VIII. On the Optical Properties of Muriate of Soda. Fluate of 

 Lime, and the Diamond, an exhibited in their action 

 upon Polarised Light. By David Brewster, LL. D.. 

 F. R. S. Lond. & Edin. & F. A. S. Edin. 



[Read Nov. 20. 1815.] 



IT has long ago been remarked by the Abbe 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 Dia- 

 mond; 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 consider- 

 ed all the crystals of this class as exercising no more action 

 upon polarised light than a mass of water. 



No explanation was offered of this singular anomaly, till 

 M. Biot discovered that all doubly refracting crystals are 



divisible 



