heated by two or three ring burners (figs. 14 and 15). The burners are
connected to the gas supply by means of _lead_ tubing, to which they are
soldered. Flasks and dishes after being put on the plate are not further
handled until solution is complete or the evaporation is carried to
dryness. The hot plate is contained in a cupboard so as to be out of the
reach of cold draughts.

[Illustration: FIG. 13.]

[Illustration: FIG. 14.]

[Illustration: FIG. 15.]

The action of the acids and other solvents is described in the chapter
on Reagents.

~Precipitation.~--In precipitating add sufficient of the reagent to
complete the reaction. The student must be on his guard against adding a
very large excess, which is the commoner error. In some reactions the
finishing point is obvious enough; either no more precipitate is formed,
or a precipitate is completely dissolved, or some well-marked colour or
odour is developed or removed.

In those cases in which there is no such indication, theoretical
considerations should keep the use of reagents within reasonable limits.
The solutions of the reagents (_see_ REAGENTS) are generally of five or
ten per cent. strength. A small excess over that demanded by theory
should be sufficient.

[Illustration: FIG. 16.]