In the analysis of the development of thermodynamics as a science, the theoretical work of Sadi Carnot, published in 1824, is generally considered to be the starting point. Carnot studied the cycle of an ideal heat engine and formulated the condition for its maximum efficiency. In this article we examine James Watt’s contributions to the formation of fundamental concepts of thermodynamics, made in the course of his work on improving the Newcomen engine and developing his own steam engine. It is shown that Watt was the first to characterize thermodynamic properties such as latent heat and vapor density. The authors prove Watt’s priority in the studies of the dependence of saturated steam pressure on temperature, in which a critical point was found when the latent heat disappears. These results of Watt anticipated by many decades the studies on the thermodynamic critical state by Th. Andrews and J. Thomson. The article also discusses Wattʼs research on thermodynamic cycles. It is shown that he was the first to study the Rankine cycle with superheated steam, known from the history of thermodynamics. Watt was also the first scientist to introduce the concept of a steam engine’ volumetric work as the product of pressure and volume, and developed a device, the steam pressure indicator, to measure its value. We show the results obtained by Watt with steam to be considerably ahead of Prescott Jouleʼs work on the cooling and condensation of gases during expansion. The article presents an interpretation of Watt’s 1769 patent that is very important as the primary source for a subsequent study and establishment of the principles of energy conversion. The factual material presented in this article suggests that Watt’s scientific research have not been properly understood or simply went unnoticed.
