10 REPORT 1860. 



with other bodies to which it has no natural affinity ; and therefore its atoms, 

 which have their similar poles lying in every possible direction, have a constant 

 tendency to recover their crystalline position as when in a state of silex. For the 

 same reason, the earths, alkalies and metals, with which the atoms of silex have 

 been constrained by fusion to enter into union, all tend to resume their crystalline 

 position and separate themselves from the silex. 



Owing to the manner in which melted glass is cooled and annealed, whether it 

 is made by flashing or blowing, or moulding, the cohesion of its parts is not the 

 same throughout the nias3 ; and consequently its particles are held together with 

 different degrees of force, varying in relation to points, lines, and surfaces. Au 

 atom of the flux, or other ingredient, may be less firmly united to an atom of silex 

 in one place than in another, depending on the degree of heat by which they were 

 combined, or upon the relative positions of the poles of the atoms themselves when 

 combined. There are some remarkable cases where flint-glass without any rude 

 exposure to the elements has become opake, and I have seen specimens iu 'which 

 the disintegration had commenced a few years after it was made. In general, how- 

 ever, the process is very slow, excepting in stables, where the prevalence of am- 

 monia hastens the decomposition and produces all the beautiful colours of the 

 soap-bubble. It is, however, from among the ruins of ancient buildings that glass 

 is found in all the stages of decomposition ; and there is perhaps no material body 

 that ceases to exist with such grace and beauty, when it surrenders itself to time 

 and not to disease. 



In damp localities, where acids and alkalies prevail in the soil, the glass rots as 

 it were by a process which, owing to the opacity of the rotten part, it is difficult to 

 study, it may be broken between the Angers of an infant ; and we often find in the 

 middle of the fragment a plate of the original glass which has not yielded to the 

 process of decay. 



In dry localities, where Eoman, Greek, and Assyrian glass has been found, the 

 process of decomposition is exceedingly interesting," and its resvdts singularly beau- 

 tiful. At one or more points in the surface of the glass the decomposition begins. 



Itextends round that point in spherical surfaces so that the first film is a minute 

 hemispherical cup of exceeding thinness. Film after film is formed in a similar 

 manner, till perhaps twenty or thirty are crowded into the 50th of an inch. They 

 now resemble the section of a pearl or of an onion, and as the films are still glass, 

 the colours of thin plates are seen when we look down through their edges which 

 form the surface of the glass. These thin edges, however, being exposed to the 

 elements, suffer decomposition. The particles of silex and the other ingredients 

 now readily separate, and the decomposition goes on downwards in films parallel 

 to the surface of the glass, the crystals of silex in one specimen forming a white 

 ring, and the other ingredients rings of a different colour. ( See the Figure.) 



Such is the process round one point, but the decomposition commences at many 

 points, and generally these points lie in lines, so that the circles of decomposition 

 meet one another and form sinuous lines. When there are only two points, these 

 circles, when they meet, surround the two points of decomposition like the rings 

 round two knots of wood ; and in like manner, when there are many points, and 

 these points near each other, the curves of decomposition unite as already mentioned, 

 and form sinuous lines. When the decomposition is uniform and the little hemi- 

 spheres have nearly the same depth, we can separate the upper film from the one 

 below it, the convexities of the one falling into the concavities of the other. 



This general description was illustrated bv drawings on the table, all of which 

 were executed by Miss Mary King, of Ballylin, now the Hon. Mrs. Ward. 



But beautiful and correct as these drawings are, they convey a very imperfect 

 idea of the brilliant colours and singular forms which characterize glass in a parti- 

 cular stage of its decomposition, and of the optical phenomena which it exhibits in 

 common and polarized light. 



When the decomposition has gone regularly on round a single point, and there 

 is no other change, a division of the glass into a number of hemispherical films 

 within one another takes place, the group of films exhibiting in the microscope cir- 

 cular cavities, which under different circumstances become elliptical and polygonal. 



In saltwater the decomposition of glass goes on more rapidly, as I have found in 



