|French chemist Antoine Lavoisier (center) shown conducting animal heat combustion experiments on his assistant, French chemist Armand Seguin; his wife chemist Marie-Anne Paulze (Madame Lavoisier) seated. |
In circa 1770s, the "phlogiston theory of animal heat", a spin-off German chemist Georg Stahl’s 1718 phlogiston theory of burning, was in full swing; although, towards the end of the decade, experimental evidence was beginning to cast doubt on this theory.
In circa 1780, French chemist Antoine Lavoisier and several of his associates began to carry out various experiments in the area of metabolic heat generation—in the midst of which was the upgrade of the phlogiston theory of heat to that of the caloric theory of heat; a theory which, in turn, was applied to explain the generation of heat in animals.
The germ of the new combustion theory of animal heat, according to American animal heat historian Everett Mendelsohn, was clearly present in Lavoisier’s early papers on respiration and combustion; the transition took place in a series of studies first by Lavoisier and Pierre Laplace and later by Lavoisier with the assistance of French chemist and physiologist Armand Seguin. 
In Lavoisier’s 1785 memoir “Les Alterations a l’Air” he suggested that vital air (oxygen + caloric) extracted some carbonaceous substance from the lungs during respiration. 
Sometime thereafter, Seguin expanded on this view to argue that a chemical combination of hydrogen with carbon (hydrogene carbone) was released by the blood into the lungs. 
In the decades to follow, modifications to the combustion theory of animal heat ensued, including: Bernard (moved the stove to the tissue); Justus Liebig (called the blood the stove, and food the fuel); Claude Berthollet (originated the theory that bodily heat is due to molecular as well as chemical changes); Marshall (respiration is the functional source of heat); Dutrochet (overthrew the combustion theory, by his discovery that vital heat of vegetables is greatest when oxygen is being exhaled); John Dalton (argued that higher bodily temperatures in animals is due to a variety of nutritive chemical processes). 
Mechanical equivalent of heat
A consequence of the Leibig school of thought, a logic taught in German universities in the early 19th century, was that when blood is warmed by combustion of carbon from food, a consequence is the changing of red arterial blood coming from the heart into the darker venous blood that carries the ashes back to the lungs in the form of carbon dioxide.  This model was taught to Liebig's student Robert Mayer while in medical school in 1838. One question from Mayer's state medical exam in Stuttgart had been: “what influence does continued damp and warm weather exert on a person’s state of health?” Mayer’s answer, according to his records, was: “the blood becomes richer in carbon, darker, and the difference between red and black blood is less.”
In 1840, while a ship’s physician aboard a Dutch merchantman ship destined for a round trip to Java, Mayer employed this "darker blood = more combustion" model when examining patients blood in the "warmer" tropical climates, and on the observation of the bright redness of the blood intuited his first formulation of the mechanical equivalent of heat.
1. Gray, G. H. (1876). “Animal Heat and its Reduction by the Use of Gelsemium”, Medical Record, 11: 378-80.
2. Mendelsohn, Everett I. (1964). Heat and Life: the Development of the Theory of Animal Heat (pg. 146-61). Harvard University Press.
3. Levere, Trevor H. (2001). Transforming Matter: a History of Chemistry from Alchemy to the Buckyball (pg. 98). JHU Press.
4. Lavoisier, Antoine. (1785). “The Alterations in the Air” (“Les Alterations a l’Air”), Publisher, pg. 573.
5. Seguin, Armand. (date). "Observations Generales", Publisher, pgs. 469-70.
6. (a) Sketch by Madame Lavoisier, first published in E. Grimauzi, Lavoisier 1743-1794, Paris, 1889, Opposite, pg. 128.
(b) Marie-Anne Pierrette Paulze – Wikipedia.
7. Baeyer, Hans C. von (1999). Warmth Disperses and Time Passes - the History of Heat (pg. 20-22). New York: The Modern Library.