In the theory of heat of the first half of the nineteenth century, heat was a substance. Mayer and Joule contradicted this thesis but developed different concepts of heat. Heat was a force for Mayer and a motion for Joule. Both Mayer and Joule determined the mechanical equivalent of heat. This result was, however, justified in accordance with those concepts of heat. Mayer's characterisation of force reappears in the very common textbook definition "energy cannot be created or destroyed but only transformed" and his theory led to a phenomenological approach to energy. Joule and Thomson's concept of heat led to a mechanistic approach to energy and to the common definition "energy is the capacity of doing work". One and the same term "energy" subsumed these two approaches. The problematic concept of energy, energy as a substance, appears then as a result of an eclectic development of the concept. Another approach, which appeared in the 1860s, is directly based on the mechanical equivalent of heat and can be characterized by the use of "principle of equivalence" instead of "principle of energy conservation". Unlike the others, this approach, which has been lost, poses no problems with the concept of energy. The problems with the energy concept as to the kind of phenomena dealt with in the present paper can, however, be overcome, as we shall see, in distinguishing between that which comes from experiments and that which is an interpretation of the experimental results within a conceptual framework.