ee
_Glass-making_, by Powell, Chance, and Harris, Chap. IV.

Glass tubes are made of various sizes. When purchasing a supply, it is
necessary to be somewhat precise in indicating to the vendor the sizes
required. I have therefore placed at the end of the book, in an
appendix, a table of numbered diagrams. In ordering tubes it will
usually only be necessary to give the numbers of the sizes wished for,
and to specify the quantity of each size required. In ordering glass
tubes by weight, it must be remembered that a great many lengths of the
smaller sizes, but very few lengths of the larger sizes, go to the
pound. Larger-sized tubes than those on the diagram are also made. In
ordering them the external diameter and thickness of glass preferred
should be stated.

=Cleaning and Preparing a Tube.=--It is frequently much easier to clean
the tube from which a piece of apparatus is to be made than to clean
the finished apparatus. A simple method of cleaning a tube is to draw a
piece of wet rag which has been tied to a string through the tube once
or twice, or, with small tubes, to push a bit of wet paper or cotton
wool through them. If the dirt cannot be removed in this way, the
interior of the tube should be moistened with a little sulphuric acid in
which some bichromate of potassium has been dissolved. In any case, it
must finally be repeatedly rinsed with distilled water, and dried by
cautiously warming it, and sucking or blowing air through it. In order
to avoid heating delicate apparatus which has become damp and needs
drying, the water may be washed out with a few drops of spirit, which is
readily removed at a low temperature.

Before using a glass tube for an operation in which it will be necessary
to blow into it, one end of it must be contracted, unless it is already
of such a size that it can be held between the lips with perfect ease;
in any case, its edges must be rounded. For descriptions of these
operations, see page 35. The other end must be closed. This may be done
by means of a cork.

=Presenting Glass to the Flame.=--Glass tubes must never be brought
suddenly into the flame in which they are to be heated. All glass is
very likely to crack if so treated. It should in all cases be held for a
little while in front of the flame, rotated constantly in the hot air
and moved about, in order that it may be warmed over a considerable
area. When it has become pretty hot by this treatment, it may be
gradually bro