flat. It is often necessary to flatten a sheet of ebonite, and of
course this is the more easily accomplished the smaller the sheet.
Consequently a bit of ebonite of about the required size is first cut
from the stock sheet by a hack-saw such as is generally used for
metals. This piece is then boiled and pressed between two planed iron
plates previously warmed to near 100 deg. C.

With pieces of ebonite such as are used for the tops of resistance
boxes, measuring, say, 20 X 8 X 11 inches, very little trouble is
experienced. The sheets when cold are found to retain the flatness
which has been forced upon them perfectly well. It is otherwise with
large thin sheets such as are used for Holtz machines. I have
succeeded fairly, but only fairly, by pressing them in a "gluing
press," consisting of heavy planed iron slabs previously heated to
100 deg. C.

I do not know exactly how best to flatten very thin and large sheets.
It is easy to make large tubes out of sheet ebonite by taking
advantage of the softening which ebonite undergoes in boiling water.
A wooden mandrel is prepared of the proper size and shape. The
ebonite is softened and bent round it; this may require two or three
operations, for the ebonite gets stiff very quickly after it is taken
out of the water. Finally the tube is bound round the mandrel with
sufficient force to bring it to the proper shape and boiled in water,
mandrel and all. The bath and its contents are allowed to cool
together, so that the ebonite cools uniformly.

Tubes made in this way are of course subject to the drawback of having
an unwelded seam, but they do well enough to wind wire upon if very
great accuracy of form is not required. If very accurate spools are
needed the mandrel is better made of iron or slate and the spool is
turned up afterwards. The seam may be strapped inside or at the ends
by bits of ebonite acting as bridges, and the seam itself may be
caulked with melted paraffin or anthracene.

Working Ebonite.

Ebonite is best worked as if it were brass, with ordinary
brass-turning or planing tools. These tools should be as hard as
possible, for the edges are apt to suffer severely, and blunt tools
leave a very undesirable woolly surface on the ebonite. In turning or
shaping ebonite sheets it is as well to begin by taking the skin off
one side first, and then reversing the sheet, finishing the second
side, and then returning to the first. This is on account of t