ranium-glass were joined; but the internal seal is most conveniently
made by sliding a length of tubing over the lead-glass and fusing this
tubing to the large diameter soda-glass tube to which the lead-glass is
already joined. The first stage of this operation is illustrated by _d_.
When this seal is completed, the end of the soda-glass tube is drawn off
and sealed as shown in _e_, and at this stage a side tube or branch is
joined on. The sealed end of the outer and large diameter soda-glass
tube is heated until it contracts and fuses to the enlargement that has
previously been joined to the lead-glass tube, and the end is burst out
as shown in _f_. Another length of soda-glass is then joined on to the
burst-out end, and this length of soda-glass tubing is drawn out to a
thin-walled contraction; the non-contracted part is expanded to form the
bulb, and a small exhaustion branch made on the side, the drawn-out
portion being cut off, and an electrode, previously prepared by coating
a part of its length with a suitable enamel, is introduced. The tube is
tilted to keep the electrode away from the drawn-out end, which is
melted off and sealed. A small perforation is made with a hot platinum
or iron wire in the sealed end, the electrode is shaken into position,
and the sealing is completed as explained on page 42.

The remainder of the tube, that is to say the lead-glass tube and the
bulb on the other side of the middle bulb, is completed in a similar
manner.

SUMMARY OF CONDITIONS NECESSARY FOR SUCCESS IN GLASS-BLOWING.

For the convenience of the student, it may be well to summarise the
chief essentials for success in glass-blowing, and at the same time to
add such brief notes on the various methods as may seem desirable.

_Adjustment of Blowpipe._--The air jet should be clean internally, and
so centered as to give a flame having a well-defined blue portion, the
tip of the flame should not be only slightly luminous but purple in
colour. In the case of a blowpipe burning oil or wax fuel the flame may
be a trifle more ragged without disadvantage.

_Bellows and Blowing._--The bellows should be adjusted to deliver air at
constant pressure, either by insertion of a tap or, better, by attention
to the wind reservoir if necessary. The movement of the foot in blowing
should be steady, not jerky.

_Heating Glass._--The tube or rod should be heated cautiously until it
has reached its softening point in its thickest part. Stea