What element did Faraday first succeed in liquefying?

The element Michael Faraday first succeeded in liquefying was chlorine.

In 1823, Faraday liquefied chlorine while investigating gases and their behaviour under pressure. The achievement helped demonstrate that gases could be understood as vapours of liquids with very low boiling points, strengthening the developing idea that gases and liquids were different states of the same substances.

The experiment also clarified a confusing material then called “solid chlorine.” Faraday showed that this substance was actually chlorine hydrate, a compound in which chlorine is trapped within water’s crystalline structure. Chlorine itself is a greenish-yellow halogen gas at ordinary conditions, so the experiment was a striking transformation of a familiar gas into a liquid.

Faraday later liquefied other gases, including carbon dioxide. This work was separate from his famous discoveries in electromagnetism and electrolysis, but it established him as an important experimental chemist as well as a physicist.