In one of his lectures Pasteur thus described his interview with M. Biot:--'He made me come to his house, where he put into my hands some paratartaric acid which he had carefully studied himself, and found perfectly neutral as regards polarised light. It was not in the laboratory of the École Normale, it was in his own kitchen, and in his presence, that I was to prepare this double salt with soda and ammonia procured by himself. The liquor was left slowly to evaporate, and at the end of ten days, when it had deposited thirty or forty grammes of crystals, he begged me to go over to the Collège de France to collect the crystals and to extract from them specimens of the two kinds, which he proposed to have placed, the one on his right hand, the other on his left, desiring me to declare if I was ready to re-affirm, that the crystals to the right would turn the plane of polarisation to the right and the others to the left. This declaration made, he said that he would charge himself with the rest of the inquiry. M. Biot then prepared the solutions in well-measured proportions, and at the moment of observing them in the polarising apparatus he invited me again to come into his study. He placed first in the apparatus the most interesting solution, that which ought to deviate to the left. Without even making any measurements, he saw, by the mere inspection of the colours of the ordinary and extraordinary images of the analyser, that there was a strong deviation to the left. Visibly moved, the illustrious old man took my arm and said, "My dear child, I have loved science so well throughout my life that this makes my heart beat."' The emotion of M. Biot was all the more profound because he had been himself the first to discover the rotation of the plane of polarisation by chemical substances, and had, for more than thirty years, affirmed that the study of these substances and of their action in regard to rotatory polarisation was, perhaps, the surest means of penetrating into the intimate constitution of bodies. His counsels were received with deference, but they had never been followed out. And now there appeared before the old man, already somewhat discouraged, a youth of twenty-five, who from his first investigation had proved himself a master, who had dissipated the obscurities of the famous German note, and created a new chapter in crystallographic chemistry. The composition and nature of paratartaric acid had been explained, and a new substance, the left-handed tartaric acid, with its truly surprising properties, had been discovered; molecular physics and chemistry had been enriched with new facts and theories of great value.