If one gram of such ore contain .5 gram of copper pyrites (= .1725 gram
copper) and .5 gram of gangue, these will contain 62,500 and say 83,500
particles respectively. Altogether 146,000 particles. With Poisson's
formula this gives the limit of sampling error as the square root of
(8?62500?83500)/146000 or 521 particles. But a variation of 521 on
62,500 is a variation of .83 per cent. The percentage of copper in the
ore is 17.25 per cent., and .83 per cent. of this is .14 per cent. The
limits of sampling error, therefore, are 17.11 per cent. and 17.39 per
cent. Again, it must be remembered that the mean sampling error would be
a little over one-quarter of this, or say from 17.2 per cent. to 17.3
per cent. The practical conclusion is that a powder of this degree of
fineness is not fine enough. In the last place let us consider a similar
iron ore containing 90 per cent. of h?matite (sp. gr. 5) and 10 per
cent. of gangue (sp. gr. 3), 1 gram of such ore will contain 90,000
particles of h?matite weighing .9 gram and containing .63 gram of iron
with say 16,500 particles of gangue weighing .1 gram. Altogether 106,500
particles.
Poisson's formula then gives the limits of variation as the square root
of (8?90000?16500)/106500 or 334 particles. But 334 on 90,000 is 0.23 on
63.0, which is the percentage of iron present.
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