What is mica, and where is it found?
B. O. B.
Answer.—Mica consists of a silicate of alumina, combined, according to species, with small proportions of potash, soda, lithia, oxide of iron, oxide of manganese, etc. The most common and serviceable variety is known as potash mica. It is a constituent of granite, gneiss, mica slate, and several other kindred rocks. It is found both disseminated and in veins. It is very widely distributed, especially in composition with other minerals; but there are comparatively few localities where it is known to exist in such quantities and form as to be mined with profit. Its most valuable form is that of muscovite, in which it appears in translucent lamina or plates. The larger and clearer these plates the greater the value of the mine or quarry. In Siberia they have been found more than three feet across, and they have been obtained of great size in Sweden and Norway. This is also the case at Acworth, Grafton, and Alstead, N. H., and mica has been found large enough for economic use in Canada. Mica is used largely for the doors of stoves and the sides of lanterns. It is employed in some countries as a substitute for window glass, and its toughness recommends it for this purpose on board vessels of war. It has also been used for spectacles. When ground it makes a cheap bronzing powder. There are some mica factories in North Carolina, near Mitchell County mica mines. In the state of a fine powder it is used to give a brilliant appearance to walls, and as a sand for drying ink on manuscripts. Lithia mica contains a small proportion of lithia, which gives it in many cases a fine rose or peachblow color, so that it is used for ornamental purposes.
WHY THE GREAT DIPPER SPINS AROUND THE POLE.
Coldwater, Mich.
Please explain the apparent revolution of the Dipper about the Polar Star.
J. C. A.
Answer.—The Great Dipper only appears to revolve, owing to the revolution of the earth. Of course, it makes an apparent revolution every twenty-four hours, the same as the sun. It is visible throughout the entire night because it is within what is called the circle of perpetual apparition, or that part of the heavens which is always in sight to the observer. This circle enlarges as one travels from the equator, where it is nothing, to the pole, where it takes in the whole northern hemisphere.