To determine more exactly what the loss is,
the following plan is recommended:--The two silver buttons are wrapped
up each in 10 grams of lead, and cupelled side by side with two other
lots of 10 grams of the original alloy. If now the two buttons I. and
II. weigh 0.1202 and 0.1203, they will have suffered in this second
cupellation an average loss of 2.5 milligrams. Suppose the two fresh
lots of alloy gave 0.1233 and 0.1235 of silver, the average loss on
these would also be 2.5 milligrams. Add this loss to each result, and
take the mean; which is in this case 0.1259.
If copper is present in the alloy as well as silver, it is necessary to
add about the same quantity of copper to the checks as is supposed, or
known, to be present in the assays. If the substance to be assayed is an
alloy of silver and copper, first cupel 0.5 gram of it, with, say, 10
grams of lead, and weigh the resulting button of silver, in order to get
an approximate knowledge of its composition. Suppose the button weighs
0.3935 gram. We know that this is below the truth: for the sake of round
numbers take it as 0.4, and assume that the rest of the alloy (0.1 gram)
was copper. Two check pieces are then weighed out, each containing 0.4
gram silver and 0.1 gram of copper wrapped in 5 grams of lead. Of course
the silver must be pure.
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