(_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 oxygen is termed _oxidation_, and the products are termed _oxides_. As these oxides have qualities differing from those which are non-oxidized, it therefore frequently becomes necessary to convert substances into oxides; or, if they are such, of a lower degree, to convert them into a higher degree of oxidation. These different states of oxidation frequently present characteristic marks of identity sufficient to enable us to draw conclusions in relation to the substance under examination. For instance, the oxidation of manganese, of arsenic, etc. The conditions necessary for oxidation, are high temperature and the free admission of air to the substance.