Hydrogen
Henry Cavendish

In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance " phlogiston " and further finding in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.
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