7 9.4200

Platinum vessels and apparatus lose, and porcelain ones slightly gain,
weight with continued use.

The special details of the work is given under each assay; certain
general instructions will be given here.

~Solution.~--It is not always necessary to get the whole of the mineral
in solution, provided the body sought for is either completely dissolved
or altogether left in the residue. It is often only by a qualitative
examination of the solution (or residue, as the case may be) that the
assayer can satisfy himself that it is free from the substance sought.
But previous experience with the same kind of ore will show to what
extent this testing is necessary.

Solution is generally best effected in flasks; but where the resulting
liquid has afterwards to be evaporated to dryness and ignited,
evaporating dishes (fig. 12) are used. With them clock glasses are used
as covers during solution to avoid loss through effervescence.
Evaporating dishes are also best when an insoluble residue has to be
collected, since it is difficult to wash out most residues from a flask.
Bumping occurs less frequently in dishes than in flasks.

[Illustration: FIG. 12.]

After the addition of the acid, and mixing by agitation, the vessel
containing the substance is heated. This is best done on the "hot
plate" (fig. 13). This consists of a slab of cast iron about half or
three-quarters of an inch thick, supported on loose fire bricks, and