The document describes the discovery timeline of various chemical elements from 1668 to 1952. It notes that elements like carbon, sulfur, iron, tin, lead, copper, mercury, silver and gold were known since ancient times. It then details the year of discovery for additional elements such as phosphorus, cobalt, nickel, hydrogen, nitrogen, oxygen, chlorine, and more, crediting the discovering scientists. The timeline shows most elements were discovered in the 18th-19th centuries by chemists from various European countries through experiments and analysis of minerals.