Firstly, the author has analyzed Mendeleev's first paper on the periodicity and has shown that the development and the interaction of the following two concepts should play the important role in Mendeleev^ path to the discovery of the law, i.e., the concepts of fundamental im of matter and of chemical classification. Secondly, in order to show the course by which Mendeleev approached the dicovery, the author has examined his scientific works from 1854, when he published his first scientific paper, up to February 17(March 1 in present calendar system), 1869, when his first periodic table was compiled The study has shown that one can divide these years into three periods: I, 1854-61:II, 61-67: III, 68-69. Period I: His early work was concerned with the physicochemical properties of chemical substances which could be used as criteria cf their classification Though he could not reach satisfactory classification, his studies helped the development of his concepts of fundamental unit of matter, such as the atomic weight, the elements and so forth. Period II: Though he succeeded in systematization of organic compounds by his "theory of limits", he encountered difficulties in understanding so-called indefinite compounds in terms of atomic theory To avoid them, he distinguished the term "element" from "simple body" to give the former an attribute of "atom" and limited the scope of the atomic weight to definite compounds. Period III: The writing of the textbook Osnovy Khitnii (The Principles of Chemistry) in 1868 made him search for the fundamental property of elements for classification The comparison of the atomic weights of two dissmimilar groups of elements led him to conclude that the atomic weight is the fundamental property (i,e.,it belongs directly to his concept of "element" and determines all the other properties of elements), and that all the elements could be systematized in the table now known as the periodic table, if their atomic weights are used as criterion• These two interrelated recognitions are what he achieved through his discovery on February 17,1869.

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