Weights of less than 0.01 gram are generally present in a box, but it is
much more convenient to work with a rider. This is a piece of wire which
in the pan weighs 0.01 gram; it is made in such a form that it will ride
on the beam, and its effective weight decreases as it approaches the
centre. If the arm of the beam is divided into tenths, then each tenth
counting from the centre outward equals 0.001 gram or 1 milligram, and
if these tenths be further subdivided the fractions of a milligram are
obtained; and these give figures in the fourth place of decimals. A
fairly good balance should be sensitive to 0.0001 gram. The weights must
never be touched with the fingers, and the forceps for moving them is
used for no other purpose. When not in actual use the box is kept
closed. The weights must not be allowed to remain on the pan of the
balance. The balance-case must not be open without some reason. It must
be fixed level, and, once fixed, must not be needlessly moved. The bench
on which it stands should be used for no other purpose, and no one
should be allowed to lean upon it.

[Illustration: FIG. 25.]

When using a balance sit directly in front of it. Ordinarily the
substance to be weighed is best put on the pan to the user's left; the
weights and the rider are then easily manipulated. Powders, &c., should
not be weighed directly on the balance; a counterpoised watch-glass or
metal scoop (fig. 25) should be used. In some cases it is advisable to
use a weighing-bottle. This is a light, well-stoppered bottle (fig. 3)
containing the powdered ore. It is first filled and weighed; then some
of the substance is carefully poured from it into a beaker or other
vessel, and it is weighed again; the difference in the two weighings
gives the weight of substance taken. A substance must always be cold
when weighed, and large glass vessels should be allowed to stand in the