From the solution, beautiful quadratic crystals separate having the composition 9WO₃,B₂O₃,2BaO₂H₂ + 18H₂O. These are purified by several recrystallizations and freed from any scales of boric acid by washing with alcohol. Any reducing action, revealed by a violet coloration of the crystals, can be avoided by adding a few drops of nitric acid. From a boiling solution of these crystals, the cadmium salt desired is obtained by treatment with the proper amount of cadmium sulfate solution to precipitate the barium. The barium sulfate is separated by filtration. The cadmium borotungstate is soluble in less than ten parts by weight of water. From this solution it is obtained in pure form by evaporation under a vacuum, or by carefully concentrating on a water-bath and cooling. A saturated solution of these crystals at 15° has a bright yellow color and a specific gravity of 3.28. If a dilute solution of the above salt be carefully evaporated on a water-bath, any violet color which may be present disappears when the specific gravity reaches 2.7. If the evaporation be continued until a crystal of augite will float on the hot liquid, crystals may be obtained on cooling which, dissolved in as little water as possible, make a solution which will almost support olivine. If the two solutions be united, the specific gravity of the mixture is 3.30–3.36. The highest attainable specific gravity; _viz._, 3.6, is produced by continuing the evaporation on a water-bath until the liquid will support olivine, and then allowing to stand in a closed place for twenty-four hours. The crystals of cadmium borotungstate thus obtained are freed as much as possible from the mother-liquor by drainage and then melted at about 75° in their own water of crystallization. A liquid is thus obtained on which spinel will float. The same concentration may also be obtained by careful heating on a water-bath. At its highest specific gravity this solution has an oily consistence and this renders its practical use in the separation of fine particles somewhat restricted. By filtering the liquor when a crystalline crust begins to form during evaporation, a cold solution of 3.360–3.365 specific gravity is obtained which is found practically useful. It has a higher specific gravity than Thoulet’s mixture, is not injurious to any of the mineral particles, not even of iron with which it is brought into contact, but the trouble of preparing it is far greater than that of the mixture of mercuric and potassium iodids.