Our thermometers come chiefly from abroad. The
common mercurial one passes through the most difficult process. It is
made upon the principle of quickly drawing out a hollow sphere into a
thin tube which keeps all the character of its original. The lump of
glass is blown hollow. An assistant fastens his punty to the round end
and pulls the lump into a short thick tube, which is pressed into an
elliptical shape. The flattened tube is then coated with another portion
of melted glass, and it is rolled on an iron slab until a cylindrical
exterior is formed around the flattened bore, leaving an elliptical
opening within. A small batch of white glass is attached to it, and the
furnace evenly distributes it over one side. Now it is a short thick
cylindrical tube, white on one side. It is drawn out into a long thin
tube; but the drawing preserves exactly the first shape and proportions,
merely reducing the size. The tube is cut off into the lengths required.
Holding one of the pieces to a blow-pipe, the workman converts the end
of it into a bulb. It is then heated to expel the air, and the open end
plunged into mercury. This is repeated until the mercury entirely
displaces the air, when the open end is hermetically sealed. It goes
thence to the graduator, who marks on it by careful tests the scale of
degrees, which are indicated by the fine flat thread of mercury against
the white background.
Most of the world's beads are Venetian. In the
island of Murano a thousand workmen are devoted to this branch. The first
process is to draw the glass into tubes of the diameter of the proposed
bead. For this purpose the glass-house at Murano has a kind of rope-walk
gallery 150 feet long. By gathering various colors from different pots and
twisting them into one mass many combinations of color are made. The tubes
are carefully sorted by diameters, and chipped into fragments of uniform size.
These pieces are stirred in a mixture of sand and ashes, which fills the
holes, and prevents the sides from closing together when they are heated.
They are next placed in a kind of frying-pan, and constantly stirred over
a fire until the edges are rounded into a globular form. When cool they
are shaken in one set of sieves until the ashes are separated, and in
another series of sieves until they are perfectly sorted by size. Then
they are threaded by children, tied in bundles, and
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exported to the ends of the earth. France has long produced the "pearl
beads" which in the finer forms are close imitations of pearls. They
are said to have been invented by M. Jaquin in 1656. The common variety
threaded for ornament is blown from glass tubes. An expert workman can
blow five or six thousand globules in a day. They are lined with
powdered fish scales and filled with wax. It takes 16,000 fish to make
a pound of the scaly essence of pearl. Until recently the heirs
of Jaquin still carried on a large factory of these mock-pearls. The
best of them are blown irregular to counterfeit nature, some in pear shape,
others like olives, and they easily pass for genuine.
Imitation gems formerly employed the chief
attention of the highest artificers in glass. They are still the chief
idea of ornamental glass in China. In the ancient and middle ages they
circulated everywhere without much danger of discovery, and their formulas
were held as precious secrets. Blancourt first published their compositions
in 1696. Now they are common property; and with the growth of science in
the past century an expert knowledge has become widely disseminated which
easily detects the paste from the real jewel, particularly as the modern
false stones are less successful copies than the old glass-makers produced.
More study is now given to artificial gems, which are true gems, being
composed of the same materials as the genuine ones, but manufactured.
Mirrors are made chiefly in Europe, the cheap ones
in Germany, which invented the tin amalgam in the fourteenth century, and
the large expensive ones in France. The silvering table is a smooth slab
of thick wood or stone fixed on a pivot so that one side may be raised,
and having a frame on three sides. The slab is placed horizontally, and
covered tightly with gray paper. A smooth thin sheet of tin-foil is laid
on the paper, and mercury is poured on its flat surface. The plate of glass
is then carefully slid into the frame. Gently it is dropped, squeezing
out the superfluous mercury, which runs off into a channel and is collected
below. The plate is then covered with flannel, loaded with weights, and
tilted on an incline. In this position it remains an entire day, while
the mercury drips off. Then it is cautiously lifted from the frame. The
amalgam has adhered to the glass, and after it
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