dark part of the flame, become reduced and, _in statu nascendi_,
evaporated; but by passing through the external part of the flame,
they become oxidized again, and impart a color to the flame. The
spirit flame is the most efficient one for the examination of
substances the nature of which we wish to ascertain through color
imparted to the flame, as that of the spirit-lamp being colorless, is,
consequently, most easily and thoroughly recognized by the slightest
tinge imparted to it.

It is necessary that in operating with such minute quantities of
substances as are used in blowpipe analysis, that they should have
some appropriate support. In order that no false results may ensue, it
is necessary that the supports should be of such a nature that they
will not form a chemical combination with the substance while it is
exposed to fusion or ignition. Appropriate supports for the different
blowpipe experiments are charcoal, platinum instruments, and glass
tubes.

(_a._) _Charcoal._--The value of charcoal as a support may be stated
as follows:

1. The charcoal is infusible, and being a poor conductor of
heat, a substance can be exposed to a higher degree of heat
upon it than upon any other substance.

2. It is very porous, and therefore allows easily fusible
substances (such as alkalies and fluxes) to pass into it,
while other substances less fusible, such as metals, to remain
unabsorbed.