They must both be weighed before the combustion is
commenced; to prevent absorption of moisture during weighing, &c., the
ends are plugged with pieces of tube and glass rod.
Fill the combustion tube to a depth of about eight inches with some
copper oxide, which has been recently ignited and cooled in a close
vessel. Put in the weighed portion for assay and a little fresh copper
oxide, and mix in the tube by means of an iron wire shaped at the end
after the manner of a corkscrew. Put in some more oxide of copper, and
clean the stirrer in it. Close loosely with a plug of recently ignited
asbestos, place in the furnace, and connect the ~U~-tube and bulbs in
the way shown in the sketch (fig. 72).
[Illustration: FIG. 72.]
See that the joints are tight, and then commence the combustion by
lighting the burners nearest the ~U~-tube; make the first three or four
inches red hot, and gradually extend the heat backwards the length of
the tube, but avoid too rapid a disengagement of gas. When gas ceases to
come off, open the pointed end of the tube and draw a current of dried
air through the apparatus.
The carbon dioxide is absorbed in the potash bulbs, and their increase
in weight multiplied 0.2727 gives the amount of carbon in the substance
taken.
The increase in weight in the calcium chloride tube will be due to the
water formed by the oxidation of the combined hydrogen.
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