These areas of enquiry were related in the period: the chemical changes produced by heat were the subject of intense investigation in an effort to provide a comprehensive account of why and how substances combined. Priestley's isolation of gases was made under the aegis of a phlogiston theory; this was superseded by Lavoisier's caloric theory, which was in turn undermined by Davy's discovery of chlorine's role in reactions. New gases were discovered - oxygen, carbon dioxide, nitrous oxide; photosynthesis was postulated; infrared and ultraviolet rays were detected. The Voltaic pile was a spectacular instrument for harnessing electricity - Davy used it to decompose substances, reveal unknown elements, and show that substances were bonded by electrochemical forces. Dalton codified the operation of these forces - and the structure of matter - in his atomic theory. Meanwhile Herschel, when not detecting infra red, was observing previously invisible points of light in the heavens with improved telescopes, while Young was proposing a wave theory of light. Towards the end of the period, Oersted and then Faraday began the pioneering work on electromagnetism that changed understanding of the interaction of forces and enabled the construction of the electric motor.