The tests are best made upon about ½ mg. of powder with HFl (pure and fuming). As soon as the fluorides begin to dry treat with dilute H_{2}SO_{4} and warm until white fumes of SO_{3} appear. In this way the HFl and SiFl_{4} are driven off and the sulphates are left. This part of the test can conveniently be made on a piece of platinum foil. Add excess of water and concentrate. Transfer a drop to a clean object glass and, while still liquid, examine it with the microscope. Do not use a cover-glass over the drop. Crystallizations Obtained by Behrens’ Method. _Potassium._ Add a little platinic chloride when octahedral crystals of K_{2}PtCl_{6} (size .18 to .30 mm.) will appear, which are clear, bright yellow in color with strong refraction. [Illustration: FIG. 82.—Potassium platinic chloride.[153] ] [Illustration: FIG. 83.—Sulphate of calcium (gypsum).[153] ] _Sodium._ Use sulphate of cerium and allow a small amount of this reagent to act through a capillary pipette upon a drop of the solution. Very small aggregates of brown crystals (size .02 mm.) of the double sulphate of cerium and sodium are formed, which are clearly visible with 600 diams. If potassium is present the double sulphate of that alkali will appear in larger, grayish grains (size .05 to .06 mm.). An excess of H_{2}SO_{4} is to be avoided. _Calcium._ After a few minutes little gypsum crystals (CaSO_{4} + 2H_{2}O) will appear. In strongly acid solutions the thin acicular crystals will be grouped in bushes or stars; in neutral solutions the crystals will have the normal shape of selenite crystals or form swallow-tailed twins. _Magnesium._ Use salt of phosphorus dissolved in water and allow it to mix with the solution (to which has been added ammonium chloride and ammonia) through a capillary pipette. From a solution containing more than 5 per cent. of magnesium are first deposited X-shaped skeletons and rudimentary crystals of Mg.NH_{4}.PO_{4} + 6H_{2}O. If the solution is more dilute beautiful, sharp, hemimorphic crystals (.10 to .20 mm. in size) of the orthorhombic system will appear. These crystals often resemble the roof of a house. The formation of the crystals is assisted by heat. Iron and manganese phosphates yield crystals of the same type, but the iron is separated on the addition of ammonia. _Aluminium._ Use chloride of cæsium. Take a drop of the solution, with excess of H_{2}SO_{4} driven off, and touch it with a platinum wire that has been dipped in the melted chloride of cæsium. Large crystals (.40-.90 mm. in size) of cæsium alum will form, which are octahedral and cubo-octahedral in shape. Iron does not interfere, as its crystallization would take place much more slowly. The solution should not be too concentrated. [Illustration: FIG. 84.—Magnesiumammonium phosphate.[154] ] [Illustration: FIG. 85.—Cæsium alum.[154] ] Special Tests.