particularly the case with the phosphates and borates. A piece of red
litmus paper will readily detect the escaping ammonia, while its odor
will indicate its presence with great certainty. The halogen compounds
of mercury, we should have mentioned, also sublime, the red iodide
giving a yellow sublimate.

The bulb is also a convenient little instrument for the purpose of
heating those substances which phosphoresce, and likewise those salts
that decrepitate.

Should the above reactions not be readily discerned, it should not be
considered as an indication that the substances are not present, for
they are frequently expelled in such combinations that the above
reactions will not take place. This is often the case with sulphur,
selenium, arsenic, and tellurium. It frequently happens, likewise,
that these substances are in such combinations that heat alone will
not sublime them; or else two or more of them may arise together, and
thus complicate the sublimate, so that the eye cannot readily detect
either substance. Sometimes sulphur and arsenic will coat the tube
with a metal-like appearance, which is deceptive. This coating
presents a metallic lustre at its lower portion, but changing, as it
progresses upward, to a dark brown, light brown, orange or yellow;
this sublimate being due to combinations of arsenic and sulphur, which
compounds are volatilized at a lower temperature than metallic
arsenic.

If certain reagents are mixed with many substances, changes are
effected which would not ensue with heat alone. _Formiate of soda_
possesses the property of readily reducing metallic oxides. When this
salt is heated, it gives off a quantity of carbonic oxide gas. This