three to twelve hours,
according to the nature of the article to be annealed. A thick article,
or one having great irregularities in thickness will need much longer
annealing than one thinner or more regular. As a rule, soda-glass will
need more annealing than lead-glass.

_Drilling Glass._--Small holes may be drilled in glass by means of a rod
of hard steel which has been broken off, thus giving a more or less
irregular and crystalline end.

There are several conditions necessary to enable the drilling of small
holes to be carried out successfully:--the first of these is that the
"drill" should be driven at a high speed. This may be done by means of a
geared hand-drill such as the American pattern drill, although a
somewhat higher speed than this will give is even more satisfactory. The
second condition is that the pressure on the drill is neither too light
nor too heavy; this is conveniently regulated by hand. The third
condition is that the drill be prevented from "straying" over the
surface of the glass; for this purpose a small metal guide is useful.
The fourth condition is that a suitable lubricant be used; a strong
solution of camphor in oil of turpentine is perhaps the most suitable.
For commercial work, a diamond drill is often used, but this is scarcely
necessary for the occasional work of a laboratory.

_Larger Holes in Glass._--The method of drilling with a hard steel rod
is not highly satisfactory for anything but small holes. When a larger
hole, say one of an eighth of an inch or more, is needed it is better to
use a copper or brass tube. This tube may be held in an American
hand-drill, but a mixture of carborundum or emery and water is supplied
to the rotating end. Tube or drill must be lifted at frequent intervals
in order to allow a fresh supply of the grinding material to reach the
end. In this case, also, a guide is quite essential in the early stages
of drilling; otherwise the end of the tube will stray. The speed of
cutting may be increased slightly by making a number of radial slots in
the end of the tube; these serve to hold a supply of the grinding
material.

_Grinding Lenses._--This is scarcely within the scope of a book on
glass-blowing for laboratory purposes, but it may be said that the lens
may be ground by means of a permutating mould of hard lead or
type-metal. The rough shaping is done with coarse carborundum or emery,
and successive stages are carried on with finer and finer materia