"
Accepting the result just arrived at that a portion of 1 gram may be
safely taken for an assay if the particles are 1-20th of a millimetre in
diameter, the further question remains as to what weight of the original
sample must be reduced to this degree of fineness. This may be answered
on the principle that the same degree of excellence should be aimed at
in each of a series of samplings. This principle is illustrated in the
table on page 2.
A fine sand, such as would pass a 40 sieve but be retained on a 60
sieve, would be fairly represented by particles one-quarter of a
millimetre in diameter. This being five times coarser, to contain the
same number of particles must be 125 times (the cube of 5) as heavy;
therefore 125 grams of it can be taken with the same degree of safety as
1 gram of the finer powder. Of such a sand about this weight should be
taken and reduced to the finer powder. If the ore were in coarse sand,
say in particles 1 millimetre in diameter, this would be four times as
coarse as that last considered, and we should have to take 64 times as
much of it: 64 times 125 grams is 8 kilos, or say roughly from 15 to 20
lbs. This should be crushed to the finer size and mixed; then from 100
to 150 grams should be taken and ground to the finest powder.
There is, however, a reason why, on the coarser stuff, a smaller
proportion may safely be used.
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