Thallium was discovered in 1861 by Sir William Crookes, through spectroscopic techniques, during his seek for tellurium, in some plant residues. Crookes found a sharp and unknown green stripe in the spectrum of some acids removed from those plants, and proposed the existence of a new element. This green stripe reminded Crookes, the color of the vegetation in Spring. For that reason, he called it thallium, that derives from the Latin word thallus, which means "in bloom". During that year, Crookes isolated a small amount of metallic thallium, and exhibited it at an international conference in London, in 1862.