Crookes' thallium samples and notebook, 1861.
© Science Museum / Science & Society Picture Library
The five bottles contain specimens of different thallium compounds, isolated by Sir William Crookes (1832-1919) in 1861. The notebook details his discovery. He could detect minute quantities of the new element because it emits a brilliant green light when in a hot flame. Thallium is a rare soft white metallic element, occurring naturally in zinc blende and some iron ores.