In this case, care must be taken to draw
off uniformly a sufficient volume of the gas during a prolonged period;
any portion of this larger volume may then be taken for the analytical
operation.
_In the case of liquids_, which mix more or less easily--and this class
includes metals, &c., in the state of fusion--more or less severe
agitation, followed by the immediate withdrawal of a portion, will yield
a fairly representative sample.
_In the case of solids_, the whole mass must be crushed, and, if not
already of fairly uniform quality, mixed, before sampling can take
place. Most of the material which a sampler is called upon to deal with,
is, however, in a more or less divided state and fairly uniform. In
practice it is assumed that 5 per cent. of the whole (= 1/20th), if
taken in portions of equal weight and at frequent and regular intervals,
will represent the mass from which it was taken. Taking a heap of ore,
A, and selecting one out of every twenty spade-, bag-, barrow-, or
wagon-fuls, according to the quantity of stuff in the heap, there is
obtained a second heap, B, containing one-twentieth of the stuff of the
heap A. If we crush the stuff in B until this heap contains
approximately the same number of stones as A did--which means, crushing
every stone in B into about twenty pieces--B will become the counterpart
of A.
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