Sir William Crookes
Contribution to the atomic theory
In 1861, Crookes had obtained the sludge left over from the sulfuric acid left by a friend. After removing all of the selenium from the sludge, he inspected it with a spectroscope to look for signs of tellurium. Instead of seeing the yellow spectral lines produced by tellurium, he saw a bright green line that no one had ever seen before. He named the new element with the green line thallium, after the greek word for 'green twig'.
2. In 1863 he became an Elected Fellow of the Royal Society.
3. In 1861 is when he found the element thallium