Probably all of us have consumed pasteurized milk. Most of us have heard that he
devised the process so French Navy could keep wine from spoiling on long voyages.
But Pasteur also made many other memorable discoveries. One of them was in
the chemistry and physics of optical isomers. This is from www.uh.edu/engines/epi604.htm

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
for hand.

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 chemical
structure.

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
mind."

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.

If I ever knew, I've forgotten how "racemic" came to mean
"optically inactive". The root is L "racemus" = a cluster of grapes.
The title is "chemical chirality" meaning ability to polarize light,
either to right hand, or to left hand. Racemic means a mixture
of both, so no nandedness can be seen.