GENERAL OPERATIONS

Quartering.—When the first mixing is finished the pile is
made approximately circular and it is then divided, by means of a
spatula, into quarters. Opposite quarters are carefully scraped
to another sheet of paper, ground finer if necessary, remixed and
quartered as before. This process of grinding, rolling, and quar-
tering is continued until a sample is finally obtained, small
enough in quantity and fine enough in texture to serve the pur-
pose of the final weighing and analysis.

Maximum Size of Particles.—The maximum size of particles
to be allowed in any particular mixing and quartering will depend
upon the total quantity of material being handled in this opera-
tion. No particle should be so large that its inclusion in any
quarter would cause the average composition of this quarter to be
appreciably different from the average composition of the entire
pile. This means that the ratio of the size of the largest particle
to the size of the quarter should not be greater than a certain
maximum value. What this maximum value shall be must be
arbitrarily determined by the nature of the sample and the degree
of accuracy required in the analysis. It is obvious that the part
can perfectly represent, in composition, the whole only when
the largest particle is infinitesimal. It is equally obvious that
this limit is impossible and unnecessary in practice and we may
say that, in general, the ratio of the largest particle to the portion
that includes it should not be greater than 0.01 per cent. If this
condition is met, then, after thorough mixing of the sample, the
chance inclusion or exclusion of any given particle cannot modify
the results of the analysis to any appreciable extent.

Other Considerations.—The maximum size of the particles to
be obtained in the final portion that is to be weighed and used in
the analysis must be determined, not only from the above con-
siderations, but also by the nature of the operation to follow the
weighing. This is usually solution or fusion. If the substance
is considered to be almost absolutely homogeneous and if it is
easily soluble (as, for example, a crystal of cupric sulphate) then
the grinding need be carried no farther than is necessary to per-
mit the easy adjustment, between fairly narrow limits, of the
weight taken for analysis. In such a case, if a sample of 0.3 to
0.5 gm is required, then no particle should weigh more than
about 0.1 gm. If, however, the process of solution or fusion is a