley best as the man who discovered oxygen, the active ingredient in our planet's atmosphere. He settled in Pennsylvania, where he continued his research until his death. His unorthodox religious writings and his support for the American and French revolutions so enraged his countrymen that he was forced to flee England in 1794. He invented carbonated water and the rubber eraser, identified a dozen key chemical compounds, and wrote an important early paper about electricity. Priestley (1733-1804) was hugely productive in research and widely notorious in philosophy. It might have endured even longer had it not been for a free-thinking English chemist and maverick theologian named Joseph Priestley. But it made excellent sense at the time, and there was so little reason to dispute it that the idea persisted until the late 18th century. That notion may seem charmingly primitive now. Some 2,500 years ago, the ancient Greeks identified air - along with earth, fire and water - as one of the four elemental components of creation. ![]()
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