The Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, built on the accepted idea that heat was something to do with matter in motion. This was the same idea put forward several years earlier by Sir Benjamin Thompson in 1798, who said he was only following on from the work of many others. Maxwell outlined four stipulations for the definition of heat. Heat:
- ... is something which may be transferred from one body to another, according to the second law of thermodynamics,
- ... a measurable quantity, and thus treated mathematically.
- ... cannot be treated as a substance, because it may be transformed into something that is not a substance, e.g., mechanical work.
- ... is one of the forms of energy.
Heat transfer can occur in a variety of ways: by conduction, radiation, convection, mass transfer or by friction.
The SI unit of heat is the joule.