Phlogiston theory

The alchemist and physician J. J. Becher proposed the phlogiston theory

The phlogiston theory (from the Ancient Greek φλογιστόν phlŏgistón "burning up", from φλόξ phlóx "fire"), first stated in 1667 by Johann Joachim Becher, is a defunct scientific theory that posited the existence of a fire-like element called "phlogiston" that was contained within combustible bodies, and released during combustion. The theory was an attempt to explain processes such as combustion and the rusting of metals, which are now understood as oxidation.

Contents

History

In 1667, Johann Joachim Becher published his Physical Education, which was the first mention of what would become the phlogiston theory. Traditionally, alchemists considered that there were four classical elements: fire, water, air, and earth. In his book, Becher eliminated fire and air from the classical element model and replaced them with three forms of earth: terra lapidea, terra fluida, and terra pinguis.[1][2] Terra pinguis was the element which imparted oily, sulphurous, or combustible properties.[3] Becher believed that terra pinguis was a key feature of combustion and was released when combustible substances were burned.[1] In 1703 Georg Ernst Stahl, professor of medicine and chemistry at Halle, proposed a variant of the theory in which he renamed Becher's terra pinguis to phlogiston, and it was in this form that the theory probably had its greatest influence.[4]

Phlogiston theory was a 17th century attempt to explain oxidation processes such as fire and rust.

Theory

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

"Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" when burned; "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."[5] Thus, phlogiston as first conceived was a sort of anti-oxygen in today's terms.

Joseph Black's student Daniel Rutherford discovered nitrogen in 1772 and the pair used the theory to explain his results. The residue of air left after burning, in fact a mixture of nitrogen and carbon dioxide, was sometimes referred to as "phlogisticated air", having taken up all of the phlogiston. Conversely, when oxygen was first discovered it was thought to be "dephlogisticated air", capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.[6]

Challenge and demise

Eventually, quantitative experiments revealed problems, including the fact that some metals, such as magnesium, gained weight when they burned, even though they were supposed to have lost phlogiston. Mikhail Lomonosov attempted to repeat Robert Boyle's celebrated experiment in 1753 and concluded that the phlogiston theory was false. He wrote in his diary:

"Today I made an experiment in hermetic glass vessels in order to determine whether the mass of metals increases from the action of pure heat. The experiment demonstrated that the famous Robert Boyle was deluded, for without access of air from outside, the mass of the burnt metal remains the same."

Some phlogiston proponents explained this by concluding that phlogiston had negative weight; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it was lighter than air. However, a more detailed analysis based on the Archimedean principle and the densities of magnesium and its combustion product shows that just being lighter than air cannot account for the increase in mass.

Still, phlogiston remained the dominant theory until Antoine-Laurent Lavoisier showed that combustion requires a gas that has weight (oxygen) and could be measured by means of weighing closed vessels. The use of closed vessels also negated the buoyancy which had disguised the weight of the gases of combustion. These observations solved the weight paradox and set the stage for the new caloric theory of combustion.

During the eighteenth century, as it became clear that metals gained weight when they were oxidized, phlogiston was increasingly regarded as a principle rather than a material substance.[7] By the end of the eighteenth century, for the few chemists who still used the term phlogiston, the concept was linked to hydrogen. Joseph Priestley, for example, in referring to the reaction of steam on iron, whilst fully acknowledging that the iron gains weight as it grabs oxygen to form a calx, iron oxide, iron also loses “the basis of inflammable air (hydrogen), and this is the substance or principle, to which we give the name phlogiston.”[8] Following Lavoisier’s description of oxygen as the oxidisng principle, hence its name (oxus = sharp, acid; geneo = I beget), Priestley described phlogiston as the alkaline principle.[9]

In some respects, the phlogiston theory can be seen as the opposite of the modern "oxygen theory". The phlogiston theory states that all flammable materials contain phlogiston that is liberated in burning, leaving the "dephlogisticated" substance in its "true" calx form. In the modern theory, on the other hand, flammable materials (and unrusted metals) are "deoxygenated" when in their pure form and become oxygenated when burned. However, the first part of the old theory requires that phlogiston has weight (since ashes weigh less), but the second requires that it have no weight or negative weight, since corroded metals weigh the same or more, depending on whether or not they are allowed to corrode in sealed chambers.

Enduring aspects

Phlogiston theory allowed chemists to bring explanation of apparently different phenomena into a coherent structure: combustion, metabolism, and formation of rust. The recognition of the relation between combustion and metabolism was a forerunner of the recognition that the metabolism of living creatures and combustion can be understood in terms of fundamentally related chemical processes.

References

  1. ↑ 1.0 1.1 Bowler, Peter J (2005) (Online). Making modern science: A historical survey. Chicago: University of Chicago Press. 
  2. ↑ Becher, Physica Subterranea p. 256 et seq.
  3. ↑ Brock, William Hodson (1993) (Hardback). The Norton history of chemistry (1st American ed.). New York: W. W. Norton. ISBN 0393035360. 
  4. ↑ Mason, Stephen F., (1962). A History of the Sciences (revised edition). New York: Collier Books. Ch. 26.
  5. ↑ James Bryan Conant, ed. The Overthrow of Phlogiston Theory: The Chemical Revolution of 1775–1789. Cambridge: Harvard University Press (1950), 14.
  6. ↑ "Priestley, Joseph". Spaceship-earth.de. http://www.spaceship-earth.de/Biograph/Priestley.htm#4A4FW. Retrieved 2009-06-05. 
  7. ↑ For a discussion of how the term phlogiston was understood during the eighteenth century, see: James R Partington & Douglas McKie; “Historical studies on the phlogiston theory;” Annals of Science, 1937, 2, 361-404; 1938, 3, 1-58; and 337-71; 1939, 5, 113-49.
  8. ↑ Joseph Priestley; Considerations on the doctrine of phlogiston, and the decomposition of water; Philadelphia, Thomas Dobson, 1796; p.26.
  9. ↑ Joseph Priestley; Heads of lectures on a course of experimental philosophy; London, Joseph Johnson, 1794.