How did Joule “disprove” caloric theory? According to the Blundells in their Concepts in Thermal Physics, they mention:
Joule let a mass tied to a string slowly descend a certain height, while the other end of the string turns a paddle wheel immersed in a certain mass of water. The turning of the paddle frictionally heats the water. After a number of descents, Joule measured the temperature rise of the water. In this way he was able to deduce the ‘mechanical equivalent of heat’. He also measured the heat output of a resistor (which, in modern units, is equal to $I^2 R$, where $I$ is the current and $R$ the resistance). He was able to show that the same heat was produced for the same energy used, independent of the method of delivery. This implied that heat is a form of energy. Joule’s experiments therefore consigned the caloric theory of heat to a footnote in history.
I can’t understand how the authors justify the disproval by Joule.