Jacobus Henricus Van't Hoff explained why two chemically indistinguishable forms of tartaric acid rotated the plane of polarized light in different ways. He realized that this phenomenon could be explained if it was asumed that the four valencies by which a carbon atom links itself with other atoms were directed to the four apices of a regular tetrahedron. Hence, two forms of the same compound, which are mirror images of each other can exist. Van't Hoff won a Nobel Prize in 1901, the first year in which the prizes were awarded.
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