The reactions of iodine are similar to those of bromine with concentrated sulphuric acid and binoxide of manganese, and with nitric acid:  The iodine is released and, if the quantity be not too great, colors the liquid brown.  If there be a considerable quantity of iodine present, it is precipitated as a dark colored powder.  Either of these, when heated, gives out the violet-color of the iodine.

With starch paste free iodine combines, producing a deep blue compound.  If, however, the iodine be in very minute quantity, the color, instead of being blue, will be light violet or rose color.

If to a solution of the sulphate of copper, to which a small portion of sulphurous acid has been added, a liquid containing iodine and bromine is poured in, a dirty, white precipitate of the subiodide of copper is produced, and the bromine remains in the solution.  The latter may then be tested for the bromine by strong sulphuric acid.

(11.) Fluorine (Fl).—­This element exists combined with sodium, calcium, lithium, aluminium, magnesium, yttrium, and cerium.  Fluorine also exists in the enamel of the teeth, and in the bones of some animals.  This element has a strong affinity for hydrogen, and, therefore, we find it frequently in the form of hydrofluoric acid.  Brazil-wood paper is the most delicate test for hydrofluoric acid, which it tinges of a light yellow color.  Phosphoric acid likewise colors Brazil paper yellow, but as this acid is not volatile at a heat sufficient to examine hydrofluoric acid, there can be no mistake.  If the substance is supposed to contain this acid, it should be placed on a slip of glass, and moistened with hydrochloric acid, when the test paper may be applied, and the characteristic yellow color will indicate the presence of the fluorine.

As hydrofluoric acid acts upon glass, this property may be used for its detection.  The substance may be put into a glass tube, and sulphuric acid poured upon it in sufficient quantity to moisten it; a slight heat applied to the tube will develop the acid, which will act upon the glass of the tube.  If the acid is retained in the mineral by a feeble affinity, and water be present, a piece of it may be put in the tube and heated, when the acid gas will be eliminated.  The test paper will indicate its presence, even before it has time to act upon the glass.  If the temperature be too high, fluosilicic acid is generated, and will form a silicious incrustation upon the cool portion of the tube.

If the fluorine is too minute to produce either of the above reactions, then the following process, recommended by Plattner, should be followed:  the assay should be mixed with metaphosphate of soda, formed by heating the microcosmic salt to dull redness.  The mass must then be placed in an open glass tube, in such a position that there will be an access of hot air from the flame.  Thus aqueous hydrofluoric acid is formed, which can be recognized by its smell being more suffocating than chlorine, and also by the etching produced by the condensation of vapor in the tube.  Moist Brazil paper, applied to the extremity of the tube, will be instantly colored yellow.