Henry Cavendish (10 October 1731 – 24 February 1810) was a British natural philosopher and theoretical chemist and physicist.
Cavendish described the density of inflammable air, in a 1766 paper "On Factitious Airs" where he noted it formed water on combustion and is considered as the discovery of hydrogen. He was the first to recognize the gas as a discrete substance and stated that it produced water when reacted with oxygen (called 'dephlogisticated air') , so establishing water as a compound, not an 'element'.
Cavendish used the language of the old phlogiston theory in chemistry. He recognized it by naming the gas from a metal acid reaction "Flammable air”. Antoine Lavoisier later reproduced Cavendish's experiment and gave the element the name Hydrogen from the Greek words "hydro" and "genes", which together means "water forming".