Some molecules, such a glucose, come in different structural forms which are termed enantiomers. These versions of the molecule differ in the ways that atoms are bonded to one another, but are similar in all other ways, such as their physical and chemical properties. It is thus very difficult to distinguish between enantiomers of a molecule since they are almost identical.
Except when exposed to polarized light.
What Biot and others found was that right-handed and left-handed enantiomers of a molecule that had a chiral center (four different groups attached to one carbon atom, for example) bent polarized light in opposite directions. This kind of behavior was unique to these kinds of chiral molecules and is nowadays called optical activity.
The enantiomer that rotates polarized in a clockwise direction is termed dextrorotatory (usually written +), while the opposite enantiomer (the mirror image version) will rotate the light in the counterclockwise direction, termed levorotatory (usually written -). For ease of writing these terms are usually abbreviated to (d) - from the Latin dexter, meaning "right" and (l) - from the Latin laevus meaning "left".
In this way all kinds of chiral compounds can be examined. The results are usually expressed in the form of a number called the specific rotation, which is calculated this way:
[Note: the wavelength of light used for these measurements is 589 nm (from a sodium lamp); the observed rotation is the angle measured in the polarimeter; the concentration is in grams/milliliter; and the length of the vessel is in decimeters].
Interestingly, if a molecule with a chrial center (such as a carbon atom) is made synthetically, in a test tube, a mixture of enantiomers is obtained in which both versions are equally represented. But chrial compounds are also found extensively in living organisms (like the organic carbohydrates) and when taken out of the living cell, they are found to be optically active, with one enantiomer either the sole form or the predominant form. This is the result of catalytic activity and the fact that active sites of enzymes have to be formed very specifically and cannot accommodate to opposite forms of the same molecule.
Life, it appears, is basically chiral!
Jean Baptiste Biot was made a chevalier of the Legion of Honor in 1814 and a commander of the order in 1849. He was also given the prestigious "Rumford medal" by the Royal Society in 1840. He died in Paris in 1862.