The Science of Made Up Substances

http://en.wikipedia.org/wiki/Phlogiston

phlogiston
In 1667, Johann Joachim Becher published his Physical Education, which was the first mention of what would become the phlogiston theory.

The theory holds that all flammable materials contain phlogiston, a substance without colour, odour, taste, or mass that is liberated in burning. Once burned, the "dephlogisticated" substance was held to be in its "true" form, the calx.

In general, substances that burned in air were said to be rich in phlogiston; the fact that combustion soon ceased in an enclosed space was taken as clear-cut evidence that air had the capacity to absorb only a definite amount of phlogiston. When air had become completely phlogisticated it would no longer serve to support combustion of any material, nor would a metal heated in it yield a calx; nor could phlogisticated air support life, for the role of air in respiration was to remove the phlogiston from the body. Thus, phlogiston as first conceived was a sort of anti-oxygen.

Phlogiston remained the dominant theory until Antoine-Laurent Lavoisier showed that combustion requires a gas which has weight (oxygen), which could be measured by means of weighing closed vessels.


luminiferous aether
In Greek mythology, Aether is one of the Protogenoi, the first-born elemental gods. He is the personification of the "upper sky," space, and heaven, and the elemental god of the "Bright, Glowing, Upper Air." He is the pure upper air that the gods breathe, as opposed to the gloomy lower air of the Earth, which mortals breathe.

Plato's Timaeus posits the existence of a fifth element (corresponding to the fifth remaining Platonic solid, the dodecahedron) called quintessence, of which the cosmos and all celestial bodies are made.

Aristotle included aether in the system of the classical elements of Ionic philosophy as the "fifth element" (the quintessence), on the principle that the four terrestrial elements were subject to change and moved naturally in straight lines while no change had been observed in the celestial regions and the heavenly bodies moved in circles. In Aristotle's system aether had no qualities (was neither hot, cold, wet, or dry), was incapable of change (with the exception of change of place), and by its nature moved in circles.

In order to explain the properties of light, Newton's Opticks (1704) postulated an "Aethereal Medium" transmitting vibrations faster than light, by which light, when overtaken, is put into "Fits of easy Reflexion and easy Transmission", which caused refraction and diffraction.

Einstein's theories of relativity have since made the aether theory largely an irrelevancy.

impetus
A certain modification to the Aristotelian theory of motion preceded Galileo, in which motion was described as a transmission of "impetus." Objects would eventually run out of impetus, which explained why a thrown stone would eventually come back down.

dark matter
In astronomy and cosmology, dark matter is hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter. Dark matter is postulated to explain the flat rotation curves of spiral galaxies and other evidence of "missing mass" in the universe. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter and dark energy account for the vast majority of the mass in the observable universe.

energy and other quantities
While things like phlogiston may sound silly from our modern view, they are not that far off from contemporary beliefs. Dark matter has already been mentioned as one gap-filler in modern physics. But much more essential concepts such as "energy" perform the same role. It represents a quantity we use in certain bookkeeping on particles, but energy is not an essence that physicists have ever observed in its pure element (nor could they). The same goes for charge and other quantities. They serve us in the same role that phlogiston and impetus once served our predecessors: as a theoretical representation of how the state of objects changes.