=265. Rohrbach’s Solution.=—The solution of barium mercuric iodid recommended by Rohrbach[171] for this purpose was originally prepared by Suchsin. The solution must be rapidly prepared on account of the tendency of the barium salt to decomposition. The solution is prepared by weighing rapidly 100 grams of barium and 130 grams of mercuric iodid, mixing the two salts well in a dry flask and adding twenty cubic centimeters of water. The mixture is raised to a temperature of 150°–200° on an oil-bath. The formation and solution of the double salt are promoted by constant stirring. After solution, the liquor is boiled for a few minutes and then evaporated on a water-bath until it will bear a crystal of epidote. On cooling, a small quantity of a yellow double salt is separated by crystallization and the resulting mother-liquor is dense enough to carry a fragment of topaz. Inasmuch as the liquor is filtered with difficulty, the clear mother-liquor should be separated by decantation after standing for several days. This solution has the disadvantage of not being dilutable with water, the addition of which causes a separation of red mercuric iodid. Were this solution not so easily decomposed, it would prove of high value in silt separation. =266. Braun’s Separating Liquid.=—In many respects the separatory solution proposed by Braun[172] is superior to those already mentioned. It is the commercial methylene iodid, CH₂I₂, which has at 16° a specific gravity of 3.32, at 5° of 3.35, and at 25° of 3.31. It is a strongly refractive liquid having a refractive index of 1.7466 for the yellow ray. As a separating medium the liquid is open to two objections; _viz._, first, it cannot be diluted with water and, second, it turns brown on heating or on long exposure to the sunlight. When dilution is necessary, it should be accomplished with benzene or xylene. To bring the diluted liquor again to its maximum density, the benzene must be removed by evaporation, which causes a considerable loss in the liquid. When this substance becomes opaque, the transparency may be restored by removing the separated iodin by shaking with potash lye, washing with pure water, drying by the addition of pieces of calcium chlorid and filtering. The same result may also be reached by freezing and separating the liquid portion. The frozen portion on melting will have the density of the original liquid.