Eminent scientists from the seventeenth and eighteenth centuries defended the phlogiston explanation of heat and chemical change. One of the best thinkers of his time, George Ernest Stahl, popularized the idea while a professor at the University of Halle from 1694 to 1716. Phlogiston was an “element” said to be contained in all substances that could be burned. It was often described as “inflammable earth.” Phlogiston was used to explain and predict all things relating to heat and fire. Indeed, Joseph Priestley, considered the father of modern chemistry, went to his grave defending the phlogiston theory. It was thought to be a material that did not just contain heat but was itself the heat. Phlogiston was without color, smell, weight, or taste. When you burned something, you were dephlogisticating it—that is, driving all of the phlogiston out of the material. Often this left behind only ash.

Here is how phlogiston seemed to work: Such chemicals as charcoal and sulfur were thought to be made almost completely of phlogiston. This was because when you burn them there is nothing left except a little ash, which was explained as the impurities in the phlogiston. After all, if you dephlogisticate a material that was made up mostly of some form of the heat itself, there will be little left behind.

Take another illustrative example: If one room was warm and the other cool and you opened a door between them, then the phlogiston, like any gas or liquid, would seek to balance itself between the two rooms. Phlogiston would flow into the cool room and out of the warm, phlogiston-filled room; and since it’s the essence of heat, it would raise the temperature of the cooler room and lower that in the warmer room. Eventually the amount of phlogiston would level out between the two rooms, and they would be the same temperature.

The remarkable thing about this amazing theory was that it seemed to have worked, and it had been used by eminent and respected early scientists for an entire century before it was finally proven wrong. It was not until science progressed to the point that researchers understood the fluid dynamics of the second example and Lavoisier explained the chemical changes that occurred in burning charcoal that the idea of phlogiston died out. This disproving was done by Antoine-Laurent de Lavoisier at the end of the eighteenth century. He did this when he discovered oxygen and determined the actual chemical reactions that occur during combustion. Phlogiston theory was perhaps the most persistent, widespread, and totally wrong mistake made by scientists all through the age when science, as we know it today, was developed.

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