Paracelsus unknowingly observed hydrogen as he reacted acids with metals, making hydrogen gas as a side product. Theodore de Mayerene, a Swiss-born physician with Parecelsian thinking, repeated Paracelsus's experiments for the reaction between metals and acids. While de Mayerne did not know the gas produced was hydrogen or even a new element, he did discover that the byproduct gas was flammable, a crucial observation in the understanding of chemical reactions in the future. Image Source: Public Domain, https://upload.wikimedia.org/wikipedia/commons/1/11/SirTheodoreDeMayerne...