“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.”

Reflections on the Motive Power of Heat (1824)

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Nicolas Léonard Sadi Carnot19

French physicist, the "father of thermodynamics" (1796–1832) 1796–1832

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“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)

“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 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 (1822–1888) German mathematical physicist

First Memoir.
The Mechanical Theory of Heat (1867)

“Science Of Energetics. Although the mechanical hypothesis just mentioned may be useful and interesting as a means of anticipating laws, and connecting the science of thermodynamics with that of ordinary mechanics, still it is to be remembered that the science of thermodynamics is by no means dependent for its certainty on that or any other hypothesis, having been now reduced, to a system of principles, or general facts, expressing strictly the results of experiment as to the relations between heat and motive power. In this point of view the laws of thermodynamics may be regarded as particular cases of more general laws, applicable to all such states of matter as constitute Energy, or the capacity to perform work, which more general laws form the basis of the science of energetics, — a science comprehending, as special branches, the theories of motion, heat, light, electricity, and all other physical phenomena.”

William John Macquorn Rankine (1820–1872) civil engineer

Source: A Manual of the Steam Engine and Other Prime Movers (1859), p. 31

“[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)

“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)

“The production of motion in steam-engines is always accompanied by… the re-establishing of equilibrium in the caloric; that is, its passage from a body in which the temperature is more or less elevated, to another in which it is lower.”

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

p, 125
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

“Philosophers are generally persuaded, that the sensations of heat and cold are occasioned by the presence or absence, in degree, of certain principle or quality denominated fire or heat… It is most probable, that all substances whatever contain more or less of this principle. Respecting the nature of the principle, however, there is a diversity of sentiment : some supposing it a substance, others a quality, or property of substance. Boerhaave, followed by most of the moderns, is of the former opinion; Newton, with some others, are of the latter; these conceive heat to consist in an internal vibratory motion of the particles of bodies.”

John Dalton (1766–1844) English chemist, meteorologist and physicist

Meteorological Observations and Essays: Mit Tabellen, 1834 p. 18

“A given phenomenon, today considered random, may tomorrow be considered determined because its causes will have been unraveled by thorough and specific study.”

Pierre-Paul Grassé (1895–1985) French zoologist

Grassé, Pierre Paul (1977); Evolution of living organisms: evidence for a new theory of transformation. Academic Press, p. 279
Evolution of living organisms: evidence for a new theory of transformation (1977)
Context: Exceptional, unforeseeable, or even inexplicable phenomena would hence be fortuitous. these very vague adjectives too often have a merely circumstancial meaning. A given phenomenon, today considered random, may tomorrow be considered determined because its causes will have been unraveled by thorough and specific study.
Biologists, whose task is not to seek moral causes or intentions, must first of all make sure that so-called random facts really are random facts; they must constantly keep in mind Poincare's (1912b, p. 65) famous phrase: "Chance is only the measure of our ignorance."

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Nicolas Léonard Sadi Carnot19

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