Figure : Origin of the magne6c force between chiral molecules. Chirality: Two polarizable chiral molecules, depicted as springs, undergo correlated fluctua6ons in their electron clouds (blue arrows). Charge: At the peak of one of these fluctua6ons, both molecules behave as charged dipoles aligned head-to-tail (6lted line). The correlated intramolecular electron currents that give rise to the transient dipole are subject to spin selec6vity, and cause a par6al excess of one spin orienta6on at one end of the molecule and a par6al deple6on at the other end. Spin: When the molecules are homochiral (enan6omer, leb), the unpaired spins at the point of intermolecular contact are of opposite polarity (singlet), causing a small a]rac6ve magne6c force. With molecules of opposite chirality (racemic, right), spins of the same polarity face each other (triplet), and the force is repulsive. Reversing the direc6on of the intramolecular current inverts spin polari6es but does not change the sign of the forces. Viscometry Typical viscometry experiments using a bob-and-cup arrangement require sample volumes of 20 ml or more. For enan6omerically pure chemicals, this quan6ty is prohibi6vely expensive for at least one enan6omer. Unusually among commonly available chemicals, lac6c acid (2-hydroxypropanoic acid) is available in quan6ty as either enan6omer with good chemical and enan6omeric purity. Pure lac6c acid is solid, but 90% solu6ons in water are liquid. Viscosity falls steeply with dilu6on, and variability in water content causes small differences in the viscosity of commercial lac6c acid solu6ons at the same nominal concentra6on. The lac6c acid enan6omers used in these experiments had slightly different viscosi6es. We did not adjust them by dilu6on insofar as the effect of mixing was unambiguous.