be about one-eighth of an inch in diameter, and cut into pieces about
five or six inches in length. These tubes should have both ends open.

Tubes are of great value in the examination of volatile substances
which require oxidizing or roasting, and heating with free access of
air. Also to ascertain whether a substance under examination will
sublimate volatile matter of a certain appearance. Such substances are
selenium, sulphur, arsenic, antimony, and tellurium. These substances
condense on a cool part of the tube, and they present characteristic
appearances, or they may be recognized by their peculiar smell. These
tubes must be made of the best kind of glass, white and difficult of
fusion, and entirely free from lead. The substance to be examined must
be put in the tube near one end, and exposed to the flame of the
blowpipe. The end containing the substance must be held lower than the
other end, and must be moved a little over the spirit-lamp before a
draught of air is produced through the tube. It is a good plan to have
a number of these tubes on hand. After having used a tube we cut off
that end of it which contained the substance, with a file, and clean
it from the sublimate, either by heating it over the spirit-lamp, or
with a piece of paper wound around a wire. It sometimes happens that
the substance falls out of the tube before it becomes sufficiently
melted to adhere to the glass. To obviate this, we bend the tube not
far from the end, at an obtuse angle, and place the substance in the
angle, whereby the tube may be lowered as much as necessary. Fig. 9
will give the student a comprehension of the processes described, and
of the manner of bending the tubes.

[Illustration: Fig. 9.]

(_i._) _Glass Tubes closed at one End._--If we wish to expose volatile