The mica crystal when perfect is a flat crystal with six straight edges. These crystals separate in thin layers parallel with the base. In color mica varies, through shades of brown, from a pale smoked pearl to black. Its luster is pearly, and it can be scratched with the thumb nail. Its distinguishing characteristic is that the thin layers into which it splits bend without breaking and endure great heat.

Mica was used in antiquity for windows. Because it is transparent and not affected by heat, it is used in the doors of stoves and furnaces and for lamp chimneys. Its strength makes it of use for automobile goggles. Diamond dust is powdered mica, as is also the artificial snow scattered over cotton batting for the decoration of Christmas trees. When ground finely, it is used as an absorbent for nitroglycerine in the manufacture of dynamite.

Mica mines are scarce in this country. There is an interesting one in North Carolina which had evidently been worked centuries before the advent of the white man in America. There are other mica mines in New Hampshire and Canada. The entire production of this mineral in the United States for the year 1908, was valued at a little more than a quarter of a million dollars. Nearly all of this output was used in the electrical industries, since mica is one of the best insulating materials known.

Lesson CCXVI

Mica

Leading thought—Mica is a crystal which flakes off in thin scales parallel with the base of the crystal. We rarely see a complete mica crystal but simply the thin plates which have split off. The ordinary mica is light colored, but there is a black form.

Method—If it is not possible to obtain a mica crystal, get a thick piece of mica which the pupils may split off into layers.

Observations—

1. Describe your piece of mica. Pull off a layer with the point of your knife. See if you can separate this layer into two layers or more.

2. Can you see through mica? Can you bend it? Does it break easily? What is the color of your specimen? What is its luster? Can you cut it with a knife? Can you scratch it with the thumb nail? What color is the streak left by scratching it with steel?

3. What are some of the uses of mica? How is it especially fitted for some uses?

4. Write a theme on how and where mica is obtained.

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