In famous publications, “An Inquiry Concerning the Source of Heat which is Excited by Friction” is a 1798 article by American-born English physicist Benjamin Thompson in which the results of experiments were published and discussed which showed that heat produced, via friction, in the boring of a cannon, could be used to make water boil in 2.5 hours time, thus laying question to French physicist Antoine Lavoisier’s 1787 caloric theory. [1] This experiment gave one of the first calculations of what would be later called the mechanical equivalent of heat.

What is heat?
These boring experiments, according to Thomson, gave “farther insight into the hidden nature of heat; and to enable us to form some reasonable conjectures respecting the existence, or non- existence, of an igneous fluid.” The following summary quote by Thomson highlights the central conclusion of the article:

“What is heat? Is there anything as igneous fluid? Is there anything that can with propriety be called caloric? That heat generated by friction [in the boring experiments] appeared, evidently, to be inexhaustible, [it] cannot possibly be a material substance; … it appears to me to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner heat was excited and communicated in these experiments, except it be MOTION.”

In short, heat was experimentally determined to be or be related to the motion of particles of bodies.

References
1. (a) Thomson, Benjamin. (1798). “An Inquiry Concerning the Source of Heat which is Excited by Friction”. Philosophical Transactions. Vol. XVIII, pg. 286.
(b) Read: before the Royal Society of London on January 25, 1798.
(c) Thomson, Benjamin. (1798). “An Inquiry Concerning the Source of Heat which is Excited by Friction” in The Complete Works of Count Rumford, (pgs. 469-93). Oxford University Press, 1870.

External links
● An Experimental Inquiry Concerning the Source of Heat which is Excited by Friction – Wikipedia.