This becomes more evident if we consider
a still coarser sample. A heap of ore in stones about 2 inches across
would be 50 times coarser than the sand, and an equivalent sample would
need to be 125,000 times heavier; this would amount to about 1000 tons.
Experienced samplers would say that under such conditions so large a
sample was hardly necessary.
This is because I have assumed in the calculations that the grains of
copper pyrites, for example, were all copper pyrites and the particles
of gangue were free from copper. This would be true or nearly so for the
very fine powder, but far from true in the case of the ore heap. In the
heap probably few of the stones would be pure ore and still fewer would
be free from copper. The stones would differ among themselves in their
copper contents only within certain comparatively narrow limits. And it
is evident that, if replacing one stone by another, instead of resulting
in the gain or loss of all the copper one or other contained, merely
affected the result to one-tenth of this amount, then a sample of
1-100th of the weight (say 10 tons) would be equally safe.
It should be remembered, however, that while the man who samples on a
large scale can safely and properly reduce the size of his samples on
this account, yet the principle is one which counts less and less as the
stuff becomes more finely divided, and ought to be ignored in the
working down of the smaller samples which come to the assayer.
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