The following results are often quoted:--Cupelling 1 grain of silver
with 10 grains of lead, the loss was 1.22 per cent.; 10 grains of silver
with 100 grains of lead, loss 1.13 per cent.; 25 grains of silver
cupelled with 250 grains of lead, lost 1.07 per cent. The proportion of
silver to lead was the same in the three experiments, and the largest
button gave the best result. Evidently, if the quantities of lead had
been the same in the three experiments (say, 250 grains in each case),
the loss on the smaller quantities of silver would appear worse in the
comparison.
In judging these results, it must be borne in mind that it is difficult
to regulate the temperature, &c., in consecutive experiments so as to
get exactly similar results, so that the range in consecutive
cupellations is greater than that in a batch cupelled side by side.
~Effect of Copper and Antimony.~--0.1 gram of silver was cupelled with
20 grams of lead, and to one batch 0.5 gram of antimony, and to another
0.5 gram of copper was added.
Loss in
Silver Lost. Range. Percentage.
Without addition 2.9 0.3 2.9
With antimony 3.2 0.2 3.2
With copper 4.9 1.7 4.9
Perhaps the antimony has so small an effect because it is eliminated in
the earlier part of the process, while the silver is still alloyed with,
and protected by, a large proportion of lead; whilst the copper on the
other hand makes its fiercest attack towards the close, when the silver
is least capable of resisting it.
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