, at every revolution, its point will make a mark which corresponds to
the 19th part of the circumference--

Circumf. c / Circumf. C = r / R

It is always necessary that the extremity of the wheel, E, and the
center-point, T, shall be at the same height in order to have the
divisions very accurate.

[Illustration: FIG. 2.--THE CIRCLE DIVIDER. ]

* * * * *

SOLUBLE GLASS.

Although the manufacture of soluble glass does not strictly belong to the
glass maker's art, yet it is an allied process to that of manufacturing
glass. Of late soluble glass has been used with good effect as a
preservative coating for stones, a fire-proofing solution for wood and
textile fabrics. Very thin gauze dipped in a solution of silicate of
potash diluted with water, and dried, burns without flame, blackens, and
carbonizes as if it were heated in a retort without contact of air. As a
fire-proofing material it would be excellent were it not that the
alkaline reaction of this glass very often changes the coloring matters
of paintings and textile fabrics. Since soluble glass always remains
somewhat deliquescent, even though the fabrics may have been thoroughly
dried, the moisture of the atmosphere is attracted, and the goods remain
damp. This is the reason why its use has been abandoned for preserving
theater decorations and wearing apparel. Another application of soluble
glass has been made by surgeons for forming a protecting coat of silicate
around broken limbs as a substitute for plaster, starch, or dextrine.

The only use where soluble glass has met with success is in the
preservation of porous stones, building materials, paintings in
distemper, and painting on glass. Before we describe these applications,
we will give the processes used in making soluble glass.

The following ingredients are heated in a reverberatory furnace until
fusion becomes quieted: 1,260 pounds white sand, 660 pounds potash of
78 deg.. This will produce 1,690 pounds of transparent, homogeneous glass,
with a slight tinge of amber. This glass is but little soluble in hot
water. To dissolve it, the broken fragments are introduced into a iron
digester charged with a sufficient quantity of water, at a high pressure,
to make a solution marking 33 deg. to 35 deg. Baume. Distilled or rain water
should be used, as the calcareous salts contained in ordinary water would
produce insoluble salts of lime, which would render the solution