and thus forms, by comparison, a graduated table, which is useful afterwards for new trials and experiments.*
In
many crystals, perpendicular planes at right angles exist, and it
appears that between these planes the cohesion is so slight that they
can be separated with greater facility there than in any other place.
This particular property is called " cleavage." In some gems it is very
easy to ascertain the point of cleavage, but in others it is found with
difficulty.
Many gems, as, for example, rock-crystal and emerald, will only break into irregular fragments.
SPECIFIC GRAVITY.
In order to determine the specific gravity of different bodies, distilled water is chosen as a means of comparison.
If it be a gem which weighs 17 carats in the air, and in the water but 12, there will be :
