Today, Louis Pasteur is prepared to see a mirror
image. The University of Houston's College of
Engineering presents this series about the machines
that make our civilization run, and the people
whose ingenuity created them.
We remember Louis Pasteur
for showing us how to kill germs in milk. Long
before that, he did something else which, by
itself, would've made him one of the greats in
When he was 25, Pasteur became puzzled by two salts
with peculiar optical properties. A solution of
tartaric acid in water bent polarized light. But
racemic acid had exactly the same chemical makeup
as tartaric acid. And his racemic acid solution did
nothing at all to polarized light.
Pasteur pored over the two salts -- supposedly
identical. He picked at the racemic acid crystals.
Then, suddenly, he saw it! There were two kinds of
crystals. One looked the same as tartaric acid. The
other was its mirror image.
He separated the crystals into two piles. Then he
made solutions of each. When he shone polarized
light through them, one bent the light clockwise,
the other bent it counterclockwise.
The process he'd used to make tartaric acid had
left all its crystals right-handed. But both kinds
of crystals turned up in his racemic acid. They
canceled each other out.
The molecular structure of the salts had a mirror
image. Just as you can't use a right glove on your
left hand, you couldn't rotate one shape into the
other. The same arrangement of the same atoms gave
two different molecules. Chemists call this
property "chirality." It comes from the Greek word
When the young Pasteur announced his discovery, an
aging Jean Baptiste Biot showed up from the French
Academy of Science. "Show me!" he demanded.
Together, Pasteur and Biot ran the experiment. When
it worked, Biot seized Pasteur's hand and, in a
voice charged with emotion, said, "I have loved
science so much during my life, that this touches
my very heart."
Of course the discovery reached far beyond those
salts. The fact that some molecules can be left or
right-handed helps set the complex rules of
Chance played its part in Pasteur's discovery. He'd
prepared his racemic acid on a cool window sill. In
a warmer climate that process wouldn't have given
him both left and right-handed molecules. That only
happens below 79° F.
Pasteur himself said something very important about
that. It was, " ... chance favors only the prepared
We've said before in this series that invention is
a form of recognition. It is seeing a thing out of
context. Pasteur changed chemistry just because his
mind was prepared to receive an unexpected idea.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
Roberts, R.M., Serendipity: Accidental
Discoveries in Science. New York: John Wiley
& Sons, Inc., 1989, Chapter 12.
Kempe, D.S. and Vellaccio, F., Organic
Chemistry. New York: Worth, 1980, pp.
Roberts adds a sobering note about chirality.
Thalidomide, which created so many birth defects in
the 1950s, is chiral. One form is a harmless
morning-sickness suppressant. The mirror image is
an active mutagen. The drug was put on the market
with both forms present, and it did untold damage.
For more on chiral molecules, see Episode 1181.
Picture of Pasteur from Munsey's Magazine,
The Engines of Our Ingenuity is
Copyright © 1988-1997 by John H.
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