From the point of view of scientific investigation, one must recognize that the nomination of Pierre Curie to the School of Physics and Chemistry retarded from the very first his experimental research. Indeed, at the time of his appointment nothing yet existed in that establishment; everything had to be created. Even the walls and the partitions were hardly yet in place. He had, therefore, to organize completely the laboratory and its work, and he acquitted himself of this task in a remarkable manner, injecting into it the spirit of precision and originality so characteristic of him. The direction of the laboratory work of the large number of students (thirty by promotion) was alone a strain on a young man, assisted as he was only by one laboratory helper. The first years were, therefore, hard years of assiduous work, of benefit chiefly to the students trained and developed by the young laboratory director. He himself profited by this enforced interruption of his experimental research by trying to complete his scientific studies and, in particular, his knowledge of mathematics. At the same time he became engrossed in considerations of a theoretical nature on the relations between crystallography and physics. In 1884 he published a memoir on questions of the order and repetition that are at the base of the study of the symmetry of crystals. This was followed in the same year by a more general treatment of the same subject. Another article on symmetry and its repetitions appeared in 1885. In that year he published, too, a very important theoretical work[3] on the formation of crystals, and the capillary constants of the different faces. This rapid succession of investigations shows how completely engrossed Pierre Curie was in the subject of the physics of crystals. Both his theoretical and his experimental research in this domain grouped itself around a very general principle, the principle of symmetry, that he had arrived at step by step, and which he only definitely enunciated in memoirs published between the years 1893 and 1895. The following is the form, already classic, in which he made his announcement: "When certain causes produce certain effects, the elements of symmetry in the causes ought to reappear in the effects produced. "When certain effects reveal a certain dissymmetry, this dissymmetry should be apparent in the causes which have given them birth. "The converse of these two statements does not hold, at least practically; that is to say, the effects produced can be more symmetrical than their causes." The capital importance of this statement, perfect in its simplicity, lies in the fact that the elements of symmetry which it introduces are related to all the phenomena of physics without exception.