Isolated in laboratory or library, Pasteur's only thought was to search, to learn, to question, and to verify. As the rule of the school leaves much to individual initiative, he devoted himself to his work with a joyful heart. This daily liberty constitutes the charm and the honour of the École Normale. Not only does it permit but it encourages individual effort; it allows the student to visit at his will the library, and to consult there the scientific journals and reviews. This free system of education develops singularly the spirit of research. There is in it an element of superiority over the École Polytechnique. Influenced by its military origin, constrained, moreover, by the number of its pupils to impose on all an exact discipline, and to introduce into their exercises a strict regularity, the École Polytechnique is, perhaps, less calculated than the École Normale to awaken in the minds of its pupils a taste for speculative science. It is certain that Pasteur owed to the freedom of work, and to the facilities for solitary reading which he there enjoyed, the first occasion for an investigation which was the starting-point to a veritable discovery. I. Unlike the old professor of physics and chemistry at Besançon, one of the lecturers in the École Normale often took pleasure, not only in answering Pasteur's questions, but in leading him on to talk over scientific subjects. M. Delafosse, whose memory remains dear to all his pupils, was one of those men who fail to do themselves justice, or who, according to the expression of Cardinal de Retz, do not fulfil all their merit. Not that circumstances have been unfavourable to them, but that an invincible modesty, and a natural nonchalance which finds in that modesty a shield against latent self-reproach, leave them in a sort of twilight in which they are content to dwell. Pupil, and afterwards fellow worker, of the celebrated crystallographer Haüy, M. Delafosse had devoted himself to questions of molecular physics. Pasteur, who had read with enthusiasm the works of Haüy, conversed incessantly with Delafosse about the arrangements of molecules, when an unexpected note from the German chemist Mitscherlich, communicated to the Academy of Sciences, came to trouble all his scientific beliefs. Here is the note:-- 'The paratartrate and the tartrate of soda and ammonia have the same chemical composition, the same crystalline form, the same angles, the same specific weight, the same double refraction, and consequently the same inclination of the optic axes. Dissolved in water, their refraction is the same. But while the dissolved tartrate causes the plane of polarised light to rotate, the paratartrate exerts no such action. M. Biot has found this to be the case with the whole series of these two kinds of salts. Here (adds Mitscherlich) the nature and the number of the atoms, their arrangement, and their distances apart are the same in the two bodies.'