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.

3. It has likewise a great reducing power.

The best kind of charcoal is that of pinewood, linden, willow, or
alderwood, or any other soft wood. Coal from the firwood sparkles too
freely, while that of the hard woods contains too much iron in its
ashes. Smooth pieces, free from bark and knots, should be selected. It
should be thoroughly burnt, and the annual rings or growths should be
as close together as possible.

If the charcoal is in masses, it should be sawed into pieces about six
inches in length by about two inches broad, but so that the
year-growths run perpendicular to the broadest side, as the other
sides, by their unequal structure, burn unevenly.

That the substance under examination may not be carried off by the
blast, small conical concavities should be cut in the broad side of
the charcoal, between the year-growths, with a conical tube of tin
plate about two or three inches long, and one quarter of an inch at
one end, and half an inch at the other. These edges are made sharp
with a file. The widest end of this charcoal borer is used for the
purpose of making cavities for cupellation.

In places where the proper kind of charcoal is difficult to procure,
it is economical to cut common charcoal into pieces about an inch
bro