d on the platinum wire
and placing it in the Bunsen flame.

31. ~Silicates.~--Silicates, when treated with microcosmic salt on a
platinum wire, suffer decomposition; the bases unite with the
phosphoric acid to form a transparent glass in which the silica may be
seen floating as a cloudy mass.

The bead must only be examined for silica while hot, since on cooling it
becomes opaque.

32. ~Sulphides.~--Many sulphides, when heated in an ignition tube,
volatilize and give a sublimate of sulphur in combination with the
metallic portion of the substance.

A very delicate test for sulphur in whatever combination it may be found
in a substance, and which may be performed with great ease, is to mix
the finely powdered assay with four parts, Na_{2}CO_{3}, and fuse in an
ignition tube. When thoroughly fused the tube is broken, and the fused
mass is placed on a bright silver coin, and a drop of water is added.
If the substance contains sulphur, a black spot will be observed on the
coin where the fused mass was placed.

Before going on to the next chapter, the student should assure himself
of his familiarity with the reactions just given, and he should practise
with various substances, the nature of which is unknown to him.

CHAPTER IV

BEHAVIOR OF SOME OF THE PRINCIPAL ORES BEFORE THE BLOWPIPE

For the sake of practice, and as a fitting introduction to
"Determinative Mineralogy," this chapter is appended. It is not intended
to give a detailed account of the minerals, but rather to set before the
student the most marked characters, such as hardness, specific gravity,
color, lustre, etc.

To determine the hardness of a mineral, we try to scratch it with the
minerals forming an arbitrary "scale of hardness," proceeding
successively from the softest to the hardest. When we say that a certain
mineral has hardness = 4, we mean that the mineral is scratched by 4 on
the scale, and that 4 on the scale is scratched by the mineral. The
scale of hardness chiefly in use is the Mohs-Breithaupt scale, which is
as follows:--

1. Talc, common laminated light green variety.

2. Gypsum, crystallized.

3. Calcareous spar, transparent variety.

4. Fluor spar, crystalline.

5. Apatite, transparent.

6. Orthoclase, white cleavable variety.

7. Quartz, transparent.

8. Topaz, transparent.

9. Sapphire, cleavable variety.

10. Diamond.

It seldom happens in determining the hardn