The history of Mendeleev's famous discovery has long been a matter of lively debate among experts. This essay proposes a new reading of this story, which differs from the well-known reconstructions made by Kedrov, Bensaude-Vincent, Graham and others. Particular attention is paid to the context of a Mendelevian thought and the analysis of the surviving outlines of his first variants of the Periodic Table. By considering Mendeleev's discovery of the Periodic Law one can identify the three principal stages in his work: 1) the composition of the ““first attempt””(pervaia proba) of the system of chemical elements and the discovery of the periodic character in dependence of the elements, properties on their atomic weights (late 1868-early 1869); 2) the composition of Attempt at a system of elements based on their atomic weights and chemical similarity as a temporary version of the Periodic Table (February 1869); 3) the composition of the Natural system of elements (November 1870).
Mendeleevian work on Attempt revealed a lack of clear chemical criteria for unifying elements of different classes——the ““natural families”” and ““transitional metals””——into a general taxomonical scheme that forced him to reject the ideal structure of the system of elements that he had formed earlier (1868). It was only by November of 1870 that Mendeleev finally solved the ““unification problem,”” formulating the basic principles of his system. This article also discusses how Mendeleev's views on the structure of the Periodic System were mediated by his convictions regarding the constitution of organic compounds.