Dmitri Ivanovich Mendeleev in his study at home in 1904. Image provided by Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.		The Periodic Table A handful of materials we now consider elements were known since ancient times, but thanks to people like Jöns Jakob Berzelius (1779–1848) and Humphry Davy (1778–1829) the number of known elements had reached over 50 by the mid-1800s. A pattern in the properties of the elements became obvious, and Julius Lothar Meyer (1830–1895) and Dmitri Ivanovich Mendeleev (1834–1907) developed useful tables that organized the elements into groups with similar properties. Mendeleev was even bolder and predicted that new elements would be discovered to fill in the gaps in his tables. When his predictions came true, the periodic table was widely adopted and became an icon of chemistry.