#OTD 1869. Dmitri Mendeleev presents the first periodic table to the Russian Chemical Society.
After becoming a teacher in 1867, Mendeleev wrote Principles of Chemistry (1868-70), which became the definitive textbook of its time. This is when he made his most important discovery. As he attempted to classify the elements according to their chemical properties, he noticed patterns that led him to postulate his periodic table. via @wikipedia
