“The conservation of ‘free heat’, in the simple mixture of bodies, is thus independent of all hypotheses no the nature of heat.”

— Antoine Lavoisier (1783), Memoir on Heat (co-author: Pierre Laplace) [9]

“If in a combination or in any change of state whatsoever there is a decrease of free heat, this heat reappears entirely when the substances return to their original state.”

— Antoine Lavoisier (1783), Memoir on Heat (co-author: Pierre Laplace) [9]

“All variations in heat, real or apparent, which a system of bodies undergoes in changing state are reproduced in inverse order when the system returns to its first state.”

— Antoine Lavoisier (1783), Memoir on Heat (co-author: Pierre Laplace) [9]

“Elective affinities, in Lavoisier’s eyes, governed all chemical reactions, from the simplest to the most complex. For him, the physical universe was teeming with continuous combinations and reductions.”

oxygen base + caloric
other base + caloric + inciting caloric

“[In any chemical equation] we must take into consideration the nature of the elements which enter into composition, the different affinities which the particles of these elements exert upon each other, and the affinity which the caloric possesses with them.”

Combustion theory
In 1780, Lavoisier introduced the combustion theory of animal heat.

Lavoisier shown conducting experiments in the 1770s with his solar furnace, an instrument that focused the heat of the sun using lenses which had a diameter of up to 1.32 meters. The lenses were made from curved sheets of glass with the internal space filled by vinegar. This furnace was intended to replace the need for fuel in experiments that needed heat, because of concerns about contamination from the fuel's combustion products. In one experiment, Lavoisier used the furnace to burn diamond in air in a glass jar. By analyzing the combustion products he was able to show that diamond was comprised solely of carbon. [1]

“To know the energies of all these forces, to succeed in giving them numerical values, to calculate them, that is the goal which chemistry ought to propose to itself. Chemistry advances slowly, but it is not impossible that it may reach the goal. Meanwhile we are obliged to content ourselves with general considerations … I only hope that the reader of this memoir will apprehend the possibility of some day applying exact calculations to chemistry. But, before all, certain data must be obtained which will serve as a foundation, and it is to that subject I mean to devote myself.”

Thermodynamics
In 1823, French physicist Sadi Carnot used Lavoisier's conception of caloric to outline a generalized theory of heat-engines, or of the production of motion by heat. In Carnot’s theory, the production of motion by heat was due to the need of bodies in contact to “re-establish equilibrium in the caloric”, such that the fall of the caloric from a hot body, through an intermediate substance, e.g. steam, to a colder body, thus causing expansion (and later contraction) in the “working substance”, was the drive or impelling power of the heat engine.

The subtle erroneous assumption implied in this theory is that in the cycle of the heat engine, in which (a) caloric particles are added to the working substance, (b) the working substance expands, (c) caloric particles are removed from the working substance, and (d) the body contracts to its original state, that “no permanent change occurs in the condition of the working body”.

In the late 1840s, German physicist Rudolf Clausius became aware of Carnot’s theory, through the work of French physicist Émile Clapeyron and English physicist William Thomson, and set out to amend Carnot’s assumption that the condition of the body never changes with the newer logic of the theory of heat in which heat is considered as the energy related to the vibratory motion or kinetic movement of particles, along with the mechanical equivalent of heat, in which the production of work by heat and heat by work are proportional, and thus, subsequently, a portion of heat is consumed by the particles of the working body as they act on each other in a cycle in an irreversible manner. As such, according to Clausius, the condition of the working body does change, and this change shows itself as a consumption of heat in the constituency of the working body as particles act or work on each other in an irreversible manner.

A photo (Ѻ) of Lavoisier’s laboratory.

To quantify this internal work, in an approximate manner, Clausius developed the model of "entropy", in which the intensive quantity of heat dQ, passing into or out of the system, is divided by the extensive measure of absolute temperature T at the surface of the working body, and this quantity, dQ/T, is then said to model the energy workings of the system particles as they act on each other, and was called by Clausius the "transformation-content" of the working body. This was the start of the science of thermodynamics.

“It took them only an instant to cut off that head, but it is unlikely that a hundred years will suffice to reproduce a similar one.”

— Joseph Lagrange (1794), comment made day after Lavoisier’s execution, May 9

“Although he lived only to the age of 51, Lavoisier revolutionized the field of chemistry. He created the first modern table of chemical elements [see: periodic table], recognized the role oxygen plays in the rusting of metals, demonstrated that water—previously considered one of the four fundamental elements—is a compound of hydrogen and oxygen, and asserted that the total weights of the products of a chemical reaction must equal the total weights of the reactants. Yet despite his remarkable importance to modern chemistry, Lavoisier's scientific work was more a hobby than a profession. In fact, because he made his living as a tax collector, his scientific work was relegated to early morning and after-dinner hours.”

— Rebecca Balinski (1996), abstract of English translation of Jean-Pierre Poirier’s 1993 Antoine Laurent de Lavoisier, 1743-1794 [8]

“We must trust to nothing but fact: these are presented to us by nature, and cannot deceive. We ought, in every instance, to submit our reasoning to the test of experiment, and never to search for truth but by the natural road of experiment and observation.”

— Antoine Lavoisier (1789), Elements of Chemistry (pg. xiii-xxxvii) (Ѻ)

“When we begin the study of any science, we are in a situation, respecting that science, similar to that of children; and the course by which we have to advance is precisely the same which nature follows in the formation of their ideas. In a child, the idea is merely an effect produced by a sensation; and, in the same manner, in commencing the study of a physical science, we ought to form no idea but what is a necessary consequence, and immediate effect, of an experiment or observation.”

— Antoine Lavoisier (1789), Elements of Chemistry (pg. xvi)

“We must trust nothing but facts: these are present to us by nature, and cannot deceive. We ought, in every instance, to submit our reasoning to the test of experiment, and never to search for truth but by the natural road of experiment and observation.”

— Antoine Lavoisier (1789), Elements of Chemistry (pg. xviii)

“By the term ‘elements’, we mean to express those simple and indivisible atoms of which matter is composed, it is extremely probable we know nothing at all. about them; but, if we apply the term elements or principles of bodies, to express our idea of the last point which analysis is capable of reaching, we must admit, as elements, all the substances into which we are able to reduce bodies by decomposition.”

— Antoine Lavoisier (1789), Elements of Chemistry (pg. xxiv)