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
balance-box a little while before being weighed.
Pages:
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106