alumina reaction.

* * * * *

Mineral. Spinel

Formula. [.R][...Al=].

Behavior

(1) in glass-bulb. --

(2) in open tube. --

(3) on charcoal. --

(4) in forceps. V.

(5) in borax. Gives a slight iron reaction.

(6) in mic. salt. As in borax.

(7) with carb. soda. Fuses partially and forms a porous mass.

(8) Special reactions. With nitrate of cobalt gives the alumina
reaction. With nitre and carbonate of soda a
slight manganese reaction.

* * * * *

SILICATES.

The presence of silica in a mineral can easily be ascertained by
treating a small fragment in a bead of microcosmic salt. The bases
will dissolve out with more or less difficulty in the salt, and the
silica being insoluble will remain suspended in the bead, retaining
the original form of the fragment. In borax, the silicates of lime and
magnesia generally dissolve with considerable ease, but those of
alumina slowly and with difficulty. The silicates of lime are moreover
frequently characterized by intumescence or ebullition, when heated in
the forceps in the blowpipe flame. The minerals presenting this
character are marked in the table. As the most convenient mode of
classifying the silicates for blowpipe examination, the following
arrangement will be adopted:

TABLE I.--ANHYDROUS SILICATES.

TABLE II.--HYDROUS SILICATES.

FUSIBILITY.

I. Readily fusible to a bead.
II. With difficulty fusible to a bead.
III. Readily fusible on the edges.
IV. With difficulty fusible on the edges.
V. Infusible.

a. Afford a fluid bead with carbonate of soda.
b. Afford a fluid bead with but little of that salt, but with a
larger quantity a slaggy mass.
c. Afford a slaggy mass only.

This classification of minerals, according to their fusibility and
their behavior with carbonate of soda, was originally proposed by
_Berzelius_, and a table of the principal oxidized minerals arranged
according to these characters is given in his handbook of the
blowpipe, and thence adopted, with some alterations by _Plattner_, in
the very excellent and detailed work already many times cited. In the
following general table I., the more important silicates only are
included, and in table II. are enumerated in alphabetical