To produce an efficient flame of oxidation, put the point of the blowpipe into the flame about one third the diameter of the wick, and about one twelfth of an inch above it.  This, however, depends upon the size of the flame used.  Blow strong enough to keep the flame straight and horizontal, using the largest orifice for the purpose.  Upon examining the flame thus produced, we will observe a long, blue flame, a b, Fig. 3, which letters correspond with the same letters in Fig. 2.  But this flame has changed its form, and contains all the combustible gases.  It forms now a thin, blue cone, which converges to a point about an inch from the wick.  This point of the flame possesses the highest intensity of temperature, for there the combustion of the gases is the most complete.  In the original flame, the hottest part forms the external envelope, but here it is compressed more into a point, forming the cone of the blue flame, and likewise an envelope of flame surrounding the blue one, extending beyond it from a to c, 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, or to the oxide of molybdenum.  The flame of oxidation will again convert this oxide into the acid, and this conversion is a good test of the progress of the student in the use of the blowpipe.  In cases where we have to separate a more oxidizable substance from a less one, we use with success the blue cone, particularly if we wish to determine whether a substance has the quality, when submitted to heat in the blue cone, of coloring the external flame.