of powder, which, on being brushed off,
reveals a more or less deep incision. But as the stones approach one
another in hardness, there will be little powder and a considerable
increase in the noise; for the harder are the stones, cutting and being
cut, the louder will be the sound and the less the powder. An example
of this difference is evident in the cutting of ordinary glass with a
"set" or "glazier's" diamond, and with a nail. If the diamond is held
properly, there will be heard a curious sound like a keen, drawn-out
"kiss," the diamond being considerably harder than the material it cut.
An altogether different sound is that produced by the scratching of
glass with a nail. In this case, the relative difference in hardness
between the two is small, so that the glass can only be scratched and
not "cut" by the nail; it is too hard for that, so the noise is much
greater and becomes a screech. Experience, therefore, makes it possible
to tell to a trifle, at the first contact, of what the stone is
composed, and in which class it should be placed, by the mere "feel" of
the scratch, the depth of it, the amount and kind of powder it leaves,
and above all, by the sound made, which, even in the tiniest scratch, is
quite characteristic.

CHAPTER VIII.

PHYSICAL PROPERTIES.

F--SPECIFIC GRAVITY.

The fixing of the specific gravity of a stone also determines its group
position with regard to weight; its colour and other characteristics
defining the actual stone. This is a safe and very common method of
proving a stone, since its specific gravity does not vary more than a
point or so in different specimens of the same stone. There are several
ways of arriving at this, such as by weighing in balances in the usual
manner, by displacement, and by immersion in liquids the specific
gravity of which are known. Cork is of less specific gravity than water,
therefore it floats on the surface of that liquid, whereas iron, being
heavier, sinks. So that by changing the liquid to one lighter than cork,
the cork will sink in it as does iron in water; in the second instance,
if we change the liquid to one heavier than iron, the iron will float on
it as does cork on water, and exactly as an ordinary flat-iron will
float on quicksilver, bobbing up and down like a cork in a tumbler of
water. If, therefore, solutions of known but varying densities are
compounded, it is possible to tell almost to exactitude the specific
gravity of any s