|Swedish chemist Torbern Bergman's 1775 listing of the symbols for phlogiston and the matter of heat, categorized as two types of "earths", of which he listed 15 types, as contrasted with the acids (25), alkalis (3), and metallic calces (16). |
called "delta", is the fourth letter of the Greek alphabet, capital form:
It is possible, etymologically, give the above ordering, noting that the prefix "ab-" equates to water (or father), that the use of the delta symbol for heat, as well as the ordering of the first four letters of the Greek, and in turn English alphabet, derive from the Heliopolis creation myth (3100BC), according to which the creation of the universe actuated as (a) flood (water, i.e. Nile river flood), (b) land (perceptual mountains rising out of the receding Nile flood at day 150), and (c) fire (sun or Ra) born out of the land, i.e. pyramid.
and can either mean "heat", if written over a reaction arrow:
meaning that the reaction under discussion is being heated; or "change" in a variable, say X:
on going from an initial state Xi, to a final state Xf, e.g. ΔH, ΔG, or ΔS.
The triangle symbol has long been associated with heat or fire.  However, there seems to be no agreed upon etymology of this symbollic use. One of the more intuitive reasons that the delta symbol “Δ” is the fire symbol is that the shape of any common nightly campfire will often take the shape of a triangle, pyramid, or delta, such as shown adjacent.
One of the earliest documented uses of the triangle shape representation for heat, fire, or warmth traces back to the Greeks. The Greek classical elements are:
Greek philosopher Plato mentions the elements as of pre-Socratic origin, a list created by the Ionian philosopher Empedocles (ca. 450 BC).
|Left: Step pyramid (2,800 BC) out of which the dead were said to have arisen at the rise of the sun, carried by a fiery bird on its head out of the tip of the mound. Center: Heat, fire, or light rising out of the benben stone at the tip of one of the pyramids at gaza out of which Ra the sun god arose into the sky. Right: a campfire, making the shape of a pyramid or triangle, the symbol of fire or heat.|
In the ancient Egyptian city of Heliopolis, the central location of the sun birth story, the tip of the pyramid was said to be a type of sacred stone called the "benben stone" shaped as a triangle (Δ), pictured adjacent, made out of gold or petrified semen, out of which, at each sunrise, a bird called the "bennu", often depicted as a heron, carried the sun into the sky on its daily cycle. This fiery bird was recorded the Greek historian Heroditus as the phoenix. 
The Greeks, beginning in 1,800 BC, later re-incorporated this pyramid sun-birth logic with the phenomenon of heat-fire (lava) release in local island volcanic eruptions, such as the c. 1400 BC eruption of Thera (Santorini), picture below, that devastated a thriving culture on that island. To explain these various types of natural heats, Empedocles says that many fires burn beneath the earth, that the solar fire was born in the bowels of the earth, and that volcanic fire shoots to the heavens and licks the stars.
| Left: Circa 1400 BC volcanic eruption of Thera (Santorini). Center: the "phoenix", the name assigned, by Herodotus, to Ra (bird) carrying the newborn sun out of the Nun (pyramid) following the birth of the earth. Right: German chemist Oswald Croll's 1609 alchemical symbols table, showing the “Δ” listed as the element of fire. |
From these mythological beginnings, the triangle, pyramid, or delta symbol “Δ” as shorthand representation of heat, fire, or light soon became the dominant symbol in science and particularly in chemistry.
In German chemist Oswald Croll's 1609 alchemical symbols table, as shown, the “Δ” is listed as the element of fire .  Prior to this, it seems, the Delta symbol has often been associated with heat or fire and in modern chemistry, when written over a reaction arrow, signifies that a chemical reaction occurs with the addition of heat or energy, such as shown below:
The triangle symbol representation of heat or fire, etymologically, seems to trace back to early Greek philosophers, and their 1,000 BC analysis of heat released from volcanoes in relation to the heat of the sun, and before that to ancient Egypt proto-scientists, with their 5,000 BC thoughts on the birth of the sun out of tips of pyramids.
The use of the delta symbol used to represent a heated chemical reaction diagrammatically came of use in the 1757 lectures of Scottish chemist and physicist William Cullen and later, significantly, in the 1775 textbook A Dissertation on Elective Attractions by Swedish chemist Torbern Bergman, who included a chart showing sixty-four reactions drawn in two dimenions.  The following diagram, for instance, is Bergman’s reaction scheme #42, where the triangle in the center of the diagram signifies that the reaction is heated or carried out in “the dry way”:
which shows that “if common salt be subjected to distillation with nitrous acid, the marine acid will rise, and the fossil alkai remain in the retort”. Modeled on this logic, in the decades to follow, chemical reactions began to incorporate the heat symbol above the reaction arrow to signify that the reaction was heated:
In 1825, Scottish physicist James Forbes contributed papers to the Edinburgh Philosophical Journal anonymously under the signature "Δ", likely in being a pseudonym for a person who is interested in heat.  Between 1836 and 1844 he published in the Trans. Roy. Soc. Ed. four series of "Researches on Heat".
In the 1850s, the "θ∆ics" symbol was used, by William Thomson, James Maxwell, and Peter Tait, as shorthand for the newly forming science of thermo-dynamics.
2. Opsopaus, John. (1998). “The Ancient Greek Esoteric Doctrine of the Elements: Fire”, University of Tennessee.
3. Croll, Oswald. (1608). "Alchemical Symbols" in Basilica Chymica.
4. Littleton, Scott C. (2002). Mythology - the Illustrated Anthology of World Myth and Storytelling. San Diego, Ca.: Thunder Bar Press.
5. Torbern Bergman's 1775 Reaction Diagrams - Department of Chemistry at Texas A&M University.
6. Proceedings of the Royal Society of London, vol. XIX, p ii.
7. Bergman, Torbern. (1775). A Dissertation on Elective Attractions. London: Frank Cass & Co.