The weight of the ash is best added to that of the crucible. The amount
of ash in filter-papers must not be neglected, although papers are now
made almost free from ash, and the amount to be deducted is found by
taking eight or ten papers and burning them until they become white, and
then weighing the ash. The amount varies from 0.004 to 0.0005 gram for
different papers. Having determined the ash, place in the balance-drawer
three of the filter-papers pinned together, with the weights marked on
them in the way shown in fig. 24, so as to be readily seen when there is
occasion to refer to them.

[Illustration: FIG. 24.]

It must be remembered that the determination of small quantities of
substances generally involves the use of reagents which are often
contaminated, as an impurity, with the body sought for. Thus, in
assaying silver, the oxide of lead or metallic lead used is rarely free
from silver; and in the case of arsenic, the acids, zinc or ferric
chloride are sure to contain arsenic. The same observation applies to
the precipitation of lead by zinc, &c. The errors caused by these
impurities are more marked in the determination of material having small
quantities of metal than in that of ores which contain larger
quantities. Errors of this kind are counteracted or neutralised by
"blank" or "blind" determinations. These consist in carrying out by the
side of and during the assay a duplicate experiment with the reagents
only, which are thereby subjected to the same processes of solution,
evaporation, filtration, &c. The final result thus obtained is deducted
from that given by the assay, the difference gives the corrected result.
In some cases, where it is desired or necessary to have a tangible
residue or precipitate, some _pure_ inert material is added.