tube, in all these _roasting_ operations, as they are termed, should
be held in an inclined position. The nearer perpendicular the tube is
held, the stronger is the draught of air that passes through it. If
but little heat is required in the open tube operation, the
spirit-lamp is the best method of applying the heat. But if a greater
temperature is required, then recourse must be had to the blowpipe.
Upon the angle of inclination of the tube depends the amount of air
that passes through it, and therefore, the rapidity of the draught
may be easily regulated at the will of the operator. The inclination
of the tube may, as a general rule, be about the angle represented in
Fig. 14.

[Illustration Fig. 14.]

The length of the tube must be about six inches, so that the portion
upon which the substance rested in a previous examination may be cut
off. The portion of the tube left will answer for several similar
operations.

When the substance is under examination, we should devote our
attention to the nature of the sublimates, and to that of the _odors_
of the gases. If sulphur be in the substance experimented upon, the
characteristic odor of sulphurous acid gas will readily indicate the
sulphur. If metallic sulphides, for instance, are experimented upon,
the sulphurous acid gas eliminated will readily reveal their presence.
As it is a property of this gas to bleach, a piece of Brazil-wood test
paper should be held in the mouth of the tube, when its loss of color
will indicate the presence of the sulphurous acid. It often happens,
too, that a slight deposition of sulphur will be observed upon the
cool portion of the tube. This is particularly the case with those