4 
POPULAR SCIENCE REVIEW. 
nearly twice as large in fluid cavities as in glass cavities. 
Cavities which are filled with crystals are called stone cavities. 
From a most careful series of experiments, Mr. Sorby also 
arrives at the following leading conclusions : — 
1. Crystals possessing only cavities containing water, more or 
less saturated with various salts, were formed by being deposited 
from solution in water. 
2. Crystals containing only glass or stone cavities were 
formed by being deposited from a substance in the state of 
igneous fusion. 
3. Crystals containing only gas cavities were formed by sub- 
limation, or by the solidification of a fused homogeneous 
substance, unless they are fluid cavities that have lost all their 
fluid. 
4. Crystals possessing fluid cavities, containing a variable 
amount of crystals, and gradually passing into gas cavities, were 
formed under the alternate presence of the liquid and a gas. 
5. Crystals in which are found both cavities containing 
water, and cavities containing glass or stone, were formed under 
great pressure by the combined action of igneous fusion and 
water. 
6. Crystals having these two last-mentioned characters com- 
bined were formed, under great pressure, by the united action 
of igneous fusion, aqueous solution, and gaseous sublimation. 
7. Other circumstances being the same, crystals containing 
few cavities were formed more slowly than those containing 
more. 
Turning now to the application of these principles, it will at 
once be seen what a light they can throw upon the origin of any 
particular crystal. By a knowledge of them, we are enabled, 
for instance, to class the calcite, so frequently crystallised in 
veins in limestone, with water-formed minerals ; we can do the 
same with gypsum, rock salt, and many other minerals ; we can 
prove that some varieties of quartz have been formed directly 
from a watery solution. Such forms of quartz as chalcedony, the 
various forms of agate and jasper, and siliceous sinter are of 
aqueous origin. Again, we might meet with a specimen of quartz 
containing water cavities, in which are found cubic crystals of 
sodic chloride, &c., and, together with these, gas cavities. Such 
quartz is met with in Cornwall and elsewhere, and we may con- 
clude that it has been “ formed above the surface of a hot liquid, 
and exposed alternately to water and air.” On the other hand, 
take such a rock as the pitchstone of Arran or the trap of the 
carboniferous period, and a microscopic examination will at onqe 
show that the cavities of such are of the glass or stone kind, 
with occasional gas cavities, and we are justified in pronouncing 
