substances to heat, with the exclusion of air as much as possible, or
to ascertain the contents of water, or other volatile fluids, or for
the purpose of heating substances which will decrepitate, we use glass
tubes closed at one end. These tubes must be about one-eighth of an
inch wide, and from two to three inches in length. They should be made
of white glass, difficult of fusion, and free from lead. They should
be closed at one end, as figured in the margin, Fig. 10.

[Illustration: Fig. 10.]

When a substance is to be examined for the purpose of ascertaining
whether it contains combustible matter, as sulphur or arsenic, and
where we wish to avoid oxidation, we use these tubes without extending
the closed end, in order that there may be as little air admitted as
possible, as is represented in tube B. But when a substance to be
examined is to be tested for water, or other incombustible volatile
matters, we employ tubes with little bulbs blown at one end, such as
represented at tube A. Here there is room for a circulation of air at
the bottom of the tube, by which the volatile matter rises more
easily. In some cases, it is necessary to draw the closed end out to a
fine point, as in the tubes C and D. Either one or the other of these
tubes is employed, depending upon the nature of the substance used.
The sublimates condense at the upper part of the tube _a_, and can be
there examined and recognized. These tubes, before being used, must be
thoroughly dried and cleaned. In experimenting with them, they should
not be exposed at once to the hottest part of the flame, but should be
submitted to the heat gradually. If the substance is of such a nature
that it will sublime at a low heat, the tube should be held more
horizontal, while a higher heat is attained by bringing the tube to a
more vertical position.