This last, of course, is the only
right way of powdering. Also it is evident that so much depends on the
manner of powdering that nothing precise can be stated as to the average
coarseness of the powder. Suppose, however, by good powdering a product
is obtained which may be represented by a uniform powder with particles
1/20th of a millimetre in diameter (say roughly 1/500 inch). Compared
with the previous powder, the diameter has been divided by 2.5; their
number, therefore, in any given weight has been increased by the cube of
2.5, which is 15.6. But the value of a sample varies as the square root
of the number of particles. Hence the reduction in size and consequent
increase in number has made the sample nearly four times better than
before; and it will be seen that this brings the sampling error within
tolerable limits.
There are one or two words of warning which should be given. In the
first place, using a 90 sieve instead of an 80 must not be too much
relied on; the powder I took in the example would pass through it. It is
a question of good powdering rather than of fine sifting. In the second
place, a set of, say half-a-dozen, assays concordant within 1 oz. where
the theory gives 4 ozs. as the limit of error does not upset the theory:
the theory itself states this as likely. It is the error you _may_ get
in one or two assays out of a hundred, not the error you are _likely_ to
get in any one assay, which is considered under the heading "limit of
error.
Pages:
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820