On this day in 1774, British chemist Sir Joseph Priestley announced that he had discovered oxygen.
On this day in 1834, slavery was abolished in the British Empire as the Slavery Abolition Act 1833 came into force.
On this day in 1941: American volunteer aviators form the "Flying Tigers," with Brig. Gen. Claire L. Chennault as commander, helped China combat Japanese air superiority in the early stages of World War II.
On this day in 2011, Syria's bloody crackdown on civilian demonstrators left 140 people dead, drawing worldwide condemnation.