“The careful experiments of Joule, who developed heat in various ways by the application of mechanical force, establish almost to a certainty, not only the possibility of increasing the quantity of heat, but also the fact that the newly-produced heat is proportional to the work expended in its production.”

— Rudolf Clausius

First Memoir.
The Mechanical Theory of Heat (1867)

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Rudolf Clausius 6

German mathematical physicist 1822–1888

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“Many facts have lately transpired which tend to overthrow the hypothesis that heat is itself a body, and to prove that it consists in a motion of the ultimate particles of bodies. If this be so, the general principles of mechanics may be applied to heat; this motion may be converted into work, the loss of vis viva in each particular case being proportional to the quantity of work produced.
These circumstances, of which Carnot was also well aware, and the importance of which he expressly admitted, pressingly demand a comparison between heat and work, to be undertaken with reference to the divergent assumption that the production of work is not only due to an alteration in the distribution of heat, but to an actual consumption thereof; and inversely, that by the expenditure of work, heat may be produced.”

Rudolf Clausius (1822–1888) German mathematical physicist

First Memoir.
The Mechanical Theory of Heat (1867)

“The steam-engine having furnished us with a means of converting heat into a motive power, and our thoughts being thereby led to regard a certain quantity of work as an equivalent for the amount of heat expended in its production, the idea of establishing theoretically some fixed relation between a quantity of heat and the quantity of work which it can possibly produce, from which relation conclusions regarding the nature of heat itself might be deduced, naturally presents itself. Already, indeed, have many successful efforts been made with this view; I believe, however, that they have not exhausted the subject, but that, on the contrary, it merits the continued attention of physicists… The most important investigation in connexion with this subject is that of S. Carnot.”

Rudolf Clausius (1822–1888) German mathematical physicist

First Memoir. On the Moving Force of Heat and the Laws which may be Deduced Therefrom
The Mechanical Theory of Heat (1867)

“The phenomenon of the production of motion by heat has not been considered from a sufficiently general point of view. We have considered it only in machines… [for which] the phenomenon is… incomplete. It becomes difficult to recognize its principles and study its laws. …[T]he principle of the production of motion by heat… must be considered independently of any mechanism or… particular agent. It is necessary to establish principles applicable not only to steam-engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated.”

Nicolas Léonard Sadi Carnot (1796–1832) French physicist, the "father of thermodynamics" (1796–1832)

Reflections on the Motive Power of Heat (1824)

“[T]he production of heat alone is not sufficient to give birth to the impelling power: it is necessary that there should also be cold; without it, the heat would be useless.”

Nicolas Léonard Sadi Carnot (1796–1832) French physicist, the "father of thermodynamics" (1796–1832)

p, 125
Reflections on the Motive Power of Heat (1824)

“[W]hen a body has experienced any changes, and when after a certain number of transformations it returns to precisely its original state, that is, to that state considered in respect to density, to temperature, to mode of aggregation—let us suppose, I say, that this body is found to contain the same quantity of heat that it contained at first, or else that the quantities of heat absorbed or set free in these different transformations are exactly compensated. This fact has never been called in question. It was first admitted without reflection, and verified afterwards in many cases by experiments with the calorimeter. To deny it would be to overthrow the whole theory of heat to which it serves as a basis. For the rest, we may say in passing, the main principles on which the theory of heat rests require the most careful examination. Many experimental facts appear almost inexplicable in the present state of this theory.”

Nicolas Léonard Sadi Carnot (1796–1832) French physicist, the "father of thermodynamics" (1796–1832)

Reflections on the Motive Power of Heat (1824)

“This law (regarding the theoretical efficiency of heat engines by Mr. Joule), and the law of the maximum efficiency of heat engines, are particular cases of a general law which regulates all transformation of energy, and is the basis of the Science of Energetics.”

William John Macquorn Rankine (1820–1872) civil engineer

Manual of Applied Mechanics, (1858) London and Glasgow : Richard Griffin and Company, p. 630

“The tree bears the intense heat on its head while it lessens the heat for those who take shelter under it.”

Kālidāsa Abhigyanashakuntalam

Abhijñānaśākuntalam (The Sign of Shakuntala)

“My memoirs "On the Mechanical Theory of Heat" are of different kinds. Some are devoted to the development of the general theory and to the application thereof to those properties of bodies which are usually treated of in the doctrine of heat. Others have reference to the application of the mechanical theory of heat to electricity. …Other memoirs… have reference to the conceptions I have formed of the molecular motions which we call heat. These conceptions, however, have no necessary connexion with the general theory, the latter being based solely on certain principles which may be accepted without adopting any particular view as to the nature of molecular motions. I have therefore kept the consideration of molecular motions quite distinct from the exposition of the general theory.”

Rudolf Clausius (1822–1888) German mathematical physicist

Preface (August, 1864)
The Mechanical Theory of Heat (1867)

“Iron and heat are… the supporters, the bases, of the mechanic arts.”

Nicolas Léonard Sadi Carnot (1796–1832) French physicist, the "father of thermodynamics" (1796–1832)

p, 125
Reflections on the Motive Power of Heat (1824)

“Other qualities of bodies are connected with simple energy of motion and energy of position. Such is heat, which we find by experiment can be turned into work. Finding it convertible with energy, we call it a form of energy. ...But as to heat, it is further established by experiment that in this case the energy of motion does really persist as such. Thus a gas consists of molecules flying about with velocity, rotating and vibrating, and so having energy motion. All this energy of motion is what we call heat, and thus heat is a repetition of a known meaning of energy. Again, heat exists between a radiating body and the thing it warms; now the intermediate space is filled by the luminiferous ether, which, being elastic, has in its ultimate parts both energy of motion energy of position. In these forms the heat exists in the space in question.”

William Kingdon Clifford (1845–1879) English mathematician and philosopher

"Energy and Force" (Mar 28, 1873)

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