Although separated from his brother, he remained bound to him by their former bond of love and confidence. During vacations, Jacques Curie would come to him that they might renew again that valuable collaboration to which both willingly sacrificed their periods of liberty. At times it was Pierre who joined Jacques, who was engaged in making a geological chart of the Auvergne country, and there they covered together the daily distances necessary to the tracing of such a map.
Here are a few memories of these long walks, extracts from a letter written to me shortly before our marriage:
"I have been very happy to pass a little time with my brother. We have been far from all immediate care, and so isolated by our manner of living that we have not even been able to receive a letter, never knowing one night where we would sleep the next. At times it seemed to me that we had gone back to the days when we lived entirely together. Then we always arrived at the same opinions about all things, with the result that it was no longer necessary for us to speak in order to understand each other. This was all the more astonishing because we differed so entirely in character."
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."