Non-fiction: Research on the Relations between Crystalline Form and Chemical Composition

Background and motivation
Louis Pasteur addressed a puzzling phenomenon: certain chemical compounds rotate the plane of polarized light while chemically identical samples sometimes do not. Tartaric acid and its salts, long known to exhibit optical activity, provided a ripe test case because samples derived from wine fermentation behaved differently from synthetic preparations. The relationship between crystal shape and optical behavior remained mysterious, inviting careful observational work.
Pasteur approached the problem at the intersection of chemistry and crystallography, seeking a concrete physical manifestation of the elusive connection between molecular arrangement and macroscopic properties. He relied on meticulous visual inspection and simple laboratory tools rather than abstract theorizing.

Experimental approach
Working with sodium-ammonium tartrate crystals obtained from natural sources, Pasteur examined their morphology under magnification and noticed two distinct but mirror-image forms. He separated these enantiomorphic crystals manually, using tweezers to collect the subtly different habits into two piles. Each set of crystals was then dissolved and tested with polarized light to measure optical rotation.
The key methodological stroke was the combination of precise crystallographic observation with straightforward optical measurement. The experiments demanded patience and fine motor control, but they produced clear, reproducible contrasts between the two crystal sets.

Key findings
Pasteur found that the two crystal forms were geometrical mirror images and that their aqueous solutions rotated plane-polarized light in equal magnitude but opposite directions. When both crystal types were mixed, the rotations canceled and the solution showed no optical activity, matching the properties of what chemists then called "racemic" mixtures. The correlation between handedness of crystal form and sign of optical rotation provided direct evidence that symmetry at the molecular level governs optical behavior.
These observations established that optical inactivity in a racemic sample can arise from the presence of equal amounts of two oppositely active species, rather than from some absence of an underlying cause. The experiments thus linked an observable macroscopic property, crystal morphology, to an intrinsic molecular property, chirality.

Interpretation and hypotheses
Pasteur interpreted the results as evidence of "molecular dissymmetry," arguing that molecules themselves could be asymmetric and that such asymmetry would be expressed in crystal form and optical activity. He proposed that the spatial arrangement of atoms accounts for the handedness of crystals and the direction of optical rotation, anticipating later formalizations of stereochemistry.
He also hinted at broader implications, noting that the prevalence of one chiral form in biological systems might reflect deeper chemical or physical selection. The suggestion that molecular asymmetry could be fundamental to life and chemistry was provocative and prescient.

Impact and legacy
The 1848 findings laid the empirical foundation for stereochemistry and the concept of enantiomers, transforming how chemists think about molecular structure. Pasteur's work directly inspired later theoretical models of chemical bonding and carbon stereochemistry developed by van 't Hoff and Le Bel in the 1870s, and it shaped experimental practice in organic chemistry and crystallography thereafter. The discovery that chirality at the molecular scale produces distinct macroscopic effects remains central to modern chemistry, pharmacology, and molecular biology.
Beyond specific theories, the study exemplifies the power of careful observation and simple experimental design to reveal deep principles. Pasteur's insight that crystal form can betray molecular structure continues to inform crystallographic methods and the exploration of molecular asymmetry across scientific disciplines.