and presenting a light bluish or brownish color. The external flame
has the highest temperature at _d_, but this decreases from _d_ to
_c_.

[Illustration: Fig. 3]

If there is a very high temperature, the oxidation is not effected so
readily in many cases, unless the substance is removed a little from
the flame; but if the heat be not too high, it is readily oxidized in
the flame, or near its cone. If the current of air is blown too
freely or violently into the flame, more air is forced there than is
sufficient to consume the gases. This superfluous air only acts
detrimentally, by cooling the flame.

In general the operation proceeds best when the substance is kept at a
dull red heat. The blue cone must be kept free from straggling rays of
the yellow or reduction flame. If the analysis be effected on
charcoal, the blast should not be too strong, as a part of the coal
would be converted into carbonic oxide, which would act
antagonistically to the oxidation. The oxidation flame requires a
steady current of air, for the purpose of keeping the blue cone
constantly of the same length. For the purpose of acquiring practice,
the following may be done: Melt a little molybdenic acid with some
borax, upon a platinum wire, about the sixteenth of an inch from the
point of the blue cone. In the pure oxidation flame, a clear yellowish
glass is formed; but as soon as the reduction flame reaches it, or the
point of the blue cone touches it, the color of the bead changes to a
brown, which, finally, after a little longer blowing, becomes quite
dark, and loses its transparency. The cause of this is, that the
molybdenic acid is very easily reduced to a lower degree of oxidation,