Until about 200 hundred years ago, there were three competing ontologies on the nature of heat.
1) Only cold is a real entity, so heat is the relative absence of cold. 2) Hot and cold are both real entities, so that heat is a mixture of hot and cold. 3) Only heat is a real entity, so cold is absence of heat. Within each ontology there were also competing theories about the nature of cold and hot, such as the caloric vs the kinetic theory of heat in the case of no.3, As we all know the third ontology has come to be regarded as the "truth". Interestingly each ontology suggests different approaches to the practical problem of heating and cooling. The first says: When cooling is desired you must add cold. When heating is desired you must remove cold.* The second says: When cooling is desired you may either remove heat and/or add cold. When heating is desired you may either remove cold and/or add heat.* The third says: When cooling is desired, heat must be removed. When heating is desired, heat must be added.* The approaches labled with a '*' are applicable to Rossi's reactor.
