OPTICAL PHENOMENA OF ANCIENT DECOMPOSED GLASS. 195 



completely closed as if it had never been made,"* the surfaces having returned 

 into optical contact. 



When we examine the surface of decomposed glass, either before or after 

 the removal of the film, we find it composed of an infinite number of cavities, in 

 the middle of each of which the decomposition has begun. At this point a 

 particle of glass, of a certain size, has detached itself, and a film is formed by the 

 aggregation of other particles of the same size. This decomposition extends itself 

 in all directions, but more quickly downwards beneath the first particle that was 

 detached. The consequence of this is the formation of a cavity or cup, which would 

 be part of a hollow sphere if the centres of decomposition were at great distances 

 and few in number. In general, however, these centres are innumerable, cover- 

 ing the whole surface of the glass. From this cause, the decomposition round 

 any one centre meets the decomposition round other centres, and the form of the 

 cavity is an irregular polygon, the shape of which depends upon the distances of 

 the centres of decomposition. The lines in which the decompositions meet one 

 another resemble the meshes of a net, or the lines which separate the small films 

 OT partitions that confine between them small quantities of gas, forming the froth 

 of champagne, beer, and other liquids. 



When the cavities of decomposition are very numerous and equally diffused, 

 they are often so minute as to be invisible, and their existence is shown by 

 different degrees of roughness, as if the surfaces of the film had been ground, and 

 sometimes by producing halos or rings round the flame of a candle. 



In several specimens the surfaces of the films are perfectly specular, showing 

 that particles of the same size have been uniformly detached from the original 

 surface, and that the decomposition has not taken place around and beneath dif- 

 ferent centres. 



When we consider the different states of fusion and cooling to which glass is 

 exposed, whether it is formed by flashing, or blowing, or moulding, it is evident 

 that the processes of decomposition must take place in spots or in lines where the 

 elements of the glass have been less perfectly combined, or where the cohesive 

 forces are more feeble. 



In many films, where the decomposition has been pretty uniform, there are 



I often numerous centres distant from each other, where very deep cavities have 



1 been formed. These cavities are perfectly circular, as in Plate X. fig. 1, and 



Plate XI. figs. 6, 7, 8, &c., and have different diameters and different degrees 



of depth. In other films these circular cavities are crowded together so closely as 



to occupy the whole of its surface, as in Plate XI. figs. 11 and 13. 



In many specimens the centres of decomposition lie in straight lines, and 

 near each other, as shown in Plate X. fig. 5. One cavity thus encroaches upon 



* Phil. Trans., 1816, p. 73. 



