Louis Pasteur was born today, December 27, in 1822. Pasteur's early work as a chemist resolved a problem concerning the nature of tartaric acid. A solution of this compound derived from living things (specifically, wine lees) rotated the plane of polarization of light passing through it. The mystery was that tartaric acid derived by chemical synthesis had no such effect, even though its reactions were identical and its elemental composition was the same.
Upon examination of the minuscule crystals of synthetic sodium ammonium tartrate, Pasteur noticed that the crystals came in two asymmetric forms that were mirror images of one another. Tediously sorting the crystals by hand gave two forms of the compound: solutions of one form rotated polarized light clockwise, while the other form rotated light counterclockwise. An equal mix of the two had no polarizing effect on light. Pasteur correctly deduced that the molecule in question was asymmetric and could exist in two different forms that resemble one another as would left- and right-hand gloves, and that the organic form of the compound consisted purely of the one type. This was the first demonstration of chiral molecules.
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