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mitted for examination various specimens of this new substance, is 

 subjoined ; giving an account of the results of his investigations of 

 its mechanical and optical properties. He found that it is composed 

 of laminee, which are sometimes separated by vacant spaces, and at 

 others, only slightly coherent j though generally adhering to each 

 other with a force greater than that of the lamina? of sulphate of lime, 

 or of mica • but less than those of calcareous spar. When the ad- 

 hering plates are separated, the internal surfaces are sometimes co- 

 lourless, especially when these surfaces are corrugated or uneven $ 

 but they are almost always covered with an iridescent film of the 

 most brilliant and generally uniform tint, which exhibits all the va- 

 riety of colours displayed by thin plates or polarizing laminae. This 

 substance, like most crystallized bodies, possesses the property of 

 refracting light doubly ; and, as in agate and mother-of-pearl, one of 

 the two images is perfectly distinct, while the other contains a con- 

 siderable portion of nebulous light, varying with the thickness of the 

 plate, and the inclination of the refracted ray. Like calcareous spar, 

 it has one axis of double refraction, which is negative j and it gives, 

 by polarized light, a beautiful system of coloured rings. It belongs 

 to the rhombohedral system, and, as in the Chaux carbonatee basee of 

 Hauy, the axis of the rhombohedron, or that of double refraction, is 

 perpendicular to the surface of the thin plates. As mother-of-pearl 

 has, like arragonite, two axes of double refraction ; this new sub- 

 stance may be regarded as having the same optical relation to cal- 

 careous spar that mother-of-pearl has to arragonite. 



The flame of a candle, viewed through a plate of this substance, 

 presents two kinds of images; the one bright and distinct, the others 

 faint and nebulous, and having curvatures, which vary as the incli- 

 nation of the plate is changed : the two kinds being constituted by 

 oppositely polarized pencils of light. On investigating the cause of 

 these phenomena, Sir David Brewster discovered it to be the imper- 

 fect crystallization of the substance ; whence the doubly refracting 

 force separates the incident light into two oppositely polarized pen - 

 cils, which are not perfectly equal and similar. In this respect, in- 

 deed, it resembles agate, mother-of-pearl, and some other substances; 

 but it differs from all other bodies in possessing the extraordinary 

 system of composite crystallization, in which an infinite number of 

 crystals are disseminated equally in every possible azimuth, through 

 a large crystalline plate ; having their axes all inclined at the same 

 angle to that of the larger plate, and producing similar phenomena 

 in every direction, and through every portion of the plate : or this re- 

 markable structure may be otherwise described, by saying that the 

 minute elementary crystals form the surfaces of an infinite number of 

 cones, whose axes pass perpendicularly through every part of the 

 larger plate. 



An examination of the phenomena of iridescence afforded by this 

 new substance, leads him to the conclusion that the iridescent films 

 are formed at those times when the dash-wheel is at rest, during the 

 night, and that they differ in their nature from the rest of the sub- 

 stance. These phenomena illustrate in a striking manner some ana- 



