it is thickened by
collecting glass as for a bulb on the ends of two tubes (Exercise No.
6), and drawing to form cones of suitable shape (_a_ and _b_, Fig. 17)
and of such relative sizes that a will slip about half way into _b_. In
order to make _a_ straight and give it the proper angle, it may be
rolled when hot, upon a hot plate of carbon. Blowing during this rolling
is often helpful to remove depressions. After _b_ has been drawn to
nearly the proper size and shape, it may be smoothed by the use of a
small carbon rod, held inside it at a slight angle, or better by the use
of a truncated hexagonal pyramid of carbon, whose edges have the proper
slant to make the inside of the cone right. The proper taper for both
these cones is the same as that used in stopcocks of similar size. The
hexagonal carbon can easily be made by carefully filing down an electric
light carbon, and finally impregnating it with paraffin or beeswax, and
is extremely useful wherever a conical surface has to be formed from the
inside of a tube.

[Illustration: FIG. 17.--Ground joint.]

The tail is allowed to remain on piece _a_, as a sort of guide in
grinding, and should therefore be in the axis of the tube and have
rather thick walls. Grind with emery or carborundum, as described under
a previous head. (Regrinding plug for stopcock.) If many such joints are
to be made, it will pay to have a little sleeve of brass made with the
proper taper, and rough down the plug _a_ in it to about the proper
size, while _b_ is roughed down by means of a brass or iron plug having
the same taper. This prevents excessive grinding of one-half of the
joint in order to remove a defect in the other half, and is the method
commercially used in making stopcocks.

SEALING IN PLATINUM WIRE

Very often it is necessary to seal platinum wire into the wall of a
tube. Professional glass-blowers usually use a special sort of glass
("Einschmelzglas") which is usually a lead glass, and is made of such
composition that it has the same or practically the same coefficient of
expansion as platinum. A little globule of this glass is sealed into the
tube in such a way that it joins the platinum to the glass of the tube.
To do this, the small globule of special glass is fused on the platinum
wire at the proper point and the tube into which the wire is to be
sealed is heated and a small tail drawn out at the point where the wire
is to be inserted. The lump of the special glass shoul