Antoine-Laurent Lavoisier
Adapted from The Ten Most Beautiful Experiments, by George Johnson, Alfred A. Knopf, 2008
Why do some substances burn, while other substances do not? In the 1700’s, Antoine Lavoisier
and most other chemists accepted the notion that some kinds of matter burned because of a
mysterious substance called “phlogiston.” The reason things burned was that they were rich in
phlogiston, and as they were consumed they released this “fire stuff” into the air. Set a piece of
wood aflame and it would stop burning only when its phlogiston was spent, leaving behind a
pile of ash. Wood, it logically followed, was made of phlogiston and ash. Likewise, heating a
metal under an intense flame, a process called “calcination,” left a whitish brittle substance, or
“calx.” Metal was thus composed of phlogiston and calx. The process also worked the other
way around. Calx, it was recognized, resembled the crude ores mined from the ground, which
were refined or reduced by heating them next to a piece of charcoal. The charcoal emitted
phlogiston, which combined with the calx to recover the lustrous metal.
With phlogiston, scientists had a consistent explanation for combustion, calcination, and
reduction. Chemistry suddenly made sense. There was however a problem: the calx left behind
after calcination weighed more than the original metal. How could removing phlogiston leave
something heavier?
The chemists of Lavoisier’s generation had also discovered that there were “different kinds of
air.”
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Noxious, or “fixed air” would extinguish a flame and suffocate a mouse. It also turned
limewater (calcium hydroxide solution) cloudy, forming a white precipitate (calcium
carbonate). But plants thrived in the gas and slowly made it breathable again.
There was another suffocating gas left behind when a candle was burned in a covered
jar. It did not precipitate limewater, and since it was evidently related to combustion, it
was called “phlogisticated” air, from the Greek word for “lifeless.”
Most mysterious of all was a volatile gas emitted when iron filings were dissolved in
dilute sulfuric acid. It was so combustible that it was named “inflammable air.” A
balloon filled with it would float high above the ground.
The question was whether these new airs were elements, or modifications of “normal” air,
produced by adding or subtracting phlogiston.
Lavoisier performed several experiments and found that burning phosphorus to make
phosphoric acid or sulfur to make sulfuric acid indeed left the substances heavier—the same
thing that happened when you burned metals. But what was causing the change? He thought
he knew the answer. Using a large magnifying glass to heat tin that had been sealed inside a
flask, he found that the entire apparatus weighed the same before and after. Slowly opening
the flask he heard air whistle in, and only then was there a gain in weight. Maybe things burned
not because they emitted phlogiston but because they absorbed some kind of air.
Magnifying Glass
If so, then reducing a substance—smelting an ore back into a pure metal—should force the air back out
again. He measured out a calx of lead and placed it, along with a piece of charcoal, on a pedestal in a
basin of water. Then he inverted a bell jar on top. Heating the calx with a magnifying glass, he could see
from the displacement of the water that a gas was coming out. Carefully withdrawing the gas from the
jar, he found that it extinguished flames and precipitated limewater. Fixed air appeared to be a product
of reduction, but was there more to the story than that?
The answer turned out to be in a reddish substance called mercurius calcinatus, or calx of
mercury (also called quicksilver), a very expensive substance sold in French drugstores as a
treatment for syphilis. Like all calxes, it could be produced by heating the pure mercury metal
over an intense flame. But when heated further it changed, against all expectations, back into
quicksilver. In other words, calx of mercury could be reduced without the presence of charcoal.
But what then was supplying the phlogiston? In 1774, Lavoisier and some colleagues from the
French Academy confirmed that caix of mercury could indeed be reduced “without addition,”
losing about one-twelfth of its weight.
Englishman Joseph Priestley was also experimenting with calx of mercury, heating it with a
magnifying glass and collecting the fumes. “What surprised me more than I can well express”
he would later report, “was that a candle burned in this air with a remarkably vigorous flame. I
was utterly at a loss how to account for it.” After finding that a laboratory mouse thrived on the
gas, he tried breathing it himself and said he “felt peculiarly light and easy for some time
afterwards.”
A gas in which the fires of combustion and respiration flourished must be a particularly good
absorber of’ phlogiston, so Priestley named it “dephlogisticated air”—air in its very purest form.
By now Lavoisier was calling the gas expelled by reducing calx of mercury “vital” air, and like
Priestley he thought it was ordinary air in its pristine form. But he had run across a
complication. When he tried reducing mercury calx with charcoal—the old-fashioned way—it
released the same gas that extinguished candies and precipitated limewater. Why would
reducing mercury calx without charcoal produce vital air while reducing it with charcoal
produced suffocating fixed air?
There was one way to find out. He took from his shelves a “flamingo flask” that was round on
the bottom with a long skinny neck which he heated and bent so that it curved down and then
up again.
Heating mercury in a “flamingo flask.” Drawing by Marie Anne Lavoisier
He poured four ounces of pure mercury into the round bottom chamber (A on the diagram) and
set it on a furnace with the neck dipping down into an open trough, also filled with mercury,
and then up into a bell jar. As he heated the liquid mercury in the chamber for twelve days, a
reddish crust appeared on the mercury. He also saw that the mercury in the bell jar had risen,
displacing some of the air that had been absorbed by the calx – about one sixth of its original
volume. When a mouse was put inside a container of the left-over air, it struggled for breath,
and when a candle was plunged into it, it was extinguished as if it had been immersed into
water. But since the gas did not precipitate limewater it had to be “phlogisticated” air, rather
than fixed air.
But what had the burning mercury taken from the air? Skimming off the red crust that had
formed on the metal, Lavoisier heated it until it turned back into pure quicksilver, emitting a
different gas. Exposed to this gas a taper burned with a dazzling splendor. It was a pivotal
moment. Burning mercury absorbed vital air from the atmosphere, leaving behind
“phlogisticated” air. Reducing mercury released the vital air again. He had separated the two
main components of the atmosphere.