Balard also realised this; he had the happy idea of taking the young _agrégé_ into his laboratory, and intervened vehemently when the Minister of Public Instruction desired--a few months later--that Pasteur should teach physics in the Tournon Lycée. It would be rank folly, Balard declared, to send 500 kilometres away from Paris a youth who only asked for the modest title of curator, and had no ambition but to work from morning till night, preparing for his doctor’s degree. There would be time to send him away later on. It was impossible to resist this torrent of words founded on solid sense. Balard prevailed. Pasteur was profoundly grateful to him for preserving him from exile to the little town in Ardèche; and, as he added to his Franc-Comtois patience and reflective mind a childlike heart and deep enthusiasm, he was delighted to remain with a master like Balard, who had become celebrated, at the age of twenty-four, as the discoverer of bromin. At the end of 1846, a newcomer entered Balard’s laboratory, a strange delicate-looking man, whose ardent eyes were at the same time proud and yet anxious. This man, a scientist and a poet, was a professor of the Bordeaux Faculty, named Auguste Laurent. Perhaps he had had some friction with his Bordeaux chiefs, possibly he merely wished for a change; at all events, he now desired to live in Paris. Laurent was already known in the scientific world, and had recently been made a correspondent of the Académie des Sciences. He had foreseen and confirmed the theory of substitutions, formulated by Dumas as early as 1834 before the Académie. Dumas had expressed himself thus: “Chlorine possesses the singular power of seizing upon the hydrogen in certain substances, and of taking its place atom by atom.” This theory of substitutions was--according to a simple and vivid comparison of Pasteur’s--a way of looking upon chemical bodies as upon “molecular edifices, in which one element could be replaced by another without disturbing the structure of the edifice; as if one were to replace, one by one, every stone of a monument by a new stone.” Original researches, new and bold ideas, appealed to Pasteur. But his cautious mind prevented his boldness from leading him into errors, surprises or hasty conclusions. “That is possible,” he would say, “but we must look more deeply into the subject.” When asked by Laurent to assist him with some experiments upon certain theories, Pasteur was delighted at this suggested collaboration, and wrote to his friend Chappuis: “Even if the work should lead to no results worth publishing, it will be most useful to me to do practical work for several months with such an experienced chemist.”