Probably the first man to describe heat as a physical, measurable entity was the scottish doctor Joseph Black (1728 - 1799).
For him heat was an imponderable, material fluid named "caloric". This fluid might to enter in any body increasing his temperature.
Black defined the unity of heat as the heat needed to increase the temperature of one pound of water of 1°Farenheith.
Benjamin Thompson (1753 - 1814).
In 1798, Benjamin Thompson (Count Rumford), an American who was then in the Bavarian service, presented a paper to the Royal Society of Great Britain, in which he stated the results of an experiment which he had recently made, proving the immateriality of heat and the transformation of mechanical into heat energy. He became interested in the subject by observing the vast quantities of heat produced by friction during the boring of a cannon. Thompson decided that heat was not a material fluid but the result of a conversion of energy.
The heat generated by friction was inexpicable whith the "caloric" theory.