Profession: mineralogist

The table of hardness for minerals that Mohs devised is still used. Werner taught him at the mining academy in Frieberg; Whewell, an important figure in modern crystallography, visited him. Mohs’ interest in physical (rather than chemical) attributes of minerals ran against current trends, but in fact very usefully complements chemical analysis in its practical directness.

Del Río was a significant pioneer in New World mineralogy. He studied in Freiberg under Werner, meeting Humboldt (a good friend and later an important collaborator) there. He also studied under Lavoisier, and worked together with Haüy, the founder of crystallography. Despite their friendship, del Río rather unfairly found it impossible to forgive Humboldt for his failure to recognise his discovery of the new element to be known as vanadium.

Valentin Haüy was his younger brother. Daubenton taught and encouraged him. Lamarck was friend as well as fellow-academician: they started a short-lived journal together. Lagrange, Laplace, Fourcroy, Berthollet, Malus and Legendre were colleagues at the Académie des Sciences. Geoffroy, a colleague and former pupil, interceded to have him freed from imprisonment during the Revolution (Haüy had to be persuaded to leave). He himself tried unsuccessfully to get Lavoisier freed (together they had earlier determined the weight of the kilogram). A suggestion of Coulomb’s led to his work on piezoelectricity.