e, it may be moved right into the blow-pipe
flame, and the latter may be properly urged.

It is not possible to give quite explicit and definite instructions,
applicable to every case, as to when the time is ripe for passing the
work into the flame, but the following notes will indicate the general
rules to be observed:-

(1) A thick tube must be warmed more slowly and raised to a higher
temperature than a thin tube.

(2) The same remark applies to a tube of large diameter, as compared
with one of small diameter, whatever the thickness.

(3) In the case of very large or thick tubes the hot air is
advantageously employed at first, and to complete the preliminary
heating, the luminous flame alone may be used. The object of this is
to enable the operator to judge, by the presence of soot, its
inability to deposit--or its burning off if deposited--of the
temperature of the glass, and of the equality of this temperature all
over the surface, for a large and thick tube might be heated quite
enough to enable it to be safely exposed to the full heat before it is
appreciably yielding to the fingers. In general, when the soot burns
off freely, or lead glass begins to show the faintest sign of
reduction, or soda glass begins to colour the flame, it is more than
safe to proceed.

In order to turn on the full flame the operator will form a habit of
holding the work in the left hand only, and he will also take care not
to let anything his right hand may be doing cause him to stop rotating
the tube with his left thumb and finger.

The preliminary adjustment of air or oxygen supply will enable the
change to a flame of maximum power to be made very quickly. The tube
having been introduced with constant rotation, it will soon soften
sufficiently to be worked. The beginner will find it best to decide
the convenient degree of softness by trial.

With soda glass it does not much matter how soft the glass becomes,
for it remains viscous, but with lead glass the viscosity persists for
a longer time and then suddenly gives place to a much greater degree
of fluidity. [Footnote: This is only drawn from my impressions
acquired in glass-working. I have never explicitly tested the matter
experimentally.]

It is just at this point that a beginner will probably meet with his
first difficulty. As soon as the glass gets soft he will find that he
no longer rotates the glass at the same speed by the right and left
hand, and, moreover, h