ly horizontal position, but sloping slightly upwards from the
mouth. If it be noticed that a bulb tends to blow out more at one
side than another, let the side of greatest protuberance be turned
down, so that it is at the lowest point, reduce the pressure for an
instant, and then blow again. It will be observed that the bulb will
now expand at the top.

The reason of this is chiefly that the under side cools most rapidly
(according to Faraday, Chemical Manipulation, Sec. 1194), and
consequently can expand no further; but also it is not unlikely that
the glass tends to flow somewhat from the upper side, which remains
hot, and consequently the bulb, when the next puff reaches it, will
tend to yield at this point. By heating several zones the tube will
become gradually expanded.

Fig. 24.

Fig. 25.

Fig. 26.

When the length of the thickened part of the tube only slightly
exceeds its diameter (Fig. 25), let the whole be brought to a
temperature which, with flint glass, should be just short of that of
perfect fluidity; and then, holding the tube horizontally and
constantly turning it, let the bulb be blown out to its full size,
noting the appearances and correcting too great protuberance on any
side by the means above mentioned. If the bulb appears pear-shaped
turn the tube so that the melted mass is directed upwards; if the
bulb have the contrary fault, correct in the corresponding manner.

The bulb when finished may be lightly tapped on the table, when, if
there is any weak place owing to inequality of thickness, the bulb
will break, and the operation may be started afresh. "A good bulb is
round, set truly on the tube, and free from lumps of thick glass or
places of excessive thinness." When the amateur has succeeded in
blowing a bulb two inches in diameter on the end of a strong bit of
thermometer tube--say for an air thermometer--he may well seek the
congratulations of his friends.

In case the bulb is not satisfactory on a first attempt, it may be
melted down again, if the following precautions are taken. Directly
creases begin to appear in the bulb let it be withdrawn from the
flame, and gently blown till the creases come out. By alternate
heating and blowing the glass can be got back to its original form, or
nearly so, but unless the operator shows great skill and judgment, the
probability is that the glass will be uneven. By heating and keeping
the thicker parts in the higher position, and bl