to the ton; the mean sampling error would be rather
more than a quarter of this, or say about 1.3 ozs. to the ton.
On the other hand, if one took for the assay a charge six times greater
(say about 20 grams), the number of particles would be 3000 and the
limits of variation would be 28 multiplied by the square root of 30, or
153 particles, which is very closely 1/20 of the silver present, or say
1.75 ozs. to the ton, whilst the mean error would amount to about .5
ozs. to the ton.
To work these examples by Poisson's formula let us assume the gangue to
have a mean sp. gr. of 3. Then 500 particles would weigh 3 milligrams;
and 3.2609[131] grams would contain 543,500 particles. There would be
then 500 of ruby silver and 543,500 of gangue, together 544,000, and the
formula gives the square root of (8?500?543500)/544000, which works out
to 63 particles as against 62 by the other method.
A practical conclusion from this is of course that either the ore must
be powdered more finely or a larger portion than 3 grams must be taken
for the assay. Moreover, it is evident that on such an ore no small
sample must be taken containing less than several million particles.
Consider now a copper ore of the same uniform fineness containing
particles of copper pyrites (sp. gr. 4) of which 1000 particles will
weigh 8 milligrams, mixed with gangue of which 1000 particles weigh 6
milligrams.
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