oft soon after it is hot enough to be appreciably luminous
in a darkened room, and at a white heat is very fluid. This fluidity,
though of advantage to the practised worker, is likely to give a
beginner some trouble.

Sec. 8. As against the advantages enumerated, there are some drawbacks.
The one which will first strike the student is the tendency of the
glass to become reduced in the flame of the blow-pipe. This can be
got over by proper adjustment of the flame, as will be explained later
on. A more serious drawback in exact work is the following. In
making a joint with lead glass it is quite possible to neglect to fuse
the glass completely together at every point; in fact, the joint will
stand perfectly well even if it be left with a hole at one side, a
thing which is quite impossible with soft soda glass, or is at least
exceedingly unusual. An accident of this kind is particularly likely
to happen if the glass be at all reduced. Hence, if a joint does not
crack when cold, the presumption is, in the case of soda glass, that
the joint is perfectly made, and will not allow of any leak; but this
is not the case with flint glass, for which reason all joints between
flint glass tubes require the most minute examination before they are
passed. If there are any air bubbles in the glass, especial care must
be exercised.

Sec. 9. Hard or Bohemian, Glass.

This is, of course, used where high temperatures are to be employed,
and also in certain cases where its comparative insolubility in water
is of importance. It is very unusual for the investigator to have to
make complicated apparatus from this glass. Fused joints may be made
between hard glass and flint glass without using enamel, and though
they often break in the course of time, still there is no reason
against their employment, provided the work be done properly, and they
are not required to last too long.

Sec. 10. On the Choice of Sizes of Glass Tube.

It will be found that for general purposes tubes about one-quarter
inch in inside diameter, and from one-twentieth to one-fortieth of an
inch thick, are most in demand. Some very thin soda glass of these
dimensions (so-called "cylinder" tubes) will be found very handy for
many purposes. For physico-chemical work a good supply of tubing,
from one-half to three-quarters of an inch inside diameter, and from
one-twentieth to one-eighth inch thick, is very necessary. A few
tubes up to three inches diamete