How did they discover hydrogen several hundred years ago?

Answer 1

Hydrogen gas, H2, was first artificially produced and formally described by Theophrastus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus— via the mixing of metals with strong acids. He was unaware that the flammable gas produced by this chemical reaction was a new chemical element. In 1671, Robert Boyle rediscovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.[3] In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by identifying the gas from a metal-acid reaction as "flammable," and further finding that the gas produces water when burned in air. Cavendish had stumbled on hydrogen when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a liberated component of the mercury rather than the acid, he was still able to accurately describe several key properties of hydrogen, including the fact that it produced water when burned. In 1783 Antoine Lavoisier gave the element its name and (with Laplace) reported that pure water is produced by burning hydrogen and oxygen.

One of the first uses of H2 was for balloons. The H2 was obtained by reacting sulfuric acid and metallic iron. Infamously, H2 was used in the Hindenburg airship that was destroyed in a midair fire.