В 1869 году открыт Периодический закон химических элементов

On Monday after breakfast, head of General and Inorganic Chemistry at the Saint Petersburg University Dmitry Mendeleyev got locked in his study being in high spirits and started laying out business cards with symbols of chemical elements and their properties written on the reverse side of them. The members of his family were walking on tiptoe and listening to the shouts from behind the door: “Look, horny-head, I will do it!” And he really succeeded – the table was ready in the evening. The professor rewrote it, put down the date and name (The Experiment of a System of Elements Based on Their Atomic Weight and Chemical Similarity), and sent to the print shop. Now, people often say that periodic system has appeared in Mendeleyev dreams when he was sleeping. Everything is possible. It was probably the reason behind his high spirits.