er should be well reblown, and if necessary left as a small bulb
or enlargement, rather than have it have too thick walls.

SPIRALS

Spirals of glass tubing are probably best made free-hand before the
blow-pipe, unless one has a great many of them to make, and extreme
accuracy is desired. To begin with, a piece of tubing of the desired
size (say 3/16 inch in diameter) and a convenient length (about two
feet) is selected, one end closed, and a right-angle bend made about six
inches from the closed end. Holding the closed end in the left hand and
the long open one in the right, the spiral is begun. The short closed
end is to be parallel to the axis of the spiral, and preferably in that
axis. Using a moderate-sized flame, of somewhat yellow color, and taking
care to heat the whole circumference of the tube, the long open end is
wound little by little into a spiral having the short end _a_ (Fig. 16)
as an axis. The bend at _b_, where the tube changes from the radius to
the circumference of the circle, must be rather short, but the tube must
not be flattened or constricted here. Especial pains is to be taken
with the first turn of the spiral (_b_ to _c_, Fig. 16), as the shape of
this determines the diameter of the whole spiral, and serves as a guide
for the rest of the turns. The winding of the tube is best accomplished,
after a portion has been softened, by slowly turning the short end _a_ a
little about its own axis, while the long open end remains where it was.
This winds the tube into a spiral, just as if there were a solid
cylinder in the center of it, and this cylinder was being turned about
its axis, and was winding up the soft glass upon its circumference. As
the cylinder is not actually there, the curve of the turns must be
carefully estimated by the eye, so that the spiral may be uniform and
moderately smooth. When the original piece of tube has been used up,
another piece is sealed on to the open end, and the operation continued
as far as may be required.

[Illustration: FIG. 16.--Making a spiral.]

GROUND JOINTS

It is sometimes required to join two pieces of tubing end to end, by
means of a ground joint. Whenever possible, a regular sealed joint
should be used instead of this ground joint, as it is quicker to make,
and more certain to be tight. Where a ground joint is necessary,
however, it is best made in the conical form shown in _c_, Fig. 17. If
the wall of the tube to be used is not very thick,