622 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
the patches sometimes reaches the yellow of the first order, and its crystallisation 
has a very singular appearance. 
When the molecules of the same body, or those of different bodies, are com- 
bined under the influence of disturbing causes, we may reasonably expect that 
their union will neither be strong nor permanent. When regular crystals are 
melted by heat, either alone or along with other bodies, their molecules are forced 
into positions of unstable equilibrium, and the natural tendency of similar poles 
to unite is aided by every mechanical vibration, and every variation of tempera- 
ture to which they are exposed. Different kinds of glass, for example, in which 
earths, alkalies, and metals may have been combined by fusion, are thus com- 
pletely decomposed by time, and the elementary particles, liberated from their 
constrained position, resume their place in crystals regularly formed. The spe- 
cimens of ancient glass found at Nineveh, and in various parts of Italy and 
Greece, have undergone the most remarkable decomposition, and some of it 
converted into a sort of indurated mass, which can be broken between the fingers. 
The character of these decompositions, and the process by which they are effected, 
T have had occasion to describe in the Appendix to Mr Layarp’s new work on 
Nineveh and Babylon.* The same principles operate in the decomposition of cir- 
cular crystals, and the same phenomena are exhibited in their restoration to their 
original state. 
In circular crystals the decomposition takes place in different ways. In those 
from borax and phosphoric acid, which I have had occasion to watch month after 
month for several years, the decomposition generally begins at the centre, which 
is dissolved, or occupied by a number of minute prisms, with their axes lying in 
every direction. These prisms sometimes are arranged in a ring round the centre, 
and I have seen them like a St Andrew’s cross. In other crystals, the decompo- 
sition goes on in radial lines or streaks, where the optical contact has not been 
complete; but in the more perfect crystals it takes place in concentric circles, 
sometimes double, the colours between each pair of circles being different. Nu- 
merous cavities are formed,—pieces of the crystal separate, and irregular crystals 
are often formed in the solution. Decomposition sometimes takes place without 
solution: the crystal preserves its form, the black circles are granulated, and the 
colours wholly disappear. In one of the specimens in my possession, every 
crystal has vanished, and their elements converted into beautiful prisms, united 
like a bunch of straw tightened at the middle. Between these groups there are 
numerous flat crystals, of considerable size, and of a perfectly uniform tint. All 
these decompositions have been the work of several years; and in the course of 
one year more there will not be found a vestige of the original crystals. 
In Manna the transformation of the circular into their component crystals 
* Discoveries in the Ruins of Nineveh and Babylon. By Austen H.Layarp, M.P. Appendix, 
p. 674-676. 

