Portrait of John Dalton (1766-1844), English chemist. Dalton left school at the age of 11, starting to teach a year later. His first love was meteorology, of which he made a life-long study. In the course of this he investigated the properties of gases. He deduced the structure of carbon dioxide from careful measurements, and this led to his proposing the idea of molecules constructed from exact numbers of atoms. He also held that all atoms of a given element were identical, but that different elements had different atoms. He was the first to classify elements by their atomic weights, thus setting the stage for a revolution in chemical thought.