ote 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
sulphides, which yield sublimates of sulphur when heated in the bulb.

_Selenium_ undergoes but slight oxidation, but it becomes readily
volatilized, and may be observed on the cool portion of the tube. At
the same time the nose, if applied close to the end of the tube, will
detect the characteristic odor of rotten horse-radish. Arsenic also
gives its peculiar alliaceous odor, which is so characteristic that it
can be easily detected. A few of the arsenides produce this odor. The
_sublimates_ should be carefully observed, as they indicate often with
great certainty the presence of certain substances; for instance, that
of arsenic. The sublimate, in this case, presents itself as the
arsenious acid, or the metallic arsenic itself. If it be the former,
it may be discerned by aid of the magnifying glass as beautiful
glittering octohedral crystals. If the latter, the metallic lustre
will reveal it.

But it will be observed that while some of the arsenides are sublimed
at a comparatively low temperature, others require a very high one.

_Antimony_ gives a white sublimate when its salts are roasted, as the
sulphide, or the antimonides themselves, or the oxide of this metal.
This white sublimate is not antimonious acid, but there is mixed with
it the oxide of antimony with which the acid is sublimed. As is the
case with arsenious acid, the antimonious acid may, by dexterous
heating, be driven from one portion of the tube to another.

_Tellurium_, or its acid and oxide, may be got as a sublimate in the
tube. The tellurious acid, unlike the arsenious and antimonious acids,
cannot be driven from one portion of the tube to another, but, on the
contrary, it fuses into small clear globules, visible to the naked eye
sometimes, but quite so with