is but a particular case of a bent glass tube. It
is scarcely possible when bending very large tubes in the manner
described on p. 29 to produce regular curves of sufficient strength.

To make a U-tube, or to bend a large tube, close one end of the tube
selected with a cork, soften and compress the glass in the flame at the
part where it is to be bent till a sufficient mass of glass for the bend
is collected, then remove the mass of glass from the flame, let it cool
a little, and simultaneously draw out the thickened glass, bend it to
the proper form, and blow the bend into shape from the open end of the
tube. Small irregularities may be partly corrected afterwards.

To make a good U-tube of large size, and of uniform diameter from end to
end, requires much practice, but to make a tolerably presentable piece
of apparatus in which the two limbs are bent round till they are
parallel, without any considerable constriction at the bend, can be
accomplished without much difficulty.[11]

[11] Large tubes may also be bent by rotating a sufficient length of the
tube in a large flame till it softens, and bending in the same manner as
in the case of smaller tubes, and after filling them with sand, closing
one end completely, and the other so that the sand cannot escape, though
heated air can do so.

=Spiral Tubes.=--These may be made by twisting a tube gradually softened
by heat round a metal cylinder. Spiral tubes made of small thin tubes
possess considerable elasticity, and have been used by Mr. Crookes for
making air-tight connections between separate pieces of apparatus when a
rigid connection would have been unnecessary and inconvenient. By the
use of such spiral tubes it is possible to combine comparatively free
movement with all the advantages attached to hermetically-sealed joints.

To make a flexible spiral tube, mount a copper cylinder on a screw, so
that the cylinder will travel in the direction of its axis when it is
rotated. Fix a fine glass tube to the cylinder, and direct a flame
towards the cylinder so as to heat and soften the glass, which will then
bend to the form of the cylinder. Gradually rotate the cylinder before
the source of heat, so that fresh portions of tube are successively
brought into position, softened, and bent. Useful spirals may also be
made by hand without a cylinder. As each length of tube is bent, a fresh
length may be united to it until the spiral is completed. The fine tubes
employed