It has long ago been remarked by the Abbé Hauy, that the property of Double Refraction is not possessed by any of those crystals the form of whose integrant molecule is distinguished by its symmetry, such as the Cube and the Regular Tetraedron. This class of minerals comprehends Fluate of Lime, Muriate of Soda, Spinelle Ruby, Muriate of Ammonia, Alum and the Diamond; and though M. Hauy had examined only a very small number of doubly refracting crystals, yet, with the exception of the Diamond, which I have found in many cases to possess the property of Double Refraction, his remark was confirmed by the experiments of Malus, Biot and myself, who considered all the crystals of this class as exercising no more action upon polarised light than a mass of water.