(_c._) _Fusion._--Many substances when exposed to a certain degree of
heat lose their solid form, and are converted into a liquid. Those
substances which do not become converted into the liquid state by
heat, are said to be infusible. It is a convenient classification to
arrange substances into those which are fusible with difficulty, and
those which are easily fusible. Very often we resort to fusion for the
purpose of decomposing a substance, or to cause it to enter into
other combinations, by which means it is the more readily detected. If
insoluble substances are fused with others more fusible (reagents) for
the purpose of causing a combination which is soluble in water and
acids, the operation is termed _unclosing_. These substances are
particularly the silicates and the sulphates of the alkaline earths.
The usual reagents resorted to for this purpose are carbonate of soda
(NaO, CO^{2}), carbonate of potash (KO, CO^{2}), or still better, a
mixture of the two in equal parts. In some cases we use the hydrate of
barytes (BaO, HO) and the bisulphate of potash (KO, 2SO^{3}). The
platinum spoon is generally used for this manipulation.

Substances are exposed to fusion for the purpose of getting a new
combination which has such distinctive characteristics that we can
class it under a certain group; or for the purpose of ascertaining at
once what the substance may be. The reagents used for this purpose are
borax (NaO, 2BrO^{3}) and the microcosmic salt (NaO, NH^{4}O, PO^{5},
HO). Charcoal and the platinum wire are used as supports for this kind
of operation.


(_d._) _Oxidation._--The chemical combination of any substance with