(5) in borax.  As alumina.  Generally gives also a slight iron
                         reaction.

  (6) in mic. salt.  As in borax.

  (7) with carb. soda.  Forms an infusible white mass.

  (8) Special reactions.  With cobalt-solution on charcoal gives the
                         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 order those which afford characteristic reactions.