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Physiology and the whole organism  ·  Article

Heat as motion versus caloric (heat-substance) theory

Two pictures of heat competed in nineteenth-century physics. One treats heat as a substance, caloric or Wärmestoff, that bodies hold and release. The other treats heat as motion of small parts. Helmholtz argued for the second in 184712.

Earliest held
1847, Helmholtz, H.
Most discussed in
The Fitness of the Environment, 1913
In the library
40 passages in 9 works
Rewritten
2026-10-30
18401865189019151940196519902015Helmholtz, H., 18471Helmholtz, H., 18472James, W., 18903James, W., 18904Leduc, S., 19115Henderson, L. J., 19136Thompson, D. A. W., 19927Wallace DC, 20138
01

Heat as a stored substance

Helmholtz, in Über die Erhaltung der Kraft (1847), described the older view of chemical heat. Heat from chemical processes had been explained as the release of Wärmestoff held latent in the combining bodies2. Helmholtz reports that this led to a law, partly confirmed by experience, that the same substances combining to the same products always yield the same heat, whatever the order or intermediate steps2. His own account differs from this one.

02

Helmholtz and heat as motion

Helmholtz did not claim to know what kind of motion heat is. He wrote that all clues were missing for deciding what kind of motion it is1. For his purpose it was enough to see that heat phenomena can be understood as motions1. He held that such motions in atoms could change molecular forces, bringing expansion and changes of state1. On his view, heat produced in chemical processes would be a quantity of living force, not a released substance2.

03

Words that outlasted the theory

James, in The Principles of Psychology (1890), used heat as an example of explanation by substitution. If heat is thought of as motion, whatever is true of motion holds for heat, and motion is far more familiar than heat3. He also quoted an author who compared mental faculties to caloric, electricity and gravity4. Those words name unknown causes of grouped phenomena, a convenient way of classifying facts4. The quoted author warns against turning such names into substantial entities4.

04

Calories and latent heat in biology

Later biological texts keep the terms without the substance. Leduc (1911) called a steam engine a transformer changing caloric energy into mechanical work5. Henderson (1913) gave the latent heat of evaporation of water as about 536 calories per gram, counted as heat needed to boil away that gram6. Thompson recorded an explanation of Brownian movement as due either to calorical changes in the fluid or to some obscure chemical action between the particles and the fluid, indirectly promoted by heat. Soon after, Wiener ascribed it to molecular action7. Wallace (2013) treats calories as a unit of heat and links tight coupling to lower heat per unit of ATP8.

SourcesEach quotation was checked word for word against the passage it opens.
  1. auck ist fitr unseren Zweck die Einsickt der Moglichkeit hinreichend, dass die Warmeersckei- nungen als Bewegungen gefasst werden konnen.Helmholtz, H., 1847 · Über die Erhaltung der Kraft · open at passage 47
  2. Man hat dieselbe bisher fiir ein Freiwerden von WarmestofF erklart, welcher in den sich. verbindenden Korpern latent vor- lianden sei.Helmholtz, H., 1847 · Über die Erhaltung der Kraft · open at passage 48
  3. Think of heat as motion, and whatever is true of motion will be true of heat ; but we have had a hundred experiences of motion for every one of heat.James, W., 1890 · The Principles of Psychology, Vols. 1-2 · open at passage 2495
  4. This method is precisely the one followed in Physics, where the words caloric, electricity, gravity, designate the unknown causes of certain groups of phenomena.James, W., 1890 · The Principles of Psychology, Vols. 1-2 · open at passage 1404
  5. A steam engine is a transformer of energy, changing caloric energy into mechanical work.Leduc, S., 1911 · The Mechanism of Life · open at passage 253
  6. The latent heat of evaporation of water is approximately 536. There is required, accordingly, as much heat to boil away one gram of water as to raise the temperature of 536 grams through 1° centigrade.Henderson, L. J., 1913 · The Fitness of the Environment · open at passage 139
  7. Nearly fifty years after Brown wrote, it was said to be ‘due, either directly to some calorical changes continually taking place in the fluidThompson, D. A. W., 1992 · On Growth and Form · open at passage 137
  8. Since calories are a unit of heat, such ‘tightly coupled’ individuals generate the minimum heat for the ATP used.Wallace DC, 2013 · Bioenergetics in human evolution and disease: implications for the origins of… · open at passage 23
Linked ideas
Treats the equivalence of heat and mechanical work, the quantitative side of the move from caloric to motion.
Helmholtz's 1847 case for heat as motion belongs to the larger argument that force is conserved.
Concerns how motion turns into heat, which the motion view of heat has to explain.
Henderson uses the latent heats of melting and evaporation of water as properties favourable to organisms.
Wallace links coupling of membrane potential to ATP with how much heat is produced per ATP.
Vitalism and mechanismin tension with
James's caution against turning faculty names such as caloric into substantial entities echoes disputes over vital forces.
Where it is discussedPassages matching Warmestoff, caloric, heat as motion, Warme Bewegung
1913The Fitness of the Environment · Henderson, L. J.20
1915Principles of General Physiology · Bayliss, W. M.10
1847Über die Erhaltung der Kraft · Helmholtz, H.3
1890The Principles of Psychology, Vols. 1-2 · James, W.2
2013Bioenergetics in human evolution and disease: implications for the origins of… · Wallace DC1
1992On Growth and Form · Thompson, D. A. W.1
2023Thoughts from the forest floor: a review of cognition in the slime mould… · Reid CR1
1911The Mechanism of Life · Leduc, S.1
2025Beyond Bioenergetics: Emerging Roles of Mitochondrial Fatty Acid Oxidation in… · Bang S, Choi SH, Jeong SM1