Open the clamp and allow from 10 to 20 c.c. of the
acid to run on to the assay. Carbonic acid will be evolved and will be
driven through the tubes. The gas should bubble through the sulphuric
acid in a moderate and regular stream. When the effervescence slackens
the clamp is opened and the greater part of the remaining acid run in.
When the effervescence has ceased the clamp is opened to its full extent
and a current of air drawn through with an aspirator. A gentle heat is
applied to the flask; but it should not be prolonged or carried to
boiling. After the removal of the heat a gentle current of air is drawn
through the apparatus for 30 or 40 minutes. The weighed ~U~-tube, which
in the early part of the operation will have become warm if much
carbonic acid was present, will by this time be cold. It is
disconnected, plugged, and weighed. The increase in weight is due to the
carbon dioxide of the sample.
_Example._--Ore taken 1 gram.
Weight of tube, before 42.6525 grams
" " after 43.0940 "
-------
Increase equals CO_{2} 0.4415 "
[Illustration: FIG. 81.]
~Indirect Gravimetric, or Determination by Loss.~--Take a Geissler's
carbonic-acid apparatus (fig. 81) and place in the double bulb some
strong sulphuric acid. Put into the other bulb, the stopcock being
closed, 3 or 4 c.
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