58 
referred to unequal contraction in cooling from a high temperature ; and, 
if so, the results would necessarily depend on a variety of circumstances. 
Now that attention has been directed to it, it will probably be found to be 
a very common peculiarity of certain classes of minerals, and serve to 
throw a good deal of light on their origin. 
Crystals surrounded by radiating cracks on a much larger scale have 
been observed by Mr. David Forbes*, and may, we think, be explained 
in a similar manner. 
The crystals formed in blowpipe beads kept hot for some time over the 
lamp, also furnish good illustrations of these facts. Phosphate of zirconia 
is deposited in cubes from a borax bead to which much microcosmic salt 
has been added ; and when examined with the microscope whilst cooling, 
cracks like those described in diamond and spinel are seen to be formed 
round many of the crystals, which are evidently due to the crystals con- 
tracting less than the surrounding material. On the contrary, the long 
prisms of borate of baryta deposited from solution in borax are seen to 
separate from the borax on cooling, and to be filled with transverse cracks, 
like those in schorl inclosed in quartz, which is clearly owing to their 
contracting more than the borax. 
Fluid-cavities in general. 
Before discussing the nature of fluid-cavities in connexion with the 
origin of the various minerals, we think it best to describe the remarkable 
properties of the liquid included in the sapphire, and to point out what 
it seems to be. Brewster, in his 'paper on the fluid-cavities in topazf, 
says that the more expansible liquid contained in them expands 
one-fourth its size, when heated from 50° to 80* F, or thirty-one and a 
quarter times as much as water, and, as already stated, he found that the 
fluid in sapphire expands about one-half when heated to 82^ F. Though 
this amount of expansion is very remarkable, yet, when the relative 
expansion at various temperatures is examined, it will be seen to be still 
more remarkable. Very fortunately the tubular cavity in sapphire, shown 
by fig. 2, is most admirably fitted for experiment. Mere inspection shows 
that its general diameter is very uniform ; and that it is really so can be 
proved by causing the liquid to pass from one end to the other; for at 
17£° C. the length of the column of liquid was -2J_ of an inch, whether 
it was at the end A or B. The total effective length of the cavity is 
* Ed. New Phil. Journ. July 1S57. 
t Trans. Boy. Soc. Edin. 1824, vol. x. p. 1. 
