Colorless, hydrous minerals (zeolites and chlorites) become cloudy in appearance. Colorless silicates, containing protoxide of iron (as olivine, or faintly colored pyroxene or amphibole), become red or reddish-brown. Colored minerals may change their color, chloritic substances becoming brown or black if heated enough. Hornblende always becomes pleochroic, and olivine sometimes becomes so. Members of the sodalite group may be turned blue, if not already of that color. The dichroism (yellow to blue) of almost colorless iolite may be developed. Carbonaceous particles may be distinguished from the iron oxides by being consumed.[141] METHODS OF ISOLATING CRYSTALS OR MINERAL FRAGMENTS FOR TESTING.[142] For the application of these methods the rock or aggregate of minerals should be reduced to homogeneous[143] grains of uniform size (preferably crystals or cleavages) and not to powder. This is best done by pounding in a metal mortar, avoiding all grinding motion. The separations required may be made by specific gravity solutions or magnetic methods, alone or combined; and in some cases may be assisted by chemical action. When other methods fail it may be necessary to separate single grains from a mixture by hand. A grooved piece of plate-glass, passed beneath the objective of the microscope, will be found useful. The desired grains in this groove can be picked out by means of a piece of fine waxed thread or a fine pointed stick moistened at the end. The _hardness_ of the homogeneous grains may be obtained by pressing them firmly into the end of a lead stamp or holder and trying the effect of scratching upon the faces of minerals of known hardness. _Specific Gravity Separation._[144] Accomplished by use of fluids of different specific gravity. These fluids can be made specifically lighter by dilution, and hence the fragments will fall to the bottom in order of decreasing density. Dilution of the heavy solutions to any specific gravity may be affected empirically until the solution will just suspend a fragment of a mineral having the desired specific gravity; or the exact specific gravity of the solution may be determined by the Westphal balance. The following _indicators_ may be employed to determine the limits of the specific gravity of the solution to be used for separation purposes (V. Goldschmidt):