The first period, whose transition into the second is the sharpest, spans the years 1800-1826. Before 1826, families of similar elements were barely preserved into the future (dark blue row at the bottom of Fig. ), a result also found in a recent account of the unfolding of the periodic system before its formulation in the 1860s . This period includes the most rapid expansion ever of the number of chemical elements (SI Fig. ) and the early days of the discovery of some of the valencies of the known elements (). The chemical space of those times was dominated by inorganic compounds with a growing presence of organic compounds , also evidenced in the high production of diverse metallic compounds, especially of K, Mg, Mn, Ba, Hg, Pb, Fe and other transition metals, as well as compounds containing S, As, C and H (Fig. ).