ficult to soften, especially in large pieces. It should
only be employed, therefore, when the low melting points of soda or lead
glass would render them unsuitable for the purpose to which the finished
apparatus is to be put. What is sold as Jena combustion tube should be
preferred when this is the case.

=Characters of good Glass.=--Glass tubes for glass-blowing should be as
free as possible from knots, air-bubbles, and stripes. They should be in
straight pieces of uniform thickness, and cylindrical bore. It is not
possible to obtain glass tubes of absolutely the same diameter from one
end to the other in large quantities, but the variations should not be
considerable.

When a sharp transverse scratch is made with a good file on a piece of
tube, and the scratch is touched with a rather fine point of red-hot
glass (this should be lead glass for a lead glass tube, and soda glass
for a tube of soda glass), the crack which is started should pass round
the glass, so that it may be broken into two pieces with regular ends.
If the crack proceeds very irregularly, and especially if it tends to
extend along the tube, the glass has been badly annealed, and should not
be employed for glass-blowing purposes. It is important that the point
of hot glass used shall be very small, however. Even good glass will
frequently give an irregular fracture if touched with a large mass of
molten glass.

Finally, glass tube which is thin and of small diameter should not
crack when suddenly brought into a flame. But larger and thicker tubes
will not often withstand this treatment. They should not crack, however,
when they are brought into a flame gradually, after having been held in
the warm air in front of it for a minute or so.

Good glass does not readily devitrify when held in the blow-pipe flame.
As devitrified glass very often may be restored to its vitreous
condition by fusion, devitrification most frequently shows itself round
the edges of the heated parts, and may be recognised by the production
of a certain degree of roughness there. It is believed to be due to the
separation of certain silicates in the crystallised form. Hard glass,
which contains much calcium, is more apt to devitrify than the more
fusible varieties.[3]

[3] The presence of silicates of calcium and aluminum are considered to
promote a tendency to devitrification in glass; and glasses of complex
composition are more apt to devitrify than the simpler varieties. S