18

QUANTITATIVE AGRICULTURAL ANALYSIS

Mixing and Dividing.—The mixing and dividing is best carried
out by using a sheet of oilcloth or paper and a spatula. In many
laboratories it is customary to use oilcloth, particularly for mixing
minerals. This is convenient but offers the possibility of con-
tamination ("salting77) of one sample by the remnant of one that
has preceded it. It is better to use a large sheet of tough, flexible
paper, which can be discarded after using. The sample, after
having been broken down to the proper maximum size of pieces,
is placed on the paper and thoroughly mixed by rolling diagonally
across the paper and alternating the direction of rolling as illus-
trated in Fig. 2. The proper rapid manipulation of the paper is

FIG. 2.—Manipulation of paper for mixing samples.
attained only after considerable practice. One precaution is
essential: the corner of the paper that is lifted must be drawn
across, low down, in such a manner that the pile of material is not
caused to slide along the paper but is turned over upon itself so
that the bottom is brought entirely to the top. In this way only
can a segregation of larger and smaller particles be prevented.
Since the larger and smaller particles usually have different com-
position it is essential that they should be thoroughly mixed.
The number of times that the sample is rolled before dividing will
depend upon the degree of homogeneity and the accuracy
required in the analysis. In the assaying of gold and silver ores
it is not unusual to require one hundred times.