 As we discussed in Section 1, by the end of the 18th century the modern conception of chemical substances began to take form in Lavoisier’s work. Contemporary looking lists of elements were being drawn up and also the notion of mass was introduced into chemistry. Despite these advances, chemists continued to develop theories about two substances which we no longer accept: caloric and phlogiston. Lavoisier famously rejected phlogiston, but he accepted caloric. It would be another 60 years until the notion of caloric was finally abandoned with the development of thermodynamics.