Pasteur reports hemihedral tartaric crystals and opposite optical rotation to the Académie des sciences

On 15 May 1848 Louis Pasteur, then a young chemist tied to the École normale, communicated to the Académie des sciences a note on tartaric and paratartaric (racemic) acid crystals. He reported that the two hemihedral forms can be sorted by hand under a lens and that solutions of the sorted crystals rotate the plane of polarised light in opposite senses. It is a crystallographic communication, not a germ theory and not a vaccine. Jean-Baptiste Biot had already measured optical rotation; Pasteur’s claim is the link from crystal hemihedry to molecular dissymmetry. This Fact stores the 1848 communication. It does not convert later tetrahedral-carbon textbooks into his autograph.

Context

French chemistry in 1848 still treated isomerism as a puzzle of composition rather than of spatial arrangement. Racemic acid from industrial tartar was known to be optically inactive; Pasteur’s sorting showed that inactivity could be a mixture of opposite activities. The laboratory was small, the politics of that spring were not. The note went to an Academy that already counted Biot, Dumas, and other patrons. Strasbourg teaching, Lille industry, and later fermentation work are other instruments. Chirality here is not pasteurization and is not rabies. Sharing credit with Biot’s polarimeter and with unnamed crystal-pickers does not erase Pasteur’s signed note.

Confidence is stated on the impact path, not hidden in adjectives. Mixed effects of Pasteur reports hemihedral tartaric crystals and opposite optical rotation to the Académie des sciences are separate paths.

Why it matters

Weight is modest because a crystal note does not staff a hospital or pasteurize milk. Confidence is high on the communication, medium on any lag into industrial resolution of racemates.

The contribution type is direct. That reading is an impact-path choice, not a property stored on the Fact itself.