The magnesite readily gives
up carbonic acid, which fills the tube and sweeps the mercury vapour
before it. Some of the mercury will have dropped into the beaker, and
some will remain as drops adhering to the upper part of the neck. Whilst
the tube is still hot cut off the neck of the tube just in front of the
asbestos plug (a drop of water from the wash bottle will do this), and
wash the mercury from the neck into the beaker. The mercury easily
collects into a globule, which must be transferred, after decanting off
the bulk of the water, to a weighed Berlin crucible. The water is
removed from the crucible, first by the help of filter paper, and then
by exposing in a desiccator over sulphuric acid, where it should be left
until its weight remains constant. It should not be warmed.
_Example_:--5 grams of an ore treated in this way gave 4.265 grams of
mercury, equivalent to 85.3 per cent. Pure cinnabar contains 86.2 per
cent.
WET METHODS.
_Solution._--Since solutions of chloride of mercury cannot be boiled
without risk of loss,[48] nitric acid solutions should be used wherever
possible. No mercury-containing minerals are insoluble in acids; but
cinnabar requires aqua regia for solution. In dissolving this mineral
nitric acid should be used, with just as much hydrochloric acid as will
suffice to take it up.
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