The limits of sampling
error then are 62.77 per cent. and 63.23 per cent. and the mean
variation is from 62.94 per cent. to 63.06 per cent.
These examples are worthy of careful consideration, and it must be
remembered that the calculations are made on the assumption that the ore
is made up of uniform particles of mineral of such fineness as would
pass easily through an 80 sieve, but which does not pretend to represent
with great exactness the fineness of the powdered ore customary in
practice. They show that having passed through such a sieve is no proof
of sufficient powdering, not that all ores powdered and so sifted are
unfit for assaying. This last would be an absurd and illogical
conclusion.
If an ore be powdered to a fairly fine sand and then be passed through a
series of sieves, say a 40, 60, and 80, in such a state that little or
none remains on the first, but the others retain a large proportion;
then of that which comes through the 80 sieve, perhaps two-thirds by
weight may be even coarser than the powder I have used in the example.
Of the rest most may be of about half this diameter; the weight of the
really fine powder may be quite inconsiderable. On the other hand, if
the grinding be continued until, on sifting, little or nothing that is
powderable remains on the sieves; then in the sifted product the
proportions will be very different.
Pages:
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819