In the first place it may be ascertained whether the mineral is soluble in water, and if so, to what extent. Then as to whether it becomes soluble in certain acids, such as hydrochloric or nitric acid. The former acid is generally used, except for metallic sulphides, and those minerals containing heavy metals, such as lead, silver, &c.; the latter is employed for the exceptions named. Several minerals, even when in a powdered state, are hardly, if at all, affected by acids. The results to be noted during the test with acids, commonly fall into the following three groups.[7] A. The mineral may dissolve quietly with or without colouring the solution; this holds good, for example, with hematite (a variety of iron), also of many of the sulphates and phosphates. B. There may be a bubbling off or effervescence of a gas, which gas is usually carbon dioxide; but may be hydrogen sulphide. Chlorine may be liberated, or reddish fumes of nitrogen. C. There may be separation of some insoluble substance as sulphur, silica, &c. We will close this chapter by stating the behaviour under blowpipe examination of various minerals, given in preceding pages, as being common in clays and earths used in brickmaking:-- _Quartz._--This is infusible, and remains undissolved, even in a microcosmic salt bead; but it fuses readily with soda, on charcoal. In the flame it splinters into fragments, which fly off with great rapidity. It is soluble in hydrofluoric acid. _Flint_, when pure, behaves in a similar manner. _Orthoclase Felspar._--Fusibility, 5; flame colouration brilliant yellow, when much sodium is present; not decomposed by hydrochloric acid. It may be distinguished from other common felspars by its high degree of fusibility. _Oligoclase Felspar._--Gives a sodium yellow flame; fusibility, 3.5; not decomposed by hydrochloric acid. _Biotite Mica._--With fluxes gives a strong iron reaction of yellowish red colour; decomposed in concentrated sulphuric acid, leaving a residue of siliceous matter. _Muscovite Mica._--When heated in a tube closed at one end, yields water which often gives fluorine reaction with brazil-wood test paper by colouring it straw-yellow; it is not decomposed by acids, and whitens and fuses only on thin edges. _Kaolin._--Is infusible; gives off water when heated in a closed tube; and with cobalt nitrate on charcoal, a fine alumina reaction is obtained. _Aluminium._--On charcoal, this becomes blue with cobalt nitrate, though if the surface is fused the reaction is not so clear. Prof. Cole advises that the soda-residue be dissolved in dilute hydrochloric acid, then evaporated to dryness, re-dissolved in that acid water, filter off any silica, and neutralise with ammonia; alumina is precipitated together with any iron present. The precipitate, if white, or nearly so, may be tested with cobalt nitrate, and the result is a fine blue colour.