air thus enclosed will often collect as a small bubble in the wall, thus
weakening it. Irregularities are of various kinds. Some of the larger
sizes of thin-walled tubing often have one half of their walls much
thicker than the other, and such tubing should only be used for the
simplest work. Some tubing has occasional knots or lumps of unfused
material. The rest of the tube is usually all right, but often the
defective part must be cut out. The presence of striations running along
the tube is generally an indication of hard, inferior glass. Crookedness
and non-uniformity of diameter are troublesome only when long pieces
must be used.

Devitrification is one of the worst faults glass can possibly have. It
is especially common in old glass, and in glass which has contained
acids. It seems to be of two sorts. One variety manifests itself on the
surface of the glass before it reaches its working temperature, but if
the glass be heated to the highest temperature of the flame it will
disappear except in the portion at the edge of the heated part. The
glass seems to work all right, but an ugly crystallized ring is left at
the edge of the portion heated. This kind appears most frequently in old
glass which was originally of good quality, but has in time been
superficially altered, probably by the loss of alkalies. The other
variety of devitrification does not appear when the glass is first
heated; but after it has been maintained at or above its working
temperature for a longer or shorter time, it will be noticed that the
outer surface has lost its smoothness, and appears to be covered with
minute wrinkles. It will also be found that the glass has become harder,
so that it becomes impossible to work it easily. Further heating only
makes the matter worse, as does the use of a higher temperature from the
start. In fact it will often be found that a piece of comparatively soft
glass which devitrifies almost at once in a "hissing" flame can be
worked without serious difficulty if care be taken to use a flame still
decidedly tinged with yellow. Even good glass will begin to devitrify in
this way if heated too long at the highest temperature of the flame, so
care should always be taken (1) _to reduce the time of heating of any
spot of glass to a minimum_; _i.e._, get the desired result at the first
attempt, if possible, or at least with the minimum of reheating and
"doctoring," and (2) _avoid keeping the glass at the highest temperat