Page:Popular Science Monthly Volume 41.djvu/681

Rh used in windows, as they were in the seventeenth century in Philadelphia, when glass was a luxury in the colonies. The sheets are also used in the peep-holes of smelting furnaces, in lanterns, in shades, and in the port-holes on board naval vessels, where the vibrations would soon demolish less elastic glass. Mica is an excellent non-conductor, and of recent years has been cut to some extent into narrow strips for use in the construction of dynamos.

The scrap mica was formerly thrown away, with the exception of a small quantity used as a lubricating material, but it has recently found a market in several new directions. Old waste heaps are being bought up, for a few dollars a ton, and their contents cleaned by being passed through a rough mill. This is simply a rotating cylinder of coarse wire screen with its axis slightly inclined to the horizontal. The scrap is fed into the upper end of the cylinder, and slowly discharges itself from the lower end. As it makes its way from end to end, the sand and trash are supposed to fall through the meshes of the screen. The cleaned scrap is then shipped to Richmond, where it is ground into a coarse powder and distributed to the various industries requiring it. Large quantities are used in the manufacture of wall-paper. The mica produces a sparkling surface which is thought to be decorative, but at best the effect is somewhat bizarre. Considerable amounts are used to produce the snow effects on Christmas cards, and in stage scenery and other tinsel; while smaller packages, under the name of diamond dust, are sold as powder for the hair. Much of the ground mica is sent to France, and this, oddly enough, when the East Indian sheet mica is pressing our own pretty heavily in the home market.

The Latin world used the mica dust to strew over the Circus Maximus, while mediaeval Europe knew the golden and silver scales as cat-gold and cat-silver.

But to go back again for a moment from the glass-house to the mines themselves, there is much of interest in the rare and beautiful minerals associated with the mica. Some of the mines are quite noted for these by-products and are as attractive to a lover of color as to the mineralogist. The mica itself is often the carrier of curious forms. Frequently a mineral makes its way between the laminae of the mica, and is thus forced to grow horizontally instead of normally in three directions. This gives us curious dendritic or tree-like forms which come out finely on holding the mica up to the light. The oxide of manganese is most prone to get caught in this way, and gives delicate tracery of dark brown or black. Sometimes it is a garnet which is thus entrapped, and then we have a brilliant little hexagonal plate of ruby glass, very beautiful and very gem-like. By carefully taking off the outer