Left: reactants in a beaker (Ѻ) being heated, an action which works to get the reactants up to the activation energy level in order to reaction to proceed to products. Right: the hot body (fire), cold body (cool water), and working body (substance in cylinder) of the 1690 Papin engine, through which heat is added then removed in the operation of the heat cycle.

See main: Matter theory of heat

See main: Motion theory of heat (kinetic theory of heat)

“It must not be thought that heat generates motion or motion heat—though in some respects this is true—but that very essence of heat or the substantial self of heat is motion and nothing else.”

English thinker Francis Bacon's famous 1620 statement that the very essence of heat is motion and nothing else. [7] This is one of the first precursor statements to the mechanical equivalent of heat.

“It now remains for me to tell your excellency, as I promised, some thoughts of mine about the proposition ‘motion is the cause of heat’, and to show in what sense this may be true. But first I must consider what it is that we call heat, as I suspect that people in general have a concept of this which is very remote from the truth. For they believe that heat is a real phenomenon, or property, or quality, which actually resides in the material by which we feel ourselves warmed. Now I say that whenever I conceive any material or corporeal substance, I immediately feel the need to think of it as bounded, and as having this or that shape; as being large or small in relation to other things, and in some specific place at any given time; as being in motion or at rest; as touching or not touching some other body; and as being one in number, or few, or many. From these conditions I cannot separate such a substance by any stretch of my imagination. But that it must be white or red, bitter or sweet, noisy or silent, and of sweet or foul odor, my mind does not feel compelled to bring in as necessary accompaniments. Without the senses as our guides, reason or imagination unaided would probably never arrive at qualities like these. Hence, I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we place them is concerned, and that they reside only in the consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated. But since we have imposed upon them special names, distinct from those of the other and real qualities mentioned previously, we wish to believe that they really exist as actually different from those.”

“Heat is a very brisk agitation of the sensible parts of the object, so that what in our sensation is heat in the object is nothing but motion.”

— John Locke (c.1670), “On the Five Senses of Touch” (Ѻ); in: The Works of John Locke in Nine Volumes, Volume Two (Elements of Natural Philosophy; Chapter 11 (Ѻ)

“Heat seems principally to consist in that mechanical property of matter we call motion.”

— Robert Boyle (c.1660), Publication; cited by Donald Cardwell (1971) in From Watt to Clausius (pg. 4)

“Rumford's boiling of water by the heat generated in the boring of a cannon, and Davy's melting of ice by friction in vacuo, were each conclusively demonstrative alike of the non-materiality of heat and of the ultimate fate of work spent in friction. The exact and formal enunciation of the equivalence of heat and work required to fill the lacuna in Newton's statement was first given by Davy in 1812.”

“If heat is not a substance, it must be a quality; and this quality can only be motion. It was Newton’s opinion, that heat consists in a minute vibratory motion of the particles of bodies, and that this motion is communicated through an apparent vacuum, by the undulations of an elastic medium, which is also concerned win the phenomena of light. If the arguments which have lately been advanced, in favor of the undulatory nature of light, be deemed valid, there will be still stronger reasons for admitting his doctrine respecting heat, and it will only be necessary to suppose the vibrations and undulations principally constituting it, to be larger and stronger than those of light, while at the same time the smaller vibrations of light, and even the blackening rays [ultraviolet light], derived from still more minute vibrations, may, perhaps, when sufficiently condensed, concur in producing the effects of heat. These effects, beginning from the blackening rays, which are invisible, are a little more perceptible in the violet, which still possess but a faint power of illumination; the yellow green afford the most light; the red give less light, but much more heat, while the still larger and less frequent vibrations [infrared light], which have no effect on the sense of light, may be supposed to give rise to the least refrangible rays, and to constitute invisible heat.”

A 2015 novel Elements of Chemistry: Heat (Ѻ) by biomedical research and writer Penny Rad on relationships, sex, heat, and chemistry, which shows an formula-background stylized cover.

See main: Social heat; Economic heat; Political heat

“What is democracy but a successful formula for controlling the chemical reactions of our 145,000,000 people, and turning the friction and heat generated by our living together into production and progress?”

“In this sentence from New Instruments it is clear that Bacon, like Descartes, Count Rumford, Humphry Davy and Young, had a more or less definite notion of the dynamic nature of heat and its convertibility into work. But the exact science which treats of heat as a mode of energy begins with the publication, in 1824, of the Reflections on Motive Power of Fire of Sadi Carnot, who Lord Kelvin calls the ‘profoundest thinker in thermodynamic philosophy’.”

— Fielding Garrison (1909), “Josiah Willard Gibbs and his Relation to Modern Science, Parts I-IV” [8]