Imbued as he was with the teachings of Haüy and Delafosse, and full of the ideas of M. Dumas in molecular chemistry, Pasteur asked himself this question: 'How can it be admitted that the nature and number of the atoms, their arrangement and distances apart, in two chemical substances are the same; that the crystalline forms are equally the same, without concluding that the two substances are absolutely identical? Is there not a profound incompatibility between the identity affirmed by Mitscherlich and the discrepancy of optic character manifested by the two compounds, tartaric and paratartaric, which form the subject of his note?' This difficulty rested in Pasteur's mind with the tenacity of a fixed idea. Received as _agrégé_ of physical science at the end of his third year at the École, and then keeping near his master, M. Balard, he had begun the study of crystals and the determination of their angles and forms, when his nomination to the professorship of physics in the Lycée of Tournon surprised and distressed him. M. Balard repaired immediately to the bureau of the Minister of Education, and spoke of his assistant in terms which caused the nomination to be cancelled. Pasteur remained in the laboratory of the École Normale. With a view to mastering the science of crystallography, he took for his guide the extensive work of M. de la Provostaye, resolving to repeat all the measurements of angles and all the other determinations of this author with a view to a comparison of their respective results. The work of M. de la Provostaye, who was distinguished by the exactitude of his researches, had for its subject the tartaric and paratartaric acids and their saline compounds. * * * * * Two or three years ago, while we were walking together along a road in the Jura, M. Pasteur, after quoting textually the note of Mitscherlich, described to me with enthusiasm the pleasure he had experienced in crystallising tartaric acid and its salts, the crystals of which, he said, rivalled in size and beauty the most exquisite of crystalline forms. 'I should have great difficulty,' I remarked, 'in following you through the labyrinth of tartaric acid, tartrates, and paratartrates. However much your other studies have attracted me, those which had for their starting-point the note of Mitscherlich and the memoir of M. de la Provostaye have appeared to me, whenever I tried to master them, difficult of access. Ah,' I added, 'you would have done well, out of consideration for those who love to speak of your labours, had you made no discoveries in this field.'