20

QUANTITATIVE AGRICULTURAL ANALYSIS

difficult one to accomplish or if the material is far from being
homogeneous; the grinding is carried much farther, in order to
provide a very large surface of contact between the particle and
the solution or flux, or in order to conform to the rule of maxi-
mum size of particles, stated above. In many cases, as with
minerals, the maximum size of particles is fixed by causing the
sample to pass through a sieve having meshes of stated dimen-
sions. A gold ore may be ground to pass a sieve having 100 or

FIG. 3.—Division by quartering.

200 meshes to the linear inch. In such a case one should not
make the mistake of grinding and sifting a portion until a suffi-
cient quantity is passed, discarding the remainder. This would
cause an error because the particles that resist grinding longest
are less brittle and have a composition different from that of the
particles which pulverize easily.