Louis Pasteur Discovers Molecular Chirality

Pasteur carefully observed the paratartaric acid under a microscope. Looking at the tiny crystals, he noticed two different types. While almost identical, they were actually mirror images of each other. Pasteur's next step required incredibly meticulous work. Again, working with the microscope, he separated the two types of crystals into two piles. After separating the crystals, Pasteur made two solutions—one with each of the piles—and tested how they interacted with polarized light. He found that both solutions rotated the light— but in opposite directions. When the two types of crystals were together in the solution of paratartaric acid the effect of rotation of the light was canceled.

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