ess of a mineral that its
hardness exactly conforms to that of some one member of the scale. In
such cases we generally estimate the hardness. For example, suppose a
mineral was harder than 4, but softer than 5, and that it was nearer 5
than 4, then we would call its hardness 4-3/4.

In order to preserve the scale some operators use a three-cornered file,
first cutting the mineral and then the scale until a number is found,
which is abraded to about the same depth as the mineral under
examination.

Since a set of minerals forming a scale of hardness is not always at
hand, the following scale given by Chapman is appended:--

1. Yields easily to the nail.

2. Yields with difficulty to the nail or just receives an
impression from it. Does not scratch a copper coin.

3. Scratches a copper coin but is also scratched by it,
being of about the same degree of hardness.

4. Not scratched by a copper coin. Does not scratch glass.

5. Scratches glass with difficulty, leaving its powder on
it. Yields readily to the knife.

6. Scratches glass easily. Yields with difficulty to the
knife.

7. Does not yield to the knife. Yields to the edge of a
file, though with difficulty.

8, 9, 10. Harder than flint.

Specific gravity cannot well be determined without the aid of a balance,
and hence its value here is not great.

As in the preceding chapter, alphabetic arrangement will be employed.

ORES OF ANTIMONY

~Stibnite~, Sb_{2}S_{3}, Sb . 71, S . 29.--[A]H = 2, G = 4.52-4.62. Of
lead-gray color and metallic lustre. Consists of a large number of
needle-shaped crystals. Brittle. Fuses in candle flame. In an ignition
tube yields a sublimate of sulphur. On Ch. before the Bp. it is
volatilized, giving antimony coating and tinges the flame pale blue.

[Footnote A: H = Hardness, G = Specific Gravity.]

ORES OF ARSENIC

~Native Arsenic, As.~--This contains traces of Sb, Ag, Fe, Co, and Ni.

H = 3.5, G = 5.7-5.8. Dark gray in color. Fracture tin-white, tarnishing
rapidly. Volatilizes before the Bp. on Ch. without melting, giving white
coating of arsenious acid and characteristic garlic odor. In ignition
tube it sublimes, giving arsenical ring.

~Realgar~, AsS, As . 70, S . 30.--H = 1.5-2, G = 3.56. Bright red to
orange-red color and resinous lustre. In an ignition tube it fuses and
finally sublimes. The sublimate when cool is red and transparent. Fuses
rea