Monday, July 9, 2012
Phlogiston, for theToastmasters
This blog is from a speech given at Toastmasters on July 2, 2012. It’s about a substance that never really was -- phlogiston! What was it good for? The phlogiston story illustrates how a wrong idea can lead to a useful concept.
Scientific knowledge begins with working hypotheses, tentative theories, that seem to fit what’s been observed. Between 1650 and 1750 there were two theories of combustion or burning. The first working hypothesis was the phlogiston theory.
The phlogiston theory stated that all combustible substances contained an invisible, odorless, weightless material called phlogiston. During burning, the phlogiston was released. (Picture the phlogiston like a flame.) After combustion, a dephlogisticated substance remained. For instance, when magnesium burned it released the phlogiston it contained to the air and left a dephlogisticated substance, magnesium’s true nature, called the calx. Air could hold only so much phlogiston, which explained why a burning candle extinguished in a closed jar.
It was also noted that a mouse died if left in a closed jar. Rather than attribute the mouse’s death to the loss of its Life Force, the phlogistians understood that the role of air in respiration was to remove phlogiston from the body. More importantly, the phlogistians realized that apparently different phenomena, the process of combustion and the respiration of living organisms, were fundamentally similar because both involved the release of phlogiston.
Joseph Priestley, minister and chemist, was a staunch supporter of the phlogiston theory. As a minister, Priestley helped to found Unitarianism in England. As a chemist, Priestley discovered a gas that kept a candle burning or a mouse alive longer than ordinary air. He named his new gas dephlogisticated air because it could suck up more phlogiston than ordinary air.
The Antiphlogiston theory, the oxygen theory, was the inverse of the phlogiston theory. Instead of phlogiston leaving a burning substance, the oxygen theory stated that a substance in the air, oxygen, combined with the burning substance to form a third substance. In the above example magnesium would combine with oxygen to form magnesium oxide, formerly seen as the calx. Another example, when the carbon in fossil fuels burns, it combines with oxygen to form carbon dioxide.
Don’t both the phlogiston theory and the oxygen theory seem plausible? Maybe the phlogiston theory even more so because it doesn’t postulate a third substance, the oxide.
The phlogiston theory had its problems from the get-go. For instance, the calx left after magnesium burned weighed more than the original magnesium. Priestley, who was a better minister than chemist, was the last phlogiston holdout against the Antiphlogistians.
Then other chemists like Lomonosov and Lavoisier started weighing compounds and gases in closed vessels before and after combustion. Something wasn’t right; phlogiston had a negative mass. Of course, something that weighs less than nothing can’t exist. Thus ended the hundred-year reign of the phlogiston theory. The Antiplogistians, the oxygen guys, triumphed. Priestly, ironically, is credited with the discovery of oxygen, the modern name for dephlogisticated air.
In the 21st century, some of us follow Priestley’s religion, but nobody believes in phlogiston anymore. However, the phlogiston theory, although wrong, produced the very important scientific insight I alluded to earlier: that is, combustion and respiration in living organisms were the same process. We now see both as processes combining a substance with oxygen. The idea that respiration in our bodies is not due to a mystic Life Force – it’s just chemistry – paved the way to modern medicine and pharmacy.