. Journal of applied microscopy and laboratory methods. niumgenerally take the form of envelope-like crystallites, or if separating from dilutesolutions appear in hemimorphic forms like those of ammonium magnesiumphosphate, but of a greater size (Fig. 49). 1248 Journal of Applied Microscopy Remarks.—If much ammonium chloride ispresent, the crystals at first formed will rapidlydisappear, or there may be no separation of thecalcium salt owing to its marked solubility insolutions of ammonium chloride. The double ammonium arsenates are iso-morphous with the double ammonium phosphates,a fact which
. Journal of applied microscopy and laboratory methods. niumgenerally take the form of envelope-like crystallites, or if separating from dilutesolutions appear in hemimorphic forms like those of ammonium magnesiumphosphate, but of a greater size (Fig. 49). 1248 Journal of Applied Microscopy Remarks.—If much ammonium chloride ispresent, the crystals at first formed will rapidlydisappear, or there may be no separation of thecalcium salt owing to its marked solubility insolutions of ammonium chloride. The double ammonium arsenates are iso-morphous with the double ammonium phosphates,a fact which is liable to give rise to errors in theinterpretation of results. Moreover it happensthat the usefulness of this elegant reaction isunfortunately restricted, since the elements ofthe magnesium group, which are often presentin mixtures to be tested for calcium, unite toform double ammonium arsenates of like crystal-line forms minute stars and tiny crystalline grains, while barium yieldsa dense precipitate amorphous in Exercises for Practice. Try the above reaction on salts of calcium, strontium and barium, first alone,then in mixtures. Try on salts of magnesium, zinc and calcium. Try a salt of calcium in the presence of much ammonium chloride. VI. Priinary Sodium Carbonate added to solutions containing Calcium causesthe separation of crystalline Calcium Carbonate. CaCl., 4- 2HNaC03 = CaCOg + 2NaCl ; CO, + H2O. Method.—Cause a concentrated solution of the reagent to flow into a drop ofa dilute neutral, or ammoniacal, solution of the calcium salt. In a short timevery small disks and rhombs of the compound CaCOg appear. Remarks.—The addition of the reagent in solid form gives nearly as goodresults. Warming the preparation increases the rapidity of tiie reaction and leads tothe formation of better crystals. Unless the test drop is quite dilute an amorphous precipitate results. Ammonium carbonate can be substituted for the sodium salt, the crys
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