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  • Every chemical substance, whether natural or artificial, falls into one of two major categories, according to the spatial characteristic of its form. The distinction is between those substances that have a plane of symmetry and those that do not. The former belong to the mineral, the latter to the living world.

    "Oeuvres de Pasteur". Book edited by Louis Pasteur Vallery-Radot, Vol. I, 331, 1922-1939.