198 Sm DAVID BREWSTER ON THE STRUCTURE AND 



gives it the appearance of granite. The iridescence of the films is, however, dis- 

 tinctly seen ; and when the crust is removed the transparent green glass appears, 

 with its excavated surface. 



The decomposition of glass goes on rapidly in water. In a wine bottle brought 

 up from the wreck of the " Royal George," very considerable films were formed 

 on its surface. Bottles of the form called " magnums," which were found in the 

 Cherwell, near Oxford, by Mr R. Thomas, were encrusted with films of consider- 

 able thickness. From one of these Mr Thomas detached a circularly-shaped film 

 about half-an-inch in diameter, which had a spiral crack proceeding from its 

 centre, and having eight or nine circumvolutions. This film, which was one of 

 unusual thickness, was a single one, as is shown by its producing none of the 

 colours of thin plates at any inclination, and by its action on common and polar- 

 ised light. Though perfectly transparent, its surface is not specular, but mottled 

 with the colours of striated surfaces, the points of decomposition by which they 

 were produced being visible by high magnifying powers. 



From the facts I have mentioned, it is evident that the decomposition of glass 

 is accelerated when it is exposed to the action of acids and alkalies in damp 

 localities, and particularly to the ammonia so abundant in stables. 



M. Brame, of Paris, having seen a notice of the decomposed glass from Nineveh, 

 which I had read at the British Association some years before, was led to submit 

 thick plates of glass to the action of powerful solvents. In a short time circles 

 were produced analogous to those which I had observed in the Nineveh specimens. 

 " I have produced," he says, " upon glass, circles regular and irregular, insulated 

 or concentric, notched in the interior (crystals incomplete or altered). To do this 

 we must immerse fragments of thick glass into a mixture of fluoride of calcium 

 and concentrated sulphuric acid, or expose them to the action of the vapour of 

 fluorhydric acid. In the centre of the circles we find almost always either a 

 small cavity, or a small nucleus. At the same time one or more fractures of 

 the glass are covered with striae and small anfractuosities, &c. 



"The observations of M. Brewster and mine seem fitted to throw much li^ht 

 on the formation of siliceous spheres (orbicules) in sandstone and agates, which are 

 the subject of a fine investigation by Alex'"', Brongniart ; and on the other 

 hand, this inquiry seems to connect itself directly, like that of the circles of glass 

 themselves, with the encyclical formations which I have discovered."* 



The colours exhibited by films of decomposed glass, when examined in ordi- 

 nary light, are of the most brilliant description. There is no colour in the world 

 of flowers which is not found, in all its beauty, in the light transmitted by these 

 films ; while in the light reflected from them the tints are equally varied, and of 

 a metallic lustre. As each film is composed of many separate films, the colours 



* Sur la structure des corps solides, par M. Ch. Brame (Lettre a M. Babinet). Comptes Rendus, 

 8fc., Nov. 1853, torn. xxxv. pp. 666, 667. 



