Selecting in the same manner 5 per cent. of B, there is got a
third heap, C. This alternate reduction and pulverising must be carried
on until a sample of suitable size is obtained. This may be expressed
very clearly thus:--
A = 1000 tons of rocks and lumpy ore.
B = 50 " " rough stones, 1/20th of A.
C = 2.5 " " small stones, 1/20th of B.
D = 0.125 " " coarse powder, 1/20th of C.
[Illustration: FIG. 1.
CONE
PARTLY REDUCED CONE
PLAN OF FRUSTRUM DIVIDED.
ELEVATION OF FRUSTRUM DIVIDED.]
If the material to be sampled is already a dry powder, 5 per cent. of it
should be heaped in a cone; each lot being added on the apex of the
cone already formed, so that it may distribute itself by falling evenly
in all directions. When the cone is completed, convert it into a low
frustrum of a cone by drawing stuff uniformly and in a direct line from
the centre to the circumference. Draw two diameters at right angles to
each other, and reserving any two alternate quarters, reject the others.
Mix; and form another cone, and proceed until a sample is got of the
bulk required.
This is the usual plan, and all samples should be treated in this way
when the stuff is fine enough to fall evenly down the sides of a cone.
Samples as they reach the assay office are seldom in a fit state for the
work of the assayer; they are generally too coarse, and ought always to
be more than he wants for any particular determination.
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