Joseph Priestley heated mercuric oxide and collected the gas released, finding that a candle burned with unusual brightness in it and that mice thrived when placed inside a jar of it. He called the substance "dephlogisticated air," unaware he had isolated what Antoine Lavoisier would soon rename oxygen, a discovery that would overturn the reigning phlogiston theory of combustion.

The Slavery Abolition Act of 1833 came into force, formally emancipating enslaved people throughout most of the British Empire, with roughly 800,000 people freed in the Caribbean colonies alone. The law required a transition period of "apprenticeship" for many former slaves and compensated slave owners — not the enslaved — with £20 million, a sum so large the British government only finished paying it off in 2015.

President Ulysses S. Grant signed the proclamation admitting Colorado as the 38th state, capping a decade-long push driven by the silver and gold rushes that had swelled its population. The territory's admission came exactly 100 years after the signing of the Declaration of Independence, earning it the enduring nickname "the Centennial State."

Sixty riders pedaled away from the Café au Réveil-Matin in Montgeron to launch the first Tour de France, a race conceived by journalist Henri Desgrange to boost sales of the sports newspaper L'Auto. The grueling event covered six stages and nearly 2,428 kilometers; Maurice Garin, a chimney sweep turned cyclist, won the overall title and became the race's first legend.

Eleven days after landing on Guam, U.S. Marine and Army units fought through dense jungle terrain to break the last organized Japanese resistance on the island. The campaign, part of the wider Mariana Islands offensive, would secure a crucial airbase from which B-29 bombers could later strike the Japanese home islands directly.

At 5 p.m., roughly 40,000 fighters of the Polish Home Army rose simultaneously across Warsaw, seizing key buildings and attacking German garrisons in a bid to liberate the capital before Soviet troops arrived. The uprising, ordered by General Tadeusz Bór-Komorowski, hoped to establish Polish sovereignty ahead of Soviet occupation — but Red Army forces halted their advance nearby and offered no support.

Leaders from 35 nations, spanning the Iron Curtain divide, signed the Helsinki Final Act at the Conference on Security and Cooperation in Europe. The accords formally recognized postwar European borders and, crucially, committed signatories to respect human rights and fundamental freedoms — language that Soviet dissidents and Eastern Bloc reformers would seize upon for years to come.