This document discusses the periodic table and chemical periodicity. It describes how Dmitri Mendeleev published one of the first periodic tables in 1869, organizing the known elements based on their chemical properties. It also notes that Julius Lothar Meyer published a similar table that year based on physical properties. The document outlines some of Mendeleev's predictions about undiscovered elements and how the periodic table was later updated based on Henry Moseley's work showing atomic number is the fundamental property.