TRANSACTIONS OF THE SECTIONS. 11 



examining one of the bottles brought up in the wreck of the ' Royal George ;' and the 

 same effect may be produced by a quicker process. M. Brame*, of Paris, having 

 seen a notice of the decomposed glass from Nineveh which I read at the xlssociation 

 some years ago, succeeded in producing, in a very short time, regular and irregular 

 circles of decomposition, in the centre of which there was always a small cavity or 

 nucleus. This effect was obtained by immersing fragments of thick glass in a 

 mixture of fluoride of calcium and concentrated sulphuric acid, or by exposing 

 them to the action of the vapour of fluorhydrique acid. 



Such are some of the general phenomena of decomposed glass when seen by light 

 reflected from its exposed surfaces ; but when we separate the films and examine 

 them in the microscope, either by common or polarized light, a series of phenomena 

 are seen of the most beautiful kind, — so various and so singular that it would be a 

 vain attempt to describe them. A general idea of them, however, may be obtained 

 from the drawings, and from a description of three varieties of these films. 



I. The first of these varieties has rough surfaces, — the roughness arising from 

 an almost infinite number of hemispherical cavities on one side of the film, and 

 hemispherical convexities on the other side. When these cavities are separated by 

 flat portions of the film, they are perfectly circular ; but when they are crowded 

 together, they are irregularly polygonal, the sides of the polygons forming a sort of 

 network, the cavities or convexities forming the meshes of the net. 



The convex and concave surfaces are not rough but specular, and reflect and 

 transmit white light, exhibiting none of the colours of thin plates. 



In polarized light, each of the cavities, whether circular or polygonal, act as 

 negative uniaxal crystals, exhibiting by the interference of the refracted and trans- 

 mitted pencils the black cross, and the white of the jirst order in Newton's scale, 

 rising sometimes to yellow or falling to the palest blue, or disappearing altogether, 

 according to the number or curvature of the films which compose it. 



II. The second variety of these films has perfectly specidar surfaces, in conse- 

 quence of having almost no cavities. They exhibit in common light, and in a very 

 beautifid manner, the colours of thin plates, the transmitted being complementary to 

 the reflected light. This variety is exceedingly rare. In a specimen on the table 

 the reflected light is blue and the transmitted yellow. In some of the fragments a 

 few insulated circular cavities with the black cross occur, the tints which surround 

 it being modified by the general tint of the film. 



III. The third variety of decomposed glass consists of films containing cavities of 

 all sizes and forms, from the 30th of an inch to such a size that they are hardly 

 visible by the microscope, giving to the film which they compose a sort of stippled 

 appearance, or an imperfectly specular surface. 



These cavities or combinations of hemispherical films are circular, elliptical, or 

 irregularly polygonal. The colours which they reflect and transmitare complementary, 

 and the tints and rings which in polarized light surround the black cross are curiously 

 modified by the general tint of the fragment, and the curvature of its component 

 films,— the black cross itself varying its shape with the form of the cavities. When 

 the cavities are flat, the black cross disappears as in thin slices of uniaxal crystals ; 

 but the tints reappear, rising to higher orders by inclining the plate. 



The cavities are often arranged in sinuous curves, and encroach upon one another, 

 so that the polarized tints appear only at the margin of the line which they form. 

 They frequently run in perfectly straight lines, and when they are very small and 

 invisible as cavities, their margins form in polarized light brilliant lines, which are 

 often grouped in bands like the stripes in a ribbon. Sometimes they are only a few 

 thousands of an inch in diameter, and might be used as micrometers in the micro- 

 scope, every trace of the cavities which form them having disappeared. These lines 

 of polarized light all disappear when they lie in the plane of polarization of the 

 incident light, or perpendicular to that plane. 



In some specimens a decomposition has taken place on several points of the con- 

 vex or concave surfaces of the cavities, so as to form new cavities ; and each of these 

 minute cavities, often ten or twelve in number, exhibit the black cross with its tints, 

 but disfiguring, of course, those of the cavity upon which they have encroached. 



In the three varieties of decomposed glass which I have described, the films are 



* Coniptes Eendus, &c, Nov. 2, 1852. 



