|Fire||350BC|| Aristotle: there are four elements out of which all is made: earth, air, water, fire. Each of the four earthly elements has its natural place; the earth at the center of the universe, then water, then air, then fire. When they are out of their natural place they have natural motion, requiring no external cause, which is towards that place; so bodies sink in water, air bubbles rise up, rain falls, flame rises in air. |
|Sulphur||c.790|| Geber: to further explain the phenomena of combustion and metallic properties, metals (copper, iron, gold, etc.) were said to be formed out of two elements: sulphur, ‘the stone which burns’, which characterized the principle of combustibility, and mercury, which contained the idealized principle of metallic properties. This evolved into the Arabic three principles: sulphur giving flammability or combustion, mercury giving volatility and its opposite, and salt giving solidity. |
|Sulphur||1524|| Paracelsus: in an attempt at a unification of Aristotle’s four element theory and Geber’s three principles, reasoning that the latter appeared in the former’s bodies, justifying this by recourse to a description of how wood burns in fire: Mercury included the cohesive principle, so that when it left in smoke the wood fell apart. Smoke represented the volatility (the mercury principle), the heat-giving flames represented flammability (sulphur), and the remnant ash represented solidity (salt). |
|Terra pinguis||1669|| Johann Becher: in modification on Paracelsus’ theory, it was argued that the constituents of bodies are air, water, and three types of earth: terra fluida, the mercurial element, which contributes fluidity and volatility; terra lapida, the solidifying element, which produces fusibility or the binding quality; and terra pinguis, the fatty element, which gives material substance its oily and combustible qualities. These three earths correspond with Geber’s three principles. A piece of wood, for instance, is composed of ash and terra pinguis; when the wood is burnt, the terra pinguis is released, leaving the ash. In other words, in combustion the fatty earth burns away. |
|Phlogiston||1703|| ϕ|| Georg Stahl: on a modification of Becher’s three earths theory, terra pinguis was renamedas phlogiston, from the Ancient Greek phlogios for ‘fiery’, which was said to be the “matter and principle of fire, and not fire itself” that escapes from burning bodies with a rapid whirling motion, and is contained in all combustible bodies and also in metals, which can be burnt to “calces”.|
In phlogiston theory, calx is a residual substance, sometimes in the form of a fine powder, that is left when a metal or mineral combusts or is calcinated due to heat. Calx, especially of a metal, is now properly defined as an ‘oxide’, i.e. a chemical compound containing an oxygen atom and other elements. In the phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in the process of combustion. The phlogiston was said to have the property that it could be restored to the original substance by supplying a replacement phlogiston from any material containing it, such as oil, wax, charcoal, or soot, which was thought to be nearly pure phlogiston.
Phlogiston was thought of as a material entity, sometimes considered as the matter of fire, sometimes as a dry earthy substance (soot), sometimes as a fatty principle, such as in sulphur, oils, fats, and resins, and sometimes as invisible particles emitted by a burning candle; contained in animal, vegetable, and mineral bodies. It could be transferred from one body to another.
|Caloric||1787||Antoine Lavoisier: a difficulty in Stahl’s theory was that calx becomes lighter when reduced to a metal by taking up phlogiston. If the phlogiston had mass, then when added to something, it should make it heavier.|
To remedy this issue, an experiment (Lavoisier 1786) proved that the matter of heat is weightless. Specifically, phosphorus burnt in air in a closed flask, with no appreciable change in weight.
A new particle was proposed, called “caloric”, replacing phlogiston, defined as ‘the repulsive cause, whatever that may be, which separated the particles of matter from each other.’ In this sense, heat was said to be an elastic, fluid-like, substance (called caloric), whereby the sensation of warmth was the accumulation of this substance in the interstices of the particles of matter and that, to explain Boerhaave’s law (1720), the more caloric particles a body had in its pores the more it would expand in volume; and conversely when caloric particles were taken out of a body, the body would contract.
|Motion||1798||Benjamin Thomson: heat produced, via friction, in the boring of a cannon, which was used to make water boil in 2.5 hours time, gave “farther insight into the hidden nature of heat; and to enable us to form some reasonable conjectures respecting the existence, or non- existence, of an igneous fluid.”|
“What is heat? Is there anything as igneous fluid? Is there anything that can with propriety be called caloric? That heat generated by friction [in the boring experiments] appeared, evidently, to be inexhaustible, [it] cannot possibly be a material substance; … it appears to me to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner heat was excited and communicated in these experiments, except it be MOTION.” 
|Caloric||1824||Carnot: “we shall assume that the quantities of caloric absorbed and emitted in these different transformations compensate each other exactly.” |
|Mechanical equivalent |
|1843||Joule: showed that heat Q and work W were inter-convertible according to a formulaic expression: “the grand agents of nature are … indestructible; and that wherever mechanical force is expended, an exact equivalent of heat is always obtained.” |
|?||1850||Clausius: stated an "expression needed" to amend Carnot's view (above) that "no permanent change occurs in the condition of the working body." |
|2.||Thermodynamic function||1854||Rankine: |
of all uncompensated transformations
|5.||Entropy||1865||Clausius: the symbol S is the "transformation-content" of the working body, but termed "entropy", so to have similarity to the word energy. |
|6.||Clausius (?): entropy of an ideal gas. |
|8.||Entropy||1873||Gibbs: the quantity η is the entropy, dη is the differential of entropy, H is the heat received by the body in passing from one state to another, and t the absolute temperature of a body in a given state.  |
|9.||1875||Carl Gottfried Neumann: |
|12.||Gibbs entropy||1901||Gibbs: |
|13.||c.1906||The expression S = k ln W is said to have been introduced by Planck.|
|16.||c.1935||Chiseled onto Boltzmann's tombstone in circa 1935 (as pictured adjacent).|
|17.||Prigogine entropy||1945|| Prigogine: (a) The entropy of a system is an extensive property: if the system consists of several parts, the total entropy is equal to the sum of the entropies of each part.|
(b) The change in entropy can be split into two parts: denoting deS as the flow of entropy, due to interactions with the exterior, and diS the contributions due to changes inside the system. 
|18||Shannon entropy||1948||Shannon: “we shall call H = – Σ pi log pi the entropy of a set of probabilities p1, ..., pn.” |
|19.||Von Neumann entropy||1955||John Neumann: |
|21.||Black hole entropy||1972||Jacob Bekenstein and Stephen Hawking: |
|23.||Tsallis entropy||1988|| Constantino Tsallis: |
|28.||Human entropy||2007||Thims: entropy of a system of ideal human molecules. |