The presence of lower oxides of nitrogen, which impart a brown colour to
the acid, is objectionable; they, however, are removed by boiling the
diluted acid before using it for parting. It is usual to keep a stock of
the acid suitably diluted to the two strengths required for the parting.
These are known as the parting acids. The _first parting acid_ is the
weaker and is used in the first attack on the metal. The specific
gravity generally recommended for it is about 1.2. It may be prepared
either by diluting the strong acid with about its own volume of
distilled water, or by suitably diluting the second parting acid which
has been already used in an assay; the small proportion of silver this
contains is not harmful for this purpose. The _second parting acid_ has
a specific gravity of about 1.3, and may be made by diluting the strong
acid with half its volume of distilled water.
_Parting in Flasks._--Flasks are most convenient for the larger
partings, as in bullion assays; and should always be used for this
purpose unless some of the special parting apparatus, like that used in
Mints, is available. Many assayers use flasks, though of a smaller size,
for the ordinary partings in assaying gold ores. The flasks are either
bulbs with long necks (Fig. 46) which ought to be heated on rose burners
of special construction; or they are small flat-bottomed conical flasks
which may be conveniently heated on a hot-plate and are, in this
respect, much easier to deal with in general work.
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