James Clerk Maxwell in his 1871  Theory of Heat outlines four stipulations for the definition of heat:
* It is something which may be transferred from one body to another, according to the second law of thermodynamics.
* It is a measurable quantity, and so can be treated mathematically.
* It cannot be treated as a material substance, because it may be transformed into something that is not a material substance, e.g., mechanical work.
* Heat is one of the forms of energy.