According to caloric theory of heat, heat was regarded as an invisible and weightless fluid called “caloric”. The two bodies at different temperatures placed in contact attain thermal equilibrium by the exchange of caloric. The caloric flows from the hot body to the cold body, till their temperature becomes equal.
However, this theory failed to explain the production of heat due to friction in the experiments conducted by Court Rumford. Rubbing our hands against each other produces heat. Joule’s paddle wheel experiment led to the production of heat by friction. These observations led to the dynamic theory of heat, according to which heat is a form of energy called thermal energy.