Racemic acid is an old name for an optically inactive or racemic form of tartaric acid. It is an equal mixture of two mirror-image isomers (enantiomers), optically active in opposing directions. The naturally occurring form of the acid is the right-handed.
Tartaric acid's sodium-ammonium salt is unusual among racemic mixtures in that during crystallization it can separate out into two kinds of crystals, each composed of one isomer, and whose macroscopic shapes are mirror images of each other. Thus, Louis Pasteur was able before 1850 to separate the two enantiomers by picking apart the crystals. Pasteur announced his intention to resolve racemic acid in:
In two modern-day re-enactments performed in Japan of the Pasteur experiment, it was established that the preparation of crystals was not very reproducible. The crystals deformed, but they were large enough to inspect with the naked eye (microscope not required).
^(On the relations that can exist between crystalline form, chemical composition, and the sense of rotary polarization), Annales de Chimie et de Physique, 3rd series, 24 (3) : 442-459.
^(Investigations into the specific properties of the two acids that compose racemic acid), Annales de Chimie et de Physique, 3rd series, 28 (3) : 56-99. Especially see Plate II. and the report of the commission that was appointed to verify Pasteur's findings, pp. 99-117.