The records show that Mendeleev discovered the periodic system on 17 February 1869. In fact, he was ill at the time so his colleague Nikolai Menschutkin made the announcement on his behalf – on 6 March at a meeting of the Russian Chemical Society.
Mendeleev’s table listed elements in rows or columns in order of atomic weight, starting a new row or column when the characteristics of the elements began to repeat. What made his version successful was the fact that he left gaps where there appeared to be an element missing that had not yet been discovered. He also thought to occasionally ignore the order suggested by the atomic weights and to switch adjacent elements where they could be better classified into chemical families. Atomic numbers were not yet known, but the atomic weights worked well enough for ordering most of the elements, and Mendeleev was able to accurately predict the properties of missing elements.
John Newlands took issue with the credit that Mendeleev was given for the periodic table in his book ‘On the Discovery of the Periodic Law’ in 1884, going so far as to quote Mendeleev as supporting him: "It is possible that Newlands has prior to me enunciated something similar to the periodic law...".
Mendeleev countered this somewhat in his book The Principles of Chemistry in 1891:
"I consider it well to observe that no law of nature, however general, has been established at once; its establishment is always preceded by many presentiments, but the acknowledgment of a law does not take place when it is recognised in all its significance, but only when it has been confirmed by experiment, which the scientific man must look to as the only proof of the correctness of his conjectures and opinions. I therefore, for my part, consider Roscoe, De Boisbaudran, Nillson, Winkler, Brauner, Carnelley, Thorpe, and others who verified the adaptability of the periodic law to chemical reality, as the true founders of the periodic law, the further development of which still awaits fresh workers."