excellent account of the properties of glass
will be found in Grove's edition of Miller's Elements of Chemistry.

Sec. 13. Cleaning Glass Tubes.

This is one of the most important arts in chemistry. If the tubes are
new, they are generally only soiled by dust, and can be cleaned fairly
easily--first by pushing a bit of cotton waste through with a cane,
or pulling a rag through with string--and then washing with sand and
commercial hydrochloric acid. I have heard of glass becoming
scratched by this process, and breaking in consequence when heated,
but have never myself experienced this inconvenience. In German
laboratories little bits of bibulous paper are sometimes used instead
of sand; they soon break into a pulp, and this pulp has a slightly
scouring action.

As soon as the visible impurities are removed and the tube when washed
looks bright and clean, it may be wiped on the outside and held
perpendicularly so as to allow the water film to drain down. If the
tube be greasy (and perhaps in other cases) it will be observed that
as the film gets thinner the water begins to break away and leave dry
spots. For accurate work this grease, or whatever it is, must be
removed; and after trying many plans for many years, I have come back
to the method I first employed, viz. boiling out with aqua regia.

For this purpose, close one end of the tube by a cork (better than a
rubber bung, because cheaper), and half fill the tube with aqua regia;
then, having noted the greasy places, proceed to boil the liquid in
contact with the glass at these points, and in the case of very
obstinate dirt--such as lingers round a fused joint which has been
made between undusted tubes--leave the whole affair for twelve hours.
If the greasiness is only slight, then simply shaking with hot aqua
regia will often remove it, and the aqua regia is conveniently heated
in this case by the addition of a little strong sulphuric acid.

The spent aqua regia may be put into a bottle. It is generally quite
good enough for the purpose of washing glass vessels with sand, as
above explained.

However carefully a tube is cleaned before being subjected to blowpipe
operations, it will be fouled wherever there is an opening during the
process of heating, unless the extreme tip only of an oxidising flame
be employed. Even this should not be trusted too implicitly unless
an oxygas or hydrogen flame is employed.

When a tube or piece of apparatus has been