0.7 = 228.67
0.03 = 9.80
0.004 = 1.31
0.0002 = 0.06
------
239.84
Comparing this with the result calculated by the first method--viz.,
240.26, we see that that was 0.38 oz., or between 7 and 8 dwts. too
high.
With ores containing "metallics" it is of great importance to powder the
whole of the selected sample without loss during the process; and of
even greater importance to well mix the sifted portion, of which the
last portions to come through the sieve are apt to be more than
ordinarily rich through the grinding down of some portions of the
metallic prills.
~Remarks on Cupellation.~--Cupellation is at once the neatest and the
most important of the dry methods of assaying. Its purpose is to remove
easily oxidisable metals, such as lead and copper, from silver and gold,
which are oxidisable with difficulty. Metals of the first class are
often spoken of as _base_, and gold and silver as _noble_ metals.
When lead is exposed to the action of air at a temperature a little
above redness, it combines with the oxygen of the air to form litharge,
an oxide of lead, which at the temperature of its formation is a
_liquid_. Consequently, if the lead rests on a porous support, which
allows the fused litharge to drain away as fast as it is formed, a fresh
surface of the lead will be continually exposed to the action of the
air, and the operation goes on until the whole of the lead has been
removed.
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