The question was ripe, as they say — there was a total mess in basic chemical concepts. Chemists had different ideas of “atom,” “molecule” and wrote chemical formulas differently. It got to the point where publishers of scientific journals demanded that articles should be accompanied by keys, as in music. For three days, the world’s leading scientists, including such Russian chemists, as Mendeleev, Zinin, and Borodin, discussed the existing problems and looked for ways to solve them. And they found — they ended their differences, agreed upon common symbols and gave the green light to the atomic and molecular theory. In his sunset years, Mendeleev recalled that the ideas voiced during the congress boosted his idea of the element systematization.